310T Truck and Coach Technician Exam — Questions and Answers

Question 1: A 310T technician is diagnosing a severe shimmy on a Class 8 truck that occurs only between 85–95 km/h and disappears above and below that speed range. Steering components, wheel bearings, and tire balance have all been inspected and are within spec. What is the MOST probable cause?

Tire conicity causing a pull that excites steering resonance at a critical frequency
A propeller shaft phasing error creating a torque pulse that couples into the steering linkage
King pin wear exceeding 1.5mm allowing oscillation at the resonant frequency of the front axle steering geometry (Correct answer)
Loose tie rod end with worn taper seat

Correct answer: King pin wear exceeding 1.5mm allowing oscillation at the resonant frequency of the front axle steering geometry

Speed-specific shimmy that disappears above and below a narrow speed band is a classic resonance phenomenon. When king pin wear reaches a critical threshold (typically >1.5mm), it introduces enough free play that road input energy at a specific wheel rotational frequency matches the natural resonant frequency of the front axle steering geometry. Below and above that speed band, the inputs do not excite the resonant frequency. Other causes like tire balance or conicity produce shimmy across a broader speed range.

Question 2: During a high-pressure common rail (HPCR) fuel system diagnosis on a heavy truck, the technician observes that commanded rail pressure is 1,400 bar but actual rail pressure oscillates between 1,200 and 1,600 bar. Injector return flow values are normal for all cylinders. The pressure limiting valve (PLV) is new. What is the MOST likely cause of the rail pressure oscillation?

A faulty rail pressure sensor with an intermittent signal producing false oscillation readings
Excessive injector opening duration commanded by the ECU during transient conditions
A cracked common rail accumulator allowing pressure to dissipate unevenly between injection events
A high-pressure pump with a worn or damaged inlet metering valve (IMV) causing erratic volumetric output (Correct answer)

Correct answer: A high-pressure pump with a worn or damaged inlet metering valve (IMV) causing erratic volumetric output

Rail pressure oscillation (actual pressure swinging above and below commanded pressure) with normal injector return values points to the high-pressure pump's metering system. The inlet metering valve (IMV), also called the fuel volume control valve, regulates how much fuel is admitted to the pump's high-pressure section. A worn or damaged IMV cannot hold a precise duty-cycle-controlled opening, causing the pump to alternately over- and under-deliver fuel to the rail. This produces the characteristic oscillating pressure pattern. A faulty rail pressure sensor would produce erratic readings but the actual pressure would be stable (sensor fault ≠ system fault). A cracked rail would cause a consistent pressure drop, not oscillation. Excessive ECU-commanded injection duration would show as overconsumption and low rail pressure, not oscillation around the setpoint.

Question 3: During a pre-trip inspection on a coach, the driver reports that the left-front turn signal flashes at normal rate but the right-front flashes at double speed. All bulbs appear to be working. A technician finds that the right-front marker light circuit shares a ground with the turn signal. What is the MOST likely explanation?

The right-front turn signal bulb has a higher wattage than specified, drawing more current and speeding up the flasher
The body controller has a software fault that independently times the left and right flash rates
The shared ground has developed high resistance, causing the right-front turn signal bulb to see reduced current and the flasher to sense a low-load condition, interpreting it as a burnt bulb (Correct answer)
The turn signal flasher relay has an internal fault causing asymmetric timing between left and right circuits

Correct answer: The shared ground has developed high resistance, causing the right-front turn signal bulb to see reduced current and the flasher to sense a low-load condition, interpreting it as a burnt bulb

Electronic and thermal flashers determine flash rate by measuring circuit resistance/load. A high-resistance shared ground reduces effective current through the bulb. The flasher interprets the reduced current as a missing or burnt-out bulb and increases flash rate as a warning. Because all bulbs illuminate (even dimly through the shared ground), the technician may not notice the fault visually. A faulty flasher would likely affect both sides; a high-wattage bulb would slow the flash rate, not increase it; software faults are a last resort diagnosis.

Question 4: When rebuilding a recirculating ball steering gear on a coach, the technician sets the worm bearing preload to spec using a torque wrench at the input shaft. After reassembly on the vehicle, the steering feels excessively heavy on-center but loosens off-center. What adjustment error was most likely made?

The hydraulic relief valve pressure was set too low
The sector shaft over-center preload was set without first properly centering the gear, creating excessive mesh load at center (Correct answer)
The worm bearing preload was set too high
The pitman arm was installed one spline off, shifting the center point

Correct answer: The sector shaft over-center preload was set without first properly centering the gear, creating excessive mesh load at center

In a recirculating ball steering gear, the sector shaft over-center adjustment must be made with the gear precisely at the centered (straight-ahead) position. If the gear is not correctly centered before the over-center adjuster is tightened, the point of maximum mesh (highest preload) ends up offset from true center. The result is heavy, binding steering near the actual center position while feeling correct or loose off-center — which is the reverse of the intended design where minimal preload exists off-center.

Question 5: During a brake adjustment inspection on a tractor with S-cam drum brakes, you find that the slack adjuster on the left-front steer axle moves 2.5 inches at the clevis pin hole before any resistance is felt, but the push rod stroke at full application is only 1.6 inches. What does this combination MOST likely indicate?

The automatic slack adjuster is correctly adjusted but the chamber has insufficient stroke travel
The automatic slack adjuster is slipping on the camshaft splines, creating false free-play measurement (Correct answer)
The foundation brake is correctly adjusted and no corrective action is required
The brake shoes are worn beyond the minimum lining thickness, causing early cam-over

Correct answer: The automatic slack adjuster is slipping on the camshaft splines, creating false free-play measurement

High free-play at the clevis combined with a short actual push rod stroke is the classic signature of an ASA slipping on camshaft splines. The adjuster rotates freely instead of advancing the cam, so the clevis feels loose but the stroke never exceeds the cam-over threshold. Worn linings would produce excessive stroke, not reduced stroke. An undersized chamber would show insufficient stroke but not the large free-play at the clevis. Foundation brake adjustment is clearly abnormal here.

Question 6: A hydraulic pump produces adequate flow at low speed but cavitates at high engine RPM. What is the MOST likely cause?

A failed pressure relief valve stuck open
Excessive pump output pressure exceeding system rating
Air trapped in the hydraulic reservoir
A restricted inlet (suction) line or clogged suction strainer (Correct answer)

Correct answer: A restricted inlet (suction) line or clogged suction strainer

Cavitation occurs when the pump cannot draw in enough fluid to fill its displacement chambers, forming vapor bubbles that collapse violently. At low RPM the demand is modest, so a partially restricted suction line is adequate; at high RPM the pump demands more fluid than the restricted line can deliver, causing cavitation. A stuck-open relief valve causes pressure loss, not cavitation. Excessive output pressure damages downstream components. Air in the reservoir contributes but the primary cause of RPM-dependent cavitation is suction restriction.

Question 7: A technician is performing a front-end inspection on a truck with I-beam front axle. Checking for king pin wear, the CORRECT method is to:

Check king pin wear only by observing tire wear patterns
Grip the tire at 9 and 3 o'clock and push-pull to check for play
Measure king pin diameter with a micrometer while installed
Support the axle to unload the wheel, then grip the tire at 12 and 6 o'clock and rock it to check for vertical play (Correct answer)

Correct answer: Support the axle to unload the wheel, then grip the tire at 12 and 6 o'clock and rock it to check for vertical play

With the axle unloaded, rocking the tire at 12 and 6 o'clock reveals king pin vertical play; 9 and 3 o'clock rocking checks wheel bearing play.

Question 8: During a road test of a vehicle with a 10-speed automated mechanical transmission (AMT), the technician notes the transmission consistently refuses to upshift from 9th to 10th gear when the vehicle is lightly loaded and traveling on a grade exceeding 2%. The shift selector is in automatic mode and no fault codes are present. What is the MOST probable explanation?

The clutch actuator is unable to generate sufficient clamping force for 10th gear torque capacity at the detected grade
A calibration error in the grade resistance algorithm preventing 10th gear selection above 2% grade
A faulty output shaft speed sensor is causing the TCM to underestimate road speed
The transmission control module is applying grade-logic shift inhibit, a designed feature to prevent hunting on grades (Correct answer)

Correct answer: The transmission control module is applying grade-logic shift inhibit, a designed feature to prevent hunting on grades

AMT systems such as the Eaton UltraShift and ZF TraXon incorporate grade-logic algorithms that intentionally inhibit upshifts into the highest overdrive gear when the ECM detects a positive road grade. This prevents the transmission from hunting between 9th and 10th — a condition that would cause repeated clutch engagement cycles, poor driveability, and clutch wear. Since no DTCs are present, the system is operating as designed. This feature is particularly pronounced when load is light because the ECM cannot rely on engine load alone to determine the appropriate gear.

Question 9: A truck's brake balance test reveals that the rear axle brakes are applying 200 ms before the front axle brakes during a hard stop. Which condition is MOST likely responsible for this timing discrepancy?

A worn front brake camshaft bushings causing mechanical lag in the S-cam rotation
A restricted air line to the rear axle causing a pressure drop under application
A faulty front relay valve with excessive delivery volume causing delayed pressure rise (Correct answer)
Excessive slack adjuster travel on the rear axle causing premature lining contact

Correct answer: A faulty front relay valve with excessive delivery volume causing delayed pressure rise

A relay valve with excessive delivery volume (worn or damaged) takes longer to reach cracking pressure and build delivery pressure, causing a delayed application on the axle it serves. The front relay valve would delay front brake application, making the rear brakes apply first. Restricted lines cause slow pressure build but also delay — however, restriction would typically appear as a slow rise rather than a consistent timing offset. Slack adjuster travel and camshaft bushings cause mechanical inefficiency but not the millisecond-precision pneumatic timing offset seen in brake balance tests.

Question 10: A technician is diagnosing a low-power complaint on a diesel engine with a high-pressure common rail (HPCR) fuel system. A fuel rail pressure test reveals that the actual pressure is significantly lower than the desired pressure while under load. Which of the following is the LEAST likely cause of this condition?

A restricted fuel return line (Correct answer)
A worn high-pressure fuel pump
A leaking high-pressure fuel injector
A clogged fuel filter

Correct answer: A restricted fuel return line

Restricting the fuel return line on an HPCR system would cause housing pressure within the high-pressure pump to build excessively, potentially blowing out seals. It would not cause low rail pressure. Leaking injectors, a worn pump, or a clogged fuel filter are all common causes of low fuel rail pressure because they either prevent the system from building adequate pressure or introduce excessive leakage.

Question 11: A heavy truck is experiencing "memory steer," where it continues to pull to one side after a turn is completed and requires manual correction to straighten out. Which of the following is the MOST likely cause?

Incorrect front axle toe-in.
Excessive steering gear lash.
Worn or binding kingpins. (Correct answer)
A failed power steering pump.

Correct answer: Worn or binding kingpins.

Memory steer is characterized by the steering system's inability to return to center on its own after a turn. This is often caused by binding in the steering knuckle pivot points, with worn or improperly lubricated kingpins being a primary culprit on heavy trucks with solid front axles. The binding restricts the free movement of the knuckle, forcing the driver to manually correct the steering.

Question 12: Which fluids should be inspected as standard practice during a preventive maintenance service on a heavy-duty truck?

Engine oil and fuel only
Engine oil, coolant, transmission fluid, power steering fluid, and brake fluid (Correct answer)
Engine oil and coolant only
Only fluids that are visibly low

Correct answer: Engine oil, coolant, transmission fluid, power steering fluid, and brake fluid

A thorough PM service requires checking all fluid levels — engine oil, coolant, transmission, power steering, and brake fluid — to detect early loss or contamination.

Question 13: A vehicle equipped with ABS experiences wheel lockup on the steer axle during a hard stop on a split-mu surface (ice on left, dry pavement on right). The ABS warning lamp does NOT illuminate. What is the MOST probable explanation?

The ABS ECU has defaulted to a select-low control strategy, limiting braking to the lower-mu side's level
The wheel speed sensor air gap on the locking side exceeds specification, generating a false speed signal (Correct answer)
The ABS modulator valve on the locking wheel has failed open, unable to dump pressure
The differential pressure across the axle has exceeded the ABS ECU's authority limit, causing passive braking

Correct answer: The wheel speed sensor air gap on the locking side exceeds specification, generating a false speed signal

If the wheel speed sensor air gap is out of specification (too large), the sensor generates a weaker signal that may read as a speed higher than actual — the ABS ECU sees the wheel as still rolling and does not modulate. This results in lockup without triggering an ABS fault code because the ECU has no indication of a sensor fault; it believes the wheel is rotating normally. A failed-open modulator valve would typically set a fault code (solenoid circuit monitoring). Select-low strategy is valid for some ABS configurations but would prevent lockup, not cause it. ABS ECUs do not have an 'authority limit' that allows lockup without fault.

Question 14: Which component prevents moisture and contaminants from entering the air brake system from the compressor?

Wet tank drain
Air dryer (Correct answer)
Pressure protection valve
Unloader valve

Correct answer: Air dryer

The air dryer removes moisture and oil vapor from compressor discharge air before it reaches the reservoirs, preventing corrosion and freezing.

Question 15: A truck's J1939 CAN bus network shows intermittent communication faults only when the ambient temperature exceeds 35°C. Voltage on the CAN High wire measures 2.8V and CAN Low measures 2.2V at idle. What is the MOST likely root cause?

A terminating resistor with a positive temperature coefficient is drifting out of spec under heat, causing impedance mismatch (Correct answer)
The body control module is entering thermal shutdown and dropping off the bus
CAN High and CAN Low wires are reversed at a connector near the firewall
The J1939 data link is overloaded with too many broadcast messages per second

Correct answer: A terminating resistor with a positive temperature coefficient is drifting out of spec under heat, causing impedance mismatch

The differential voltage (CAN H 2.8V, CAN L 2.2V) is within spec at rest but the temperature-dependent behavior points to a passive component failure. A terminating resistor that drifts above 120Ω under heat raises bus impedance, causing signal reflections and intermittent faults. Reversed wires would cause constant faults; a module in thermal shutdown would set its own DTCs; bus overload is unrelated to temperature.

Question 16: A tractor-trailer exhibits a delayed brake release on the rear trailer axles only after extended downhill operation. The service brake chambers are within spec and air pressure is normal. Which condition is MOST likely causing this symptom?

Brake lining thermal glazing increasing drum-to-lining coefficient of friction after cool-down
Trailer supply valve not fully opening due to a worn O-ring on the spool valve
Heat-induced spring brake modulator valve sticking due to thermal expansion of the valve body
Relay valve delivery port restricted by contaminated desiccant particles from the air dryer (Correct answer)

Correct answer: Relay valve delivery port restricted by contaminated desiccant particles from the air dryer

Desiccant particles from a failing or saturated air dryer can migrate downstream and lodge in relay valve delivery ports. During extended downhill braking, heat causes the valve body to expand slightly, trapping particles. The resulting restriction delays air exhaust from brake chambers, causing slow release specifically on the trailer axles served by that relay valve. Thermal glazing affects application force, not release timing, and spring brake modulators are not involved in normal service brake release.

Question 17: A technician finds gray, milky residue on the oil filler cap of a diesel engine. What is the most probable cause?

Excessive fuel dilution of the oil
Coolant intrusion into the engine oil (Correct answer)
Normal blow-by from worn rings
Condensation from short trip driving

Correct answer: Coolant intrusion into the engine oil

A milky or gray emulsion on the oil cap indicates coolant mixing with engine oil, typically from a head gasket failure or cracked cylinder head.

Question 18: When inspecting a hydraulic cylinder that is leaking past the rod seal, a technician notices the rod surface has a spiral groove worn into it. What caused this damage?

Over-pressurization of the cylinder
Incorrect rod material for the application
Excessive extension speed causing hydraulic hammer
Contaminated hydraulic fluid containing abrasive particles (Correct answer)

Correct answer: Contaminated hydraulic fluid containing abrasive particles

A spiral groove on a hydraulic rod is a classic signature of abrasive contamination in the fluid. Dirt or metal particles caught between the rod and wiper/seal rotate as the rod travels, cutting a helical pattern. Over-pressurization causes seal extrusion, not spiral scoring; incorrect material would cause uniform corrosion or pitting; hydraulic hammer damages end caps and hoses, not the rod surface in a spiral pattern.

Question 19: A truck's cruise control disengages randomly at highway speeds. After ruling out brake switch and clutch switch faults, what should the technician check NEXT?

Throttle position sensor calibration
Transmission gear selector position switch
Fuel injector pulse width
Vehicle speed sensor signal quality and wiring (Correct answer)

Correct answer: Vehicle speed sensor signal quality and wiring

Cruise control systems rely on a steady vehicle speed signal; an erratic VSS signal caused by a worn reluctor ring or damaged harness will cause random disengagement.

Question 20: A coach equipped with an Allison World Transmission (WT) series reports harsh 4-3 downshifts only when the retarder is active and coolant temperature exceeds 95°C. All hydraulic pressures are within specification at normal operating temperature. What is the MOST probable cause?

C3 clutch piston seal extrusion caused by elevated sump temperature reducing seal elasticity
The transmission ECU defaulting to a fixed-pressure shift schedule because the retarder temperature sensor has exceeded its calibration range
Retarder modulator valve sticking due to varnish deposits from thermally degraded transmission fluid (Correct answer)
Torque converter lockup clutch slip caused by reduced hydraulic circuit pressure at elevated fluid viscosity

Correct answer: Retarder modulator valve sticking due to varnish deposits from thermally degraded transmission fluid

When ATF thermally degrades, varnish and lacquer deposits preferentially accumulate on close-tolerance valve spools. The retarder modulator valve controls hydraulic pressure blending during retarder-on downshifts; if it sticks, the transmission cannot smoothly reduce retarder apply pressure before executing the clutch-to-clutch shift, resulting in a torque spike and harsh engagement. This fault is temperature-conditional because varnish softens and becomes tacky above ~90°C, causing intermittent sticking that does not appear during normal cold or moderate temperature operation. ECU sensor defaults and seal extrusion would produce different, broader fault patterns.

Question 21: What is the primary function of an air filter restriction indicator on a diesel engine?

To monitor turbocharger speed
To measure fuel rail pressure
To detect exhaust backpressure leaks
To warn when air filter restriction has reached its service limit (Correct answer)

Correct answer: To warn when air filter restriction has reached its service limit

The restriction indicator signals when the pressure drop across the air filter exceeds acceptable limits, indicating the filter must be replaced.

Question 22: A truck's ABS control module sets a fault code for 'wheel speed sensor signal erratic' on the left front sensor only during highway driving above 90 km/h. At lower speeds the sensor reads correctly. The sensor air gap and tone ring are within specification. What is the MOST probable cause?

The tone ring has a cracked tooth that only becomes detectable at high rotational speeds due to centrifugal expansion
The ABS module has a faulty internal comparator that misreads high-frequency signals
The sensor wiring has excessive capacitance from a poor repair splice, causing signal roll-off at high frequencies
The wheel bearing has excessive end-play causing the rotor to wobble and vary the air gap dynamically at high speed (Correct answer)

Correct answer: The wheel bearing has excessive end-play causing the rotor to wobble and vary the air gap dynamically at high speed

Wheel bearing end-play creates a dynamic variation in the air gap between the sensor and tone ring. At low speeds the variation is slow enough that the ABS module can compensate, but at higher rotational speeds the gap changes rapidly, causing signal amplitude to fluctuate beyond the module's tolerance window — resulting in an 'erratic' fault. A cracked tone ring tooth would cause a consistent missing pulse at any speed. Excessive wiring capacitance would cause a gradual roll-off at all speeds above a threshold, not a sharp onset at 90 km/h tied to wheel bearing dynamics.

Question 23: What is the correct procedure for disposing of used engine oil after completing a PM service?

Dispose of it in an approved solid waste landfill
Store it in an approved container and deliver it to a certified recycling facility (Correct answer)
Mix it with diesel fuel and burn it in the engine
Pour it into a floor drain connected to a separator

Correct answer: Store it in an approved container and deliver it to a certified recycling facility

Used engine oil is a regulated hazardous waste and must be collected in approved containers and recycled through a licensed used-oil collection facility under EPA regulations.

Question 24: What federal agency regulates commercial vehicle safety standards that truck technicians must be familiar with for compliance-related career roles?

Federal Motor Carrier Safety Administration (FMCSA) (Correct answer)
Occupational Safety and Health Administration (OSHA)
Environmental Protection Agency (EPA)
National Highway Traffic Safety Administration (NHTSA)

Correct answer: Federal Motor Carrier Safety Administration (FMCSA)

The FMCSA establishes and enforces safety regulations for commercial motor vehicles, and technicians in compliance or inspection roles must be well-versed in FMCSA standards.

Question 25: A diesel engine's DPF (diesel particulate filter) requires frequent active regeneration cycles. What underlying condition most likely causes this?

Cold ambient temperatures reducing exhaust temperature
High engine load operation that burns soot quickly
Excessive oil consumption or fuel contamination causing excess soot production (Correct answer)
Using low-sulfur diesel fuel

Correct answer: Excessive oil consumption or fuel contamination causing excess soot production

Excessive soot from burning engine oil or fuel dilution overloads the DPF faster than normal, triggering more frequent regeneration events.

Question 26: A diesel engine on a Class 8 truck exhibits black smoke under load but clears at idle. Fuel pressure and injector return flow tests are within spec. A compression test shows all cylinders within 10% of each other. Which condition is MOST likely causing this symptom?

A faulty fuel pressure regulator allowing excess fuel delivery at idle
A restricted charge air cooler reducing boost density at high load (Correct answer)
Worn piston rings allowing blow-by only at elevated cylinder pressures
A cracked cylinder head causing intermittent coolant intrusion into combustion

Correct answer: A restricted charge air cooler reducing boost density at high load

Black smoke under load that clears at idle indicates a rich air-fuel ratio specifically when demand is high. A restricted charge air cooler (CAC/intercooler) reduces the density and volume of intake air reaching the cylinders under boost conditions, creating a fuel-rich mixture and black smoke. Compression is acceptable, injectors are fine, and the symptom being load-dependent points to the air-supply side, not fuel-side over-delivery.

Question 27: What is a potential long-term career opportunity for certified technicians?

Switching to public relations
Becoming an academic researcher
Starting their own repair business (Correct answer)
Transitioning to retail sales

Correct answer: Starting their own repair business

With extensive technical knowledge, practical experience, and potentially some business acumen, certified technicians have the foundation to establish their own repair shops. This entrepreneurial path allows them to be self-employed and manage their own operations. It represents a significant long-term career opportunity leveraging their specialized skills.

Question 28: A coach is brought in with the complaint that the spring parking brakes will not release. The air system builds to the correct cut-out pressure, and there are no audible leaks. Which of the following is a plausible cause for this failure?

Excessive slack adjuster free play
A malfunctioning inversion valve (Correct answer)
A failed service brake relay valve
A clogged air dryer filter

Correct answer: A malfunctioning inversion valve

The inversion valve controls the application and release of the spring brakes. It uses pilot air pressure to hold off the spring brakes. If the inversion valve malfunctions, it can prevent air pressure from being supplied to the spring brake chambers to compress the powerful springs, thus keeping the parking brakes applied even when the dash valve is pushed in. The other components listed are related to the service brakes or air supply, not specifically the release of the spring brakes.

Question 29: When a hydraulic power steering pump is pressure-tested and output pressure is low at idle but reaches spec when the engine speed is increased, what is indicated?

A collapsed high-pressure hose
A leaking power steering gear
A faulty pressure relief valve
A worn or faulty flow control valve (Correct answer)

Correct answer: A worn or faulty flow control valve

A worn flow control valve allows flow to bypass at low pump speeds, reducing pressure at idle while higher RPM compensates through increased volume.

Question 30: While inspecting a heavy truck's recirculating-ball steering gear, a technician measures 0.008 inches (0.20 mm) of worm bearing preload turning torque at the input shaft with the gear on-center, and 0.004 inches (0.10 mm) of over-center preload. The manufacturer specifies worm bearing preload of 4–8 in-lb and over-center preload (total on-center) of 10–14 in-lb. What is the correct interpretation?

The worm bearing preload is insufficient and requires adjustment of the worm bearing adjuster plug
The over-center preload is insufficient and requires adjustment of the sector shaft adjuster screw (Correct answer)
Both adjustments are out of specification; the gear must be replaced
Both preloads are acceptable; the gear can be returned to service

Correct answer: The over-center preload is insufficient and requires adjustment of the sector shaft adjuster screw

Recirculating-ball steering gear adjustments are made in a specific sequence: worm bearing preload is set first (via the adjuster plug), then over-center preload is set (via the sector shaft/pitman shaft adjuster screw) with the total on-center torque specified. The question gives the measurements in inches but indicates they correspond to specific in-lb values through context—the worm bearing reading of 4–8 in-lb is within spec. The over-center total must be 10–14 in-lb; the reading of 0.10 mm (representing the increment above worm preload) brings the total below 10 in-lb, meaning the sector shaft adjuster must be tightened. Since worm preload is within range, the worm adjuster plug is left alone.

Question 31: A technician is diagnosing a cab pressurization complaint on a highway coach. The cab pressure differential to atmosphere is only 25 Pa instead of the specified 50–75 Pa. Fresh air intake, recirculation dampers, and blower speed all check out. A smoke test reveals leakage at the windshield lower seal and door seals. After sealing these areas, the pressure differential only rises to 35 Pa. What additional system component should the technician inspect?

The cab overpressure relief valve, which may be set too low or stuck partially open (Correct answer)
The fresh air intake actuator for proper damper closure in pressurization mode
The evaporator condensate drain check valve, which may be stuck open allowing outside air in-flow
The return air duct for restrictions reducing blower efficiency

Correct answer: The cab overpressure relief valve, which may be set too low or stuck partially open

With seal leaks addressed, the remaining deficit in pressurization pressure (35 Pa vs. 50–75 Pa target) points to a component that is actively limiting how high the pressure can build. The cab overpressure relief valve is designed to open at a set differential to prevent excessive positive pressure. If it is calibrated too low, corroded in a partially open position, or has a weakened spring, it bleeds off pressure before the target differential is reached. The condensate drain check valve prevents rain ingestion and is not a major source of pressure loss. Fresh air actuator and return air restrictions affect flow but would have been identified during the initial checks.

Question 32: On a truck with an engine-off auxiliary HVAC system using a dedicated scroll compressor driven by a battery bank, the system cools well for the first 45 minutes but then progressively loses capacity over the next 30 minutes until it can no longer maintain setpoint. The battery voltage remains above 12.2V throughout. What is the MOST likely cause?

Progressive thermal saturation of the condenser coil due to loss of airflow from the condenser fan motor overheating (Correct answer)
Refrigerant migrating to the compressor crankcase during the cool-down period, causing liquid slugging
The high-pressure cutout cycling the compressor as ambient heat soaks into the condenser shroud
Battery internal resistance increase under sustained load causing voltage drop under dynamic current draw

Correct answer: Progressive thermal saturation of the condenser coil due to loss of airflow from the condenser fan motor overheating

In an APU/auxiliary HVAC system with no vehicle airflow, the condenser relies entirely on its own fan for heat rejection. If the condenser fan motor overheats due to sustained operation and thermally limits or fails progressively, the condenser cannot reject heat efficiently. As condenser temperatures rise, high-side pressure climbs, compressor work increases, and cooling capacity degrades in a predictable time-based pattern — matching the symptom. Static battery voltage above 12.2V does not rule out voltage drop under load, but that would affect compressor speed uniformly, not progressively. Refrigerant migration to the crankcase would cause slugging at startup, not 45 minutes in. The high-pressure cutout would cause abrupt cycling, not a gradual capacity reduction.

Question 33: During a charging system analysis on a heavy-duty truck with the engine running, a technician notices the headlights are dim and the system voltage is 12.5V. Which of the following conditions is the MOST likely cause?

A fully charged battery with high internal resistance.
A faulty voltage regulator or failing alternator. (Correct answer)
Excessive resistance in the battery ground cable.
An open circuit in the starter solenoid.

Correct answer: A faulty voltage regulator or failing alternator.

A healthy charging system should maintain a voltage between approximately 13.5 and 14.5 volts to charge the batteries and power the vehicle's electrical loads. A voltage of 12.5V is essentially surface charge from the battery and indicates the alternator is not producing adequate output, which is most often caused by a failed internal voltage regulator or other alternator component failure.

Question 34: A technician is installing a new ECM on a heavy-duty truck. After installation, the engine will not start. The MOST likely reason is:

The replacement ECM requires programming or key-matching to the vehicle (Correct answer)
The ECM is not properly grounded
The battery voltage is too high
The fuel pump relay is faulty

Correct answer: The replacement ECM requires programming or key-matching to the vehicle

Most modern truck ECMs must be programmed with the vehicle's VIN, engine calibration, and security parameters before the engine will start.

Question 35: A 13-speed manual transmission equipped truck is experiencing gear skip-out specifically from 7th gear (direct drive) under sustained highway load. All synchronizer rings test within spec. What is the MOST probable root cause?

Worn detent ball and spring on the main shift rail for 7th/8th
Contaminated shift tower oil causing high-speed hydraulic lock in the range cylinder
A cracked shift fork that only flexes under sustained torsional stress
Excessive end-play in the mainshaft allowing axial movement under torque load (Correct answer)

Correct answer: Excessive end-play in the mainshaft allowing axial movement under torque load

In direct drive (7th gear), the mainshaft and countershaft rotate as a coupled unit, placing maximum axial thrust loads on the mainshaft bearings. Excessive mainshaft end-play allows the gear to walk off its engagement teeth under sustained load even when the synchronizer passes a static test, because the ring is checked without the dynamic axial force present at highway cruise. A worn detent would cause skip-out across multiple gears and at low speed. A cracked shift fork and range cylinder issues would present differently — fork cracks typically cause grinding/noise, and range cylinder problems affect all high-range gears.

Question 36: What does a hydraulic system relief valve do when system pressure reaches its set point?

It bypasses excess fluid back to the reservoir to limit maximum system pressure (Correct answer)
It shuts off the pump motor to prevent over-pressurization
It opens a secondary circuit to share load with another pump
It closes the main control valve to stop cylinder movement

Correct answer: It bypasses excess fluid back to the reservoir to limit maximum system pressure

A relief valve is a pressure-limiting safety device: when system pressure hits the set point, the valve opens and diverts fluid back to the reservoir (tank), holding pressure at the maximum set value without damaging components. It does not stop the pump motor, engage a secondary circuit, or close control valves — those are functions of separate components.

Question 37: A coach equipped with a series-sequential twin-turbocharger system has low power complaints at low speeds but acceptable high-speed power. Both turbos and all plumbing are intact. Which component failure BEST explains this symptom pattern?

The primary turbocharger compressor wheel has erosion damage reducing its low-RPM efficiency
The bypass valve (series valve) that should keep the small primary turbo active at low RPM is stuck open, prematurely engaging the large secondary turbo (Correct answer)
The intercooler outlet temperature sensor is sending a high-temperature signal, causing ECM to retard injection timing
The large secondary turbocharger wastegate is stuck closed, over-boosting at high RPM

Correct answer: The bypass valve (series valve) that should keep the small primary turbo active at low RPM is stuck open, prematurely engaging the large secondary turbo

In a series-sequential twin-turbo system, a small primary turbo is designed to spool quickly at low RPM, providing good low-end response. A larger secondary turbo takes over or assists at higher RPM. If the bypass valve (which normally routes exhaust through the primary at low RPM) is stuck open, exhaust flow is immediately split or directed to the larger turbo, which spools slowly and provides poor boost at low speeds. High-speed performance is adequate because the large turbo eventually reaches efficiency. A stuck closed wastegate would cause over-boost at high RPM, not low-RPM weakness. Sensor errors or compressor erosion present differently.

Question 38: On a tractor-trailer combination, the trailer's emergency line loses pressure rapidly but the service line remains pressurized. The red glad hand is secure. Where is the fault MOST likely located?

A defective trailer brake relay valve that is cross-porting service pressure into the emergency circuit
A failed tractor protection valve that has closed the emergency supply to the trailer
An internal failure of the trailer's relay emergency valve causing it to exhaust the emergency port (Correct answer)
A ruptured trailer emergency reservoir causing rapid pressure loss through the check valve

Correct answer: An internal failure of the trailer's relay emergency valve causing it to exhaust the emergency port

The trailer relay emergency valve (also called the trailer control valve or emergency relay valve) contains both emergency and service functions. An internal failure — specifically a cracked or displaced piston or failed seat — can cause the valve to exhaust the emergency supply port to atmosphere while still allowing service pressure to operate normally through a separate circuit path. Since the tractor protection valve monitors the emergency line pressure to decide whether to close, a rapid trailer-side exhaust can trigger tractor protection but the fault originates at the trailer valve. A failed tractor protection valve would cut BOTH emergency and service to the trailer. A ruptured reservoir would typically be gradual and audible at the trailer.

Question 39: Which type of slack adjuster requires manual repositioning when brake lining clearance exceeds specifications?

Manual slack adjuster (Correct answer)
Automatic slack adjuster (ASA)
Self-energizing adjuster
Stroke-sensing adjuster

Correct answer: Manual slack adjuster

Manual slack adjusters must be physically repositioned by a technician using a wrench to restore proper pushrod stroke, unlike ASAs which adjust automatically.

Question 40: During a 310T engine diagnostic, a technician measures blowby using a manometer at the crankcase breather. The reading is excessive. Compression tests on all cylinders return within specification. What is the MOST probable cause of the elevated blowby?

A cracked cylinder head allowing combustion gases to enter the coolant passages
Worn valve stem seals permitting oil consumption without affecting blowby
Worn piston ring grooves allowing ring flutter at high RPM despite acceptable static compression (Correct answer)
A faulty positive crankcase ventilation (PCV) valve stuck in the open position

Correct answer: Worn piston ring grooves allowing ring flutter at high RPM despite acceptable static compression

Worn piston ring grooves can cause ring flutter — where rings oscillate in their grooves at high RPM — allowing combustion gases to bypass into the crankcase. Static compression tests may read normal because the rings seal adequately during the slow cranking of a compression test but fail dynamically. A cracked head causes coolant contamination, not blowby. Worn valve stem seals cause oil consumption, not blowby. A stuck-open PCV valve would affect crankcase pressure readings but is not a source of blowby gases.

Question 41: A technician measures 450 V DC between the positive and negative HV bus terminals after removing the manual service disconnect. What should the technician do?

Cover the terminals with electrical tape and complete the repair quickly to minimize exposure
Proceed with repairs — the disconnect was already removed so the voltage will discharge on its own
Immediately reinstall the MSD and consult the OEM service manual before continuing (Correct answer)
Short the two bus terminals together through a 10-ohm resistor to force rapid discharge

Correct answer: Immediately reinstall the MSD and consult the OEM service manual before continuing

Residual voltage of 450 V after MSD removal indicates that the capacitors have not yet discharged or there is an isolation fault — either way, the system is still lethally dangerous. The correct action is to reinstall the MSD (restore safety isolation), wait the full OEM-specified discharge time, and consult the service manual before re-approaching. Proceeding with repairs at 450 V risks electrocution. Shorting with a resistor without OEM authorization risks arc flash and component damage. Taping terminals and working quickly is entirely unsafe.

Question 42: What is the primary function of a Variable Geometry Turbocharger (VGT) at low engine speeds?

To open its vanes to maximize exhaust flow and prevent over-speeding
To bypass exhaust gas around the turbine to limit maximum boost pressure
To act as an exhaust brake by creating high backpressure
To close its vanes to increase exhaust gas velocity, improve turbine response, and build boost quickly (Correct answer)

Correct answer: To close its vanes to increase exhaust gas velocity, improve turbine response, and build boost quickly

At low engine speeds, the vanes inside a VGT move to a more closed position, narrowing the passage for exhaust gases. This restriction increases the velocity of the exhaust gas hitting the turbine wheel, causing it to spool up faster and generate boost more quickly, which reduces turbo lag and improves low-end torque.

Question 43: On a tractor-trailer combination, the trailer supply valve (also called the tractor protection valve control) is normally what color?

Blue
Red (Correct answer)
Green
Yellow

Correct answer: Red

The trailer supply valve knob is red (octagonal) and controls the emergency/supply air line to the trailer; yellow is the trailer park control.

Question 44: A fleet technician notices that a coach's Allison transmission defaults to manual mode and will not auto-shift. Which system failure would MOST likely cause this?

Low transmission fluid temperature
Loss of communication between the TCM and engine ECM (J1939 datalink fault) (Correct answer)
Blown fuse to the shift selector
Failed output speed sensor

Correct answer: Loss of communication between the TCM and engine ECM (J1939 datalink fault)

Allison transmissions rely on J1939 CAN bus communication with the engine ECM for throttle position and load data; a datalink fault causes the TCM to enter a limp-home mode.

Question 45: A technician measures 0.8 V drop across a battery-to-starter cable during a crank attempt. SAE specification allows a maximum of:

1.0 V
0.1 V
0.5 V (Correct answer)
0.2 V

Correct answer: 0.5 V

SAE J541 specifies a maximum 0.5 V drop across any single heavy-gauge cable in the starting circuit; 0.8 V indicates excessive resistance.

Question 46: While diagnosing an ABS modulator valve, you cycle the valve electrically and observe correct inlet solenoid function (holds pressure) but the exhaust solenoid fails to vent the chamber quickly enough, resulting in a 300ms longer release than specification. The solenoid resistance measures 6.2 ohms (spec: 5.5–7.0 ohms). What is the MOST likely cause of the slow exhaust?

Incorrect supply voltage to the ABS ECU causing reduced solenoid drive duty cycle
A failing wheel speed sensor providing incorrect speed data and causing premature exhaust solenoid activation
A contaminated or partially blocked exhaust port orifice in the modulator valve body (Correct answer)
Electrical resistance too high, causing insufficient current to fully open the exhaust solenoid plunger

Correct answer: A contaminated or partially blocked exhaust port orifice in the modulator valve body

Since the solenoid coil resistance is within the 5.5–7.0 ohm specification, the electrical side of the exhaust solenoid is functioning correctly. Slow exhaust despite a properly energized solenoid points to a mechanical restriction — specifically a contaminated or partially blocked exhaust port orifice. Desiccant debris, oil contamination from compressor ring blow-by, or ice formation can partially block the small exhaust orifice, slowing chamber depressurization without triggering an electrical fault code. A wheel speed sensor issue would cause different fault patterns, not a repeatable slow exhaust on bench cycle testing.

Question 47: Which of the following symptoms is most commonly associated with a clogged fuel filter in a heavy-duty truck?

Excessive black smoke under load
Engine overheating
Loss of power, especially during acceleration or when climbing grades (Correct answer)
A loud knocking sound from the engine

Correct answer: Loss of power, especially during acceleration or when climbing grades

A clogged fuel filter restricts the flow of fuel to the engine. This fuel starvation becomes most apparent when the engine's demand for fuel is highest, such as during acceleration or when under heavy load (e.g., climbing a hill), resulting in a noticeable loss of power.

Question 48: When conducting a governed speed test on a diesel truck engine using diagnostic software, the technician notices the actual governed RPM is 47 RPM higher than the ECM's programmed value, and the discrepancy appears only under no-load conditions. The most likely cause is:

A faulty throttle demand sensor providing a higher-than-commanded signal to the fuel control module
Injector trim codes not properly loaded after an injector replacement, causing fueling imbalance
A miscalibrated crankshaft position sensor causing the ECM to misread actual RPM
Governor droop intentionally programmed to allow RPM rise under no-load as a design characteristic (Correct answer)

Correct answer: Governor droop intentionally programmed to allow RPM rise under no-load as a design characteristic

Governor droop (sometimes called 'speed droop') is an intentional ECM parameter that allows engine speed to rise slightly above the rated governed speed under no-load or light-load conditions. As load increases, the engine settles to the programmed rated speed. A small no-load overspeed of 30–60 RPM above the rated governed speed is entirely normal and by design in most diesel engines — it ensures a smooth transition as load is applied. This is a subtle but important distinction: the governed speed is typically the full-load rated speed, not the no-load speed. A CPS fault would manifest across load conditions and would trigger a fault code.

Question 49: A truck equipped with a Selective Catalytic Reduction (SCR) system is experiencing increased Diesel Exhaust Fluid (DEF) consumption and has an active P20EE fault code. What is the MOST likely cause of this issue?

Low-quality or contaminated DEF
A failing turbocharger actuator
A clogged Diesel Particulate Filter (DPF)
A malfunctioning upstream NOx sensor (Correct answer)

Correct answer: A malfunctioning upstream NOx sensor

The P20EE code indicates that the SCR catalyst efficiency is below the required threshold. The aftertreatment control module monitors NOx levels using both upstream (engine-out) and downstream (tailpipe-out) NOx sensors. If the upstream NOx sensor provides inaccurately high readings, the system will inject an excessive amount of DEF to compensate, leading to increased consumption and eventually triggering an efficiency code when the downstream sensor does not report the expected reduction in NOx.

Question 50: A technician measures 0.006 inches of clutch brake squeeze clearance on a pull-type clutch during a routine inspection. The OEM specification is 0.125 inches. What is the CORRECT interpretation and immediate consequence of this condition during operation?

The release bearing will contact the pressure plate fingers prematurely, causing clutch chatter during engagement from a stop
The clutch brake will drag continuously, slowing the transmission input shaft before complete pedal travel is reached, causing grinding when engaging gear from neutral (Correct answer)
Excessive clutch pedal free play results, preventing the release bearing from contacting the clutch brake entirely
The clutch will slip during high-load engagement because the pressure plate cannot achieve full clamping force with insufficient release bearing travel

Correct answer: The clutch brake will drag continuously, slowing the transmission input shaft before complete pedal travel is reached, causing grinding when engaging gear from neutral

On a pull-type clutch, the clutch brake is designed to engage during the last 1–2 inches of pedal travel to decelerate the transmission input shaft for clash-free gear engagement from neutral. The 0.006-inch clearance (versus the 0.125-inch spec) means the release bearing is sitting nearly against the clutch brake at rest. Even moderate pedal application will contact and drag the clutch brake well before full disengagement, causing the input shaft to slow prematurely and the clutch brake to wear rapidly. Gear clashing when selecting from neutral is the operational symptom. Insufficient clearance is never a slip or free-play issue — those result from excess clearance.

Question 51: On a tandem-axle suspension, what is the main purpose of longitudinal and transverse torque rods?

To absorb road vibrations for a smoother ride.
To support the vertical load of the vehicle.
To control axle positioning and absorb braking and acceleration forces. (Correct answer)
To provide a mounting point for the air springs.

Correct answer: To control axle positioning and absorb braking and acceleration forces.

Torque rods are critical links that connect the axle housing to the frame. They prevent the axle from rotating during acceleration and braking (axle wrap) and also maintain the axle's lateral and longitudinal alignment, which is essential for proper handling and driveline angles.

Question 52: A trailer's tail lights work but the brake lights do not activate. The tractor's brake lights work correctly. Which 7-way trailer connector pin should a technician test first?

Pin 3 (Left turn / Stop) (Correct answer)
Pin 1 (Ground)
Pin 7 (Reverse / Aux)
Pin 4 (Electric Brakes / Battery charge)

Correct answer: Pin 3 (Left turn / Stop)

On the standard SAE J560 7-way connector, Pin 3 carries the left turn/stop signal and Pin 4 carries the right turn/stop signal. Since tail lights (Pin 5 and 6) work but stop lights do not, the brake light signal circuit (Pins 3 and 4) is suspect. Pin 1 is ground — if it were faulty, tail lights would also fail. Pin 7 is reverse/auxiliary. Starting with Pins 3 and/or 4 targets the failed function directly.

Question 53: A trailer's spring-applied, air-released parking brakes release when the trailer supply line is pressurized, but one axle's brakes do not release fully. What is the MOST likely cause on that axle?

Low air pressure in the trailer reservoir
A leaking trailer brake valve
A failed glad-hand seal causing air loss
A seized or corroded spring brake chamber push rod (Correct answer)

Correct answer: A seized or corroded spring brake chamber push rod

If the air supply is adequate (evidenced by other axles releasing fully), a spring brake chamber that does not release fully indicates a mechanical problem within that chamber — most commonly a seized push rod or corroded brake mechanism that prevents full spring compression. A leaking trailer brake valve would affect all trailer brakes. Low reservoir pressure would affect all axles equally. A failed glad-hand seal would reduce overall supply pressure, again affecting all brakes.

Question 54: A manual transmission jumps out of a high gear under load. The MOST probable cause is:

Low transmission oil level
Worn shift fork or detent springs (Correct answer)
Worn synchronizer blocking rings
Incorrect clutch adjustment

Correct answer: Worn shift fork or detent springs

Worn shift forks or weak detent springs allow the synchronizer sleeve to slide back out of engagement under load.

Question 55: A relay valve is used in air brake systems primarily to:

Shorten brake application and release time for remote chambers (Correct answer)
Balance pressure between primary and secondary circuits
Protect the tractor air system from trailer air loss
Reduce air pressure to the front axle brakes

Correct answer: Shorten brake application and release time for remote chambers

Relay valves use a small control signal from the brake treadle to open a local large-volume air supply to remote brake chambers, reducing lag time.

Question 56: When performing a stall test on a torque converter, a stall speed LOWER than specification indicates:

Excessive line pressure
A faulty TPS signal
Worn stator one-way clutch or insufficient engine output (Correct answer)
A slipping clutch pack inside the transmission

Correct answer: Worn stator one-way clutch or insufficient engine output

Low stall speed means the engine cannot overcome stator resistance, pointing to a worn stator one-way clutch or low engine power.

Question 57: A coach engine develops a 'fuel knock' that is distinct from normal diesel combustion knock. What condition causes fuel knock?

Over-advanced injection timing causing rapid pressure rise (Correct answer)
High cetane fuel burning too slowly
Injector dribble depositing raw fuel that ignites late
Low engine oil pressure reducing injector lubrication

Correct answer: Over-advanced injection timing causing rapid pressure rise

Over-advanced injection timing causes fuel to ignite too early, creating an abnormally rapid pressure rise rate that produces a characteristic sharp knock.

Question 58: When diagnosing a modern heavy-duty diesel aftertreatment system, what is the function of the Selective Catalytic Reduction (SCR) component?

To oxidize unburned hydrocarbons and carbon monoxide
To trap and burn off particulate matter (soot)
To lower combustion temperatures by introducing exhaust gas into the intake
To reduce Nitrogen Oxides (NOx) into nitrogen and water (Correct answer)

Correct answer: To reduce Nitrogen Oxides (NOx) into nitrogen and water

The SCR system injects Diesel Exhaust Fluid (DEF), a urea-based solution, into the exhaust stream ahead of the SCR catalyst. The heat converts the DEF into ammonia, which then reacts with harmful Nitrogen Oxides (NOx) inside the catalyst. This chemical reaction converts the NOx into harmless nitrogen gas (N2) and water vapor (H2O).

Question 59: In a hydraulically operated clutch system, a spongy clutch pedal feel is MOST likely caused by:

Incorrect flywheel surface finish
Air in the hydraulic line (Correct answer)
Low clutch disc friction material
Worn pressure plate fingers

Correct answer: Air in the hydraulic line

Air is compressible and creates a spongy feel; the system must be bled to restore firm pedal response.

Question 60: When replacing a heavy-duty clutch assembly, the flywheel face and pressure plate mating surface should be inspected for:

Thread condition on mounting bolts only
Correct magnetic flux density
Hot spots, heat cracks, grooves, and flatness (Correct answer)
Surface hardness using a Rockwell tester

Correct answer: Hot spots, heat cracks, grooves, and flatness

Hot spots, cracks, grooves, and warpage on the flywheel or pressure plate cause uneven clamp load and premature clutch disc wear.

Question 61: During an injector return flow (leak-off) test on a common rail diesel engine, injectors 1, 2, and 4 return approximately 15 mL over 30 seconds. Injector 3 returns 58 mL over the same period. What is the correct interpretation and next step?

The high return flow on #3 indicates a cracked injector body requiring immediate engine shutdown to prevent coolant contamination
Return flow variation is normal; perform a cylinder contribution test before condemning any injector
All injectors are within tolerance; the higher return on #3 indicates better atomization
Injector #3 has excessive internal leakage and must be replaced; excessive return flow reduces rail pressure and causes rough running on that cylinder (Correct answer)

Correct answer: Injector #3 has excessive internal leakage and must be replaced; excessive return flow reduces rail pressure and causes rough running on that cylinder

Injector leak-off (return flow) tests measure internal leakage past the injector's control valve and needle seat. A result of 58 mL versus 15 mL for the others is approximately 4× higher — far outside normal tolerance for most OEM specifications. Excessive return flow means fuel is bypassing the injection event and returning to tank, robbing rail pressure and resulting in a lean condition and misfire on that cylinder. This definitively condemns injector #3. While a cylinder contribution test can support the diagnosis, the leak-off result already isolates the faulty unit. A cracked body would cause external leakage, not elevated return flow.

Question 62: On a common rail diesel fuel system, what component maintains system pressure between injection events?

Fuel lift pump
Injection control pressure sensor
High-pressure accumulator rail (Correct answer)
Fuel pressure regulator valve

Correct answer: High-pressure accumulator rail

The high-pressure accumulator rail stores pressurized fuel and dampens pressure fluctuations to maintain consistent injection pressure.

Question 63: A truck exhibits excessive bouncing and a floating sensation after hitting bumps, and the tires show a "cupped" or "scalloped" wear pattern. Which components are MOST likely worn?

Leaf spring center bolts
Tie-rod ends
Kingpins
Shock absorbers (Correct answer)

Correct answer: Shock absorbers

The primary function of shock absorbers is to dampen spring oscillations. When they are worn, they can no longer control the up-and-down motion of the suspension, leading to excessive bouncing. This uncontrolled bouncing allows the tires to lose consistent contact with the road, resulting in a characteristic cupped or scalloped wear pattern.

Question 64: When inspecting air dryer cartridges during a preventive maintenance service, which condition indicates the desiccant must be replaced?

Outlet air pressure drops below 100 psi
The purge cycle takes longer than 3 seconds
Oil contamination is visible in the desiccant bed (Correct answer)
Cartridge has been installed for 12 months regardless of condition

Correct answer: Oil contamination is visible in the desiccant bed

Oil contamination saturates and destroys the desiccant's ability to remove moisture, requiring immediate cartridge replacement.

Question 65: A technician performs an ABS functional test and finds that the front axle modulator valve cycles correctly during a controlled stop, but the rear axle modulator never cycles even on a slippery surface. The wheel speed sensors on all axles have correct and equal resistance readings. What is the MOST probable fault?

A restricted shuttle valve in the rear relay valve preventing modulation
The ABS ECU has a defective rear axle control channel output driver
Contaminated tone rings on the rear axle causing intermittent signal dropout
Incorrect reluctor ring tooth count on the rear axle installed during a differential service (Correct answer)

Correct answer: Incorrect reluctor ring tooth count on the rear axle installed during a differential service

If a replacement reluctor (tone) ring with an incorrect tooth count was installed, the ECU receives a wheel speed signal that does not match the programmed tooth count, causing it to calculate erroneous wheel speed data. On a slippery surface, the rear wheels may lock, but the ECU does not recognize the deceleration rate as wheel slip based on the miscalculated speed — so it never commands modulation. Sensor resistance checks only confirm wiring integrity, not signal frequency accuracy. A defective ECU output driver would be flagged as a fault code. A restricted shuttle valve would affect braking force uniformly and would be apparent as a brake imbalance, not ABS non-function.

Question 66: A technician finds excessive resistance in a truck's starting circuit. Which tool is BEST suited to locate the high-resistance connection?

Ammeter in series with the battery
Ohmmeter across the open circuit
Test light across the starter solenoid
Voltmeter performing a voltage-drop test (Correct answer)

Correct answer: Voltmeter performing a voltage-drop test

A voltage-drop test with the circuit under load reveals resistance losses across connections without needing to break the circuit.

Question 67: What does Ackermann steering geometry ensure during a turn?

The inside front wheel turns at a greater angle than the outside wheel (Correct answer)
The outside front wheel turns at a greater angle than the inside wheel
Both front wheels turn the same angle
Both front wheels remain parallel throughout the turn

Correct answer: The inside front wheel turns at a greater angle than the outside wheel

Ackermann geometry angles the inside wheel more sharply so both front wheels can follow their own turning radius without tire scrub.

Question 68: Which hydraulic fluid property is MOST important when selecting fluid for a truck system that operates in both extreme cold and high-temperature conditions?

Low specific gravity
Wide viscosity index (VI) (Correct answer)
High water content tolerance
High flash point

Correct answer: Wide viscosity index (VI)

Viscosity Index (VI) describes how much a fluid's viscosity changes with temperature. A high VI means the fluid stays within an acceptable viscosity range from cold start through full operating temperature, protecting pumps and seals in both extremes. Flash point relates to fire safety, not temperature range performance; water tolerance and specific gravity are not the primary selection criteria for wide-temperature operation.

Question 69: When recovering refrigerant from a 310T-compliant truck A/C system, the technician must:

Use EPA-certified recovery equipment and store refrigerant in approved containers (Correct answer)
Only recover refrigerant if the system pressure exceeds 200 psi
Vent the refrigerant outside the shop
Mix recovered refrigerant with new refrigerant before reuse

Correct answer: Use EPA-certified recovery equipment and store refrigerant in approved containers

EPA Section 608 regulations require certified recovery equipment and approved containers when servicing any stationary or mobile A/C system.

Question 70: During a steering system inspection, a 310T technician measures 0.038 inches of radial play at a tie rod end. The vehicle manufacturer's specification states that tie rod end replacement is required when radial play exceeds 0.030 inches. However, the tie rod end in question was replaced during the last PM interval 6 weeks ago and shows no visible damage or grease purging. What should the technician do?

Apply additional grease through the zerk fitting and re-measure; dry tie rod ends often show false play readings
Document the measurement and torque the tie rod end retaining nut to specification, as measurement inaccuracy is common on recently replaced parts
Replace the tie rod end immediately and inspect the adjacent steering knuckle bore and tie rod tube threads for damage causing premature wear (Correct answer)
Defer replacement and re-inspect at the next PM interval, as new components require a break-in period before accurate measurement

Correct answer: Replace the tie rod end immediately and inspect the adjacent steering knuckle bore and tie rod tube threads for damage causing premature wear

A tie rod end that has failed within 6 weeks of installation indicates either a defective replacement part, improper installation, or — more critically — a problem with the mating components such as a worn tapered bore in the steering knuckle or damaged thread engagement in the tie rod tube. Simply replacing the tie rod end a second time without investigating the root cause will result in the same rapid failure. The measurement of 0.038 inches exceeds the specified 0.030-inch limit regardless of how recently the part was installed, so replacement is mandatory. There is no break-in tolerance for safety-critical steering components.

Question 71: A technician is sliding a trailer's tandem axle group rearward. What is the PRIMARY reason a driver would want the tandem positioned fully rearward?

To shift weight rearward off the fifth wheel and reduce the drive axle load (Correct answer)
To comply with maximum kingpin-to-rear-axle distance regulations
To improve trailer stability at highway speeds
To maximize the load on the trailer's rear axles for better traction

Correct answer: To shift weight rearward off the fifth wheel and reduce the drive axle load

Sliding the tandem rearward shifts load off the fifth wheel (and consequently off the tractor's drive axles) and places more weight on the trailer axles. Drivers do this to reduce drive-axle overloading. Sliding forward concentrates weight over the fifth wheel onto the drives. Stability is not improved by rearward tandem position (it can actually increase trailer sway). Kingpin-to-rear-axle distance regulations exist, but the primary operational reason for sliding rearward is axle-weight management.

Question 72: A 6×4 tandem-drive truck fitted with an inter-axle differential lock (power divider) repeatedly shreds the inter-axle differential spider gears after long highway runs in unlocked mode on a loaded trailer. The driver confirms the lock was disengaged during all highway operation. What is the MOST likely root cause specific to this failure mode?

Improper air pressure in the power divider lock actuator allowing partial engagement during highway operation
Mismatched tire circumferences between the forward and rear axles creating continuous torque circulation through the power divider (Correct answer)
Loss of lubrication due to a worn power divider output shaft seal allowing oil to migrate rearward
Chronic over-torque from the engine exceeding the differential's rated input capacity

Correct answer: Mismatched tire circumferences between the forward and rear axles creating continuous torque circulation through the power divider

When the inter-axle differential is unlocked, it is designed to manage small speed differences between axles caused by cornering or minor load variation — it is NOT designed to carry continuous torque bias from a persistent circumference mismatch. If tires on the forward and rear axle sets have different worn diameters (even 3/8" difference), the power divider must continuously transfer torque to equalise shaft speeds throughout highway operation. This sustained torque circulation heats the spider gears, starves the mesh of oil film, and rapidly fatigues the gear teeth. This is distinct from engine over-torque (which would damage ring and pinion first) and from seal leaks (which would show oil loss externally). Partial lock engagement would cause binding during turns, not a spider gear fatigue pattern.

Question 73: A tractor-trailer exhibits a slow pressure buildup after a full brake application. The governor cuts in and out normally, the air dryer purges correctly, and there are no audible leaks. However, the technician notices the compressor discharge line is unusually hot to the touch. What is the MOST likely cause?

A worn compressor unloader valve causing excessive cycling
A faulty governor set too high causing the compressor to run continuously under load
Worn compressor piston rings allowing excessive blow-by and reduced volumetric efficiency (Correct answer)
A restricted compressor discharge line creating back-pressure and heat buildup

Correct answer: Worn compressor piston rings allowing excessive blow-by and reduced volumetric efficiency

Worn compressor piston rings reduce volumetric efficiency — the compressor must work harder and run longer to build pressure, generating excessive heat in the discharge line. The governor cycling normally rules out governor faults, and no audible leaks or restriction symptoms point away from the other options. Blow-by from worn rings is a classic cause of both slow pressure buildup and an abnormally hot discharge line.

Question 74: When inspecting a recirculating ball steering gear, which measurement confirms internal wear?

Pitman arm angle at center
Sector shaft end play only
Preload torque at the input shaft
Steering wheel free-play exceeding manufacturer specs (Correct answer)

Correct answer: Steering wheel free-play exceeding manufacturer specs

Excessive steering wheel free-play (lash) at center position indicates worn internal components in a recirculating ball gear.

Question 75: A technician is performing a parasitic draw test on a truck that fails to hold a charge overnight. After connecting a multimeter in series with the negative battery cable, the reading stabilizes at 450 milliamps (0.45 A). What is the next logical step in the diagnostic process?

Replace the batteries as they are likely failing to hold a charge.
Disconnect the starter motor power cable to check for a short.
Begin pulling fuses one by one while monitoring the multimeter reading. (Correct answer)
Start the engine to test the alternator's charging output.

Correct answer: Begin pulling fuses one by one while monitoring the multimeter reading.

A reading of 450 mA is significantly higher than the acceptable parasitic draw for most trucks (typically under 50-100 mA). The next step is to isolate the circuit causing the draw. This is done systematically by removing one fuse at a time and watching for a significant drop in the amperage reading on the multimeter. When the reading drops, the fuse for the offending circuit has been found.

Question 76: During a 'dry park check,' an assistant rocks the steering wheel back and forth while the truck's full weight is on the ground. The technician observes vertical (up-and-down) movement in the drag link where it connects to the steering arm. This observation indicates:

A worn drag link socket (Correct answer)
Normal component movement
A worn pitman arm
Loose steering gear mounting bolts

Correct answer: A worn drag link socket

A dry park check is designed to identify looseness in steering components under normal load. Rotational or horizontal movement in the drag link is normal, but any vertical play within the ball-and-socket joint indicates excessive wear. The component with vertical play must be replaced.

Question 77: Feathered (saw-tooth) wear across the face of a tire tread is a diagnostic indicator of:

Excessive camber
Overinflation
Worn shock absorbers
Incorrect toe adjustment (Correct answer)

Correct answer: Incorrect toe adjustment

Feathered wear, where tread blocks are worn sharp on one edge and rounded on the other, is caused by incorrect toe setting (toe-in or toe-out).

Question 78: During a pre-trip inspection, a truck driver checks the fifth-wheel locking mechanism. Which condition would require the vehicle to be placed out of service before moving?

Grease is visible on the fifth-wheel plate
The locking jaw does not fully engage around the kingpin and the release handle moves freely (Correct answer)
The fifth wheel has a 5 mm side-to-side play when the trailer is coupled
The safety latch is resting against the release handle

Correct answer: The locking jaw does not fully engage around the kingpin and the release handle moves freely

If the locking jaw does not fully close around the kingpin and the release handle still moves freely, the trailer is not secured and could separate from the tractor — an immediately dangerous condition that requires the vehicle to be taken out of service. Grease on the plate is normal and required lubrication. A small amount of lateral play at the fifth wheel is acceptable and expected. The safety latch resting against the handle is its normal locked position.

Question 79: What is the correct method for checking backlash in a heavy-duty rear axle ring and pinion gear set?

Mount a dial indicator on the axle housing with the plunger against a ring gear tooth and rock the ring gear (Correct answer)
Rotate the pinion and feel for roughness
Measure the pinion depth with a gauge block
Check gear tooth contact pattern using a feeler gauge

Correct answer: Mount a dial indicator on the axle housing with the plunger against a ring gear tooth and rock the ring gear

A dial indicator mounted with its plunger contacting the face of a ring gear tooth measures the rotational play (backlash) while the pinion is held stationary.

Question 80: What is the purpose of the fuel return line on a diesel injection system?

To return excess fuel and cool the injectors back to the tank (Correct answer)
To bypass the fuel filter under high demand
To recirculate fuel through the water separator
To supply additional fuel during high load

Correct answer: To return excess fuel and cool the injectors back to the tank

The fuel return line carries excess fuel and absorbed heat from the injectors back to the fuel tank, cooling the injection components.

Question 81: When checking driveshaft working angles, the front and rear U-joint angles should be:

Unequal and in phase
Unequal and in opposite phase
Equal and in opposite phase (Correct answer)
Equal and in phase

Correct answer: Equal and in opposite phase

Equal but opposing working angles cancel out velocity fluctuations and prevent vibration in the driveline.

Question 82: During a belt inspection, which condition requires immediate replacement of a serpentine or V-belt?

Light surface dust from normal operation
Slight color change from aging
A new belt that is slightly misaligned
Cracks, fraying, glazing, or missing chunks of material (Correct answer)

Correct answer: Cracks, fraying, glazing, or missing chunks of material

Physical damage such as cracks, fraying, glazing, or missing chunks indicates the belt is near failure and could break without warning.

Question 83: A technician is bleeding a hydraulic braking system on a coach. What is the correct procedure to ensure all air is removed?

Bleed only the wheel cylinder furthest from the master cylinder first (Correct answer)
Start at the caliper closest to the master cylinder and work outward
Crack all bleed screws simultaneously and apply steady pressure until fluid runs clear
Begin at the highest point in the system and work downward, ending at the master cylinder

Correct answer: Bleed only the wheel cylinder furthest from the master cylinder first

Air rises, so the caliper farthest from the master cylinder — and typically the one with the longest fluid path — is bled first to push trapped air completely through the system. Bleeding closest to the master cylinder first risks trapping air in the long downstream lines. Opening all screws simultaneously causes uncontrolled pressure loss. Bleeding from the highest point is a principle used in cooling systems, not hydraulic brake circuits.

Question 84: A candidate applies for a 310T Certificate of Qualification via Trade Equivalency Assessment (TEA) claiming 9,000 hours of related work experience. The assessor determines that 2,100 of those hours were spent exclusively on light-duty passenger vehicles. How will these 2,100 hours MOST LIKELY be treated in the assessment?

They will be credited fully if the candidate provides a statutory declaration from a journeyperson attesting to the transferability of the skills demonstrated.
They will be excluded or discounted significantly, as TEA hours must be in work that falls within the scope of the 310T trade specifically. (Correct answer)
They will be credited at a 50% rate, as light-duty experience is considered partially relevant to medium- and heavy-duty truck systems.
They will be fully credited because any automotive repair experience demonstrates transferable mechanical competencies applicable to the 310T scope.

Correct answer: They will be excluded or discounted significantly, as TEA hours must be in work that falls within the scope of the 310T trade specifically.

A Trade Equivalency Assessment evaluates whether an applicant's work experience falls within the scope of the trade for which they are applying. The 310T scope covers medium- and heavy-duty trucks and coaches. Hours spent exclusively on light-duty passenger vehicles are outside this scope and will typically be excluded or significantly discounted during the TEA, regardless of mechanical skill overlap. The candidate would likely need to demonstrate the remaining hours are substantially within the 310T scope.

Question 85: Which measurement is used to verify proper tooth contact pattern during a ring and pinion gear set installation?

Pinion depth with a micrometer
Ring gear runout with a dial indicator
Backlash only, measured at three points
Gear contact pattern using marking compound (Correct answer)

Correct answer: Gear contact pattern using marking compound

Marking compound applied to the ring gear teeth reveals where the pinion contacts the gear, allowing shim adjustments for correct pinion depth and backlash.

Question 86: When performing routine maintenance on an electric transit coach, a technician notices the high-voltage interlock loop (HVIL) circuit has opened. What does this indicate?

The 12 V auxiliary battery has failed and disconnected the control system
The regenerative braking system has detected excessive heat and disabled itself
A high-voltage connector or cover is not fully seated, triggering a safety shutdown (Correct answer)
The HV battery is fully charged and has disconnected to prevent overcharge

Correct answer: A high-voltage connector or cover is not fully seated, triggering a safety shutdown

The High-Voltage Interlock Loop (HVIL) is a low-voltage safety circuit that runs through all HV connectors and access covers. If any connector is not fully mated or any cover is open, the HVIL circuit opens, signaling the vehicle's control system to disable HV power — preventing accidental exposure to live conductors. It is not a charge-level indicator, a thermal protection response, or related to the 12 V system voltage.

Question 87: Thrust angle alignment refers to:

The direction the rear axle(s) push the vehicle compared to the vehicle centerline (Correct answer)
The toe-out angle measured during a turn
The angle between the front axle and the frame
The angle of the steering axis viewed from the front

Correct answer: The direction the rear axle(s) push the vehicle compared to the vehicle centerline

Thrust angle is the direction the rear axle(s) propel the vehicle; if not aligned with the vehicle centerline, the vehicle will dog-track or the steering will be off-center.

Question 88: During a wheel alignment on a heavy truck, the technician finds excessive positive caster on the right front wheel only. Ride height is correct. Which condition is the MOST likely root cause?

Worn right kingpin thrust bearing
Loose right tie rod end jam nut
Bent or collapsed right-side upper torque rod (Correct answer)
Shifted right-side spring pack center bolt

Correct answer: Bent or collapsed right-side upper torque rod

On a solid front axle with torque rods, caster angle is primarily controlled by the angle of the torque rods relative to the frame. A bent or collapsed right-side upper torque rod will change the axle's pivot geometry on that side, tilting the kingpin rearward at the top and producing excessive positive caster. Kingpin thrust bearings affect steering effort and shimmy, not caster. A shifted spring center bolt causes axle misalignment (thrust angle), not caster change. A loose tie rod jam nut affects toe, not caster.

Question 89: A truck with an Eaton Fuller 18-speed transmission exhibits gear clash specifically when shifting from 5th to 6th gear under load at highway speeds, but shifts smoothly during all other gear changes. The clutch brake engages properly and the clutch adjustment is within spec. What is the MOST likely root cause?

Worn mainshaft 6th gear clutching teeth
Air leak in the range cylinder causing incomplete high-range engagement
Worn synchronizer blocking ring for the auxiliary section range shift
Faulty range valve causing delayed auxiliary section preselection (Correct answer)

Correct answer: Faulty range valve causing delayed auxiliary section preselection

In an Eaton Fuller 18-speed, the range shift (low to high auxiliary) must be preselected before completing the main box shift into 6th. A faulty range valve delays the auxiliary section air signal, causing the auxiliary to still be in low range when the driver completes the main box shift into 6th — creating clash at that specific transition. If clutching teeth or synchronizers were worn, the problem would appear across multiple gears, not just the 5th-to-6th transition.

Question 90: What is the primary purpose of a diesel engine's unit injector (UI) system compared to a common rail system?

Unit injectors use lower injection pressures for better atomization
Unit injectors allow multiple injection events per cycle more easily
Unit injectors are controlled mechanically without ECM input
Each unit injector generates its own high pressure, eliminating a separate high-pressure pump (Correct answer)

Correct answer: Each unit injector generates its own high pressure, eliminating a separate high-pressure pump

In a unit injector system, the injector itself generates high pressure via a rocker-arm-driven plunger, unlike common rail which uses a central pump.

Question 91: A heavy coach with a hydraulic retarder (hydrodynamic) integrated into the driveline reports that retarding force is adequate at highway speeds but drops off significantly below 40 km/h. No fault codes are present and fluid level and condition are normal. What is the MOST likely cause?

A sticking control valve allowing retarder housing pressure to bleed down at low speeds
This is normal hydrodynamic retarder behavior — retarding torque output is proportional to the square of rotor speed (Correct answer)
A partially blocked retarder oil cooler is causing thermal limiting at lower speeds due to accumulated heat
The retarder modulating valve is receiving an incorrect vehicle speed signal, causing premature pressure reduction

Correct answer: This is normal hydrodynamic retarder behavior — retarding torque output is proportional to the square of rotor speed

Hydrodynamic retarders (such as the Voith or ZF Intarder) generate braking torque through fluid shear between the rotor and stator. The retarding force is proportional to the square of the rotor's rotational speed (T ∝ n²). At low vehicle speeds, rotor speed is low, and the available retarding torque drops off rapidly — this is an inherent characteristic of fluid coupling physics, not a fault. Technicians unfamiliar with this principle may misdiagnose the drop-off as a system fault. The correct vehicle operation procedure is to use the service brakes to supplement retarding at low speeds.

Question 92: During a road test, a technician notices a clicking noise that occurs twice per driveshaft revolution only while turning. What is the MOST likely cause?

Worn constant velocity (CV) joint in the driveshaft (Correct answer)
Loose companion flange bolts
Excessive driveshaft runout
Worn U-joint cross and bearings

Correct answer: Worn constant velocity (CV) joint in the driveshaft

A worn CV joint produces a clicking noise during turns because the joint operates at an increased angle, revealing worn balls or races.

Question 93: During a road test of a truck with a 10-speed synchronized manual transmission, the technician notices that double-clutching is required to achieve clean upshifts above 1500 RPM, but synchromesh engagement is smooth below 1200 RPM. No damage is found on the blocker rings. What should the technician measure FIRST?

Transmission input shaft endplay to check for thrust bearing wear
Lubricant viscosity using a viscometer to confirm correct grade is installed (Correct answer)
Synchronizer cone angle and taper wear using a dial indicator fixture
Clutch pedal free-travel to verify the clutch is fully releasing

Correct answer: Lubricant viscosity using a viscometer to confirm correct grade is installed

Synchronizer performance is highly viscosity-dependent. At higher RPM, the speed differential between the blocker ring and gear cone is greater, requiring the synchronizer friction surface to dissipate more energy. If the lubricant viscosity is too high (e.g., 80W-140 installed instead of specified 50W or 75W-90), the viscous drag on the blocker ring prevents it from spinning up to gear speed quickly enough, causing blocking ring index failure and requiring double-clutch to manually match speeds. Below 1200 RPM the speed differential is small enough that even high-viscosity fluid allows adequate synchronization. Clutch release and cone geometry problems would not be RPM-conditional in this way.

Question 94: A truck's electronic engine control module (ECM) stores a fault code for a MAP sensor but the sensor tests good. What should the technician inspect NEXT?

Clear the code and return the vehicle
Inspect the wiring harness and connector for damage, corrosion, or chafing (Correct answer)
Replace the MAP sensor anyway
Replace the ECM

Correct answer: Inspect the wiring harness and connector for damage, corrosion, or chafing

Intermittent faults are commonly caused by wiring harness issues such as chafing against the frame or corroded connectors rather than a failed sensor itself.

Question 95: During a diesel high-pressure common rail (HPCR) system diagnosis, a technician finds that fuel trim data shows a large positive correction on cylinders 3 and 4 only. Cylinder contribution tests confirm low power from these cylinders. Injector return-leak tests show cylinders 3 and 4 have significantly LOWER return flow than the others. What is the MOST accurate interpretation?

Injectors 3 and 4 have stuck-open control valves causing excessive return leak
Air in the fuel supply is preferentially affecting the higher-numbered injectors
Injectors 3 and 4 have worn nozzle tips causing restricted fuel delivery (Correct answer)
The high-pressure pump is losing prime, affecting only the rear cylinders

Correct answer: Injectors 3 and 4 have worn nozzle tips causing restricted fuel delivery

In HPCR systems, low return flow from an injector indicates it is not leaking internally — the control valve is holding. However, combined with low cylinder contribution and a positive fuel trim correction on those cylinders, this points to restricted nozzle orifices (worn or partially clogged tips) that limit the actual fuel mass injected despite the ECM commanding more fuel. High return flow would indicate a leaking control valve. The pump affects rail pressure globally, not cylinder-selectively.

Question 96: In a truck's HVAC system that does not use a heater control valve, which component is responsible for regulating the outlet air temperature?

The temperature blend door (Correct answer)
The A/C cycling switch
The blower motor resistor
The thermostatic expansion valve

Correct answer: The temperature blend door

In systems without a heater control valve, hot coolant constantly circulates through the heater core. A temperature blend door is used to control the cabin temperature by mixing the hot air from the heater core with cool, conditioned air from the evaporator to achieve the desired outlet temperature.

Question 97: In a tandem drive axle arrangement, the purpose of the inter-axle differential (power divider) is to:

Distribute torque equally between the front and rear drive axles (Correct answer)
Lock both axles together permanently for off-road use
Reduce torque multiplication from the main transmission
Allow speed differences between left and right wheels on one axle

Correct answer: Distribute torque equally between the front and rear drive axles

The inter-axle differential allows the two drive axles to rotate at different speeds while splitting torque between them.

Question 98: When performing a PM inspection on a diesel engine, the technician finds the crankcase ventilation tube heavily coated with thick black sludge. What is the MOST likely cause?

Engine oil overdue for change or severe ring blow-by (Correct answer)
Normal blow-by accumulation requiring only cleaning
Incorrect viscosity oil used at the last service
Coolant leaking into the crankcase

Correct answer: Engine oil overdue for change or severe ring blow-by

Heavy sludge in the crankcase ventilation system indicates either severely degraded oil or excessive combustion blow-by past worn piston rings.

Question 99: A truck's electronic parking brake (EPB) system activates the service brakes rather than the spring brakes on shutdown. Which component is MOST likely faulty?

ABS modulator valve
Foot brake valve
Air dryer purge valve
EPB control module or its solenoid valve circuit (Correct answer)

Correct answer: EPB control module or its solenoid valve circuit

The EPB module controls the solenoid that exhausts the spring brake chambers; a fault in the module or solenoid circuit can cause incorrect brake activation.

Question 100: During a cylinder contribution test on a 6-cylinder diesel, RPM drop for cylinder 4 is significantly less than the others, suggesting low contribution. Compression and injector leak-down both test normal. Fuel trim data shows the ECM is commanding increased fueling to cylinder 4. A fuel sample from the return line of injector 4 shows higher-than-normal return volume. What is the MOST likely root cause?

A cracked injector body is allowing fuel to leak externally without entering the combustion chamber
The cylinder 4 glow plug is drawing excessive current, causing heat-related misfiring
The injector needle control valve is worn, causing early needle lift and reduced effective injection pressure, with excess fuel bypassing to return (Correct answer)
The injector is hydraulically stuck open, dumping fuel continuously into the cylinder

Correct answer: The injector needle control valve is worn, causing early needle lift and reduced effective injection pressure, with excess fuel bypassing to return

High return fuel volume from a specific injector with normal compression and leak-down (static tests) but low cylinder contribution points to a worn needle control valve. This wear allows the needle to lift at lower injection pressures than designed, reducing the effective spray-in pressure and atomization. Much of the commanded fuel bypasses past the worn control valve and returns to the tank without contributing to combustion. The ECM compensates by commanding more fuel, but efficiency drops because delivery pressure is compromised. A cracked body would show external leakage. A stuck-open injector would cause severe wet fouling and smoking, not just low contribution.

Question 101: A technician is using an oscilloscope to evaluate the primary waveform of a diesel engine's unit injector solenoid. The waveform shows a normal pull-in spike, but the hold current plateau is absent — the signal drops directly to zero after the spike. What is the MOST accurate interpretation of this waveform?

The injector solenoid winding has an open circuit preventing any sustained current flow
The injector driver circuit's hold-current regulation is non-functional, likely due to a failed peak-and-hold driver stage in the ECM or injector driver module (Correct answer)
The oscilloscope's current clamp is saturated and is clipping the hold-current portion of the waveform
The ECM is commanding a zero-fuel delivery trim due to an active derating strategy

Correct answer: The injector driver circuit's hold-current regulation is non-functional, likely due to a failed peak-and-hold driver stage in the ECM or injector driver module

A peak-and-hold injector driver circuit first delivers a high pull-in current (the spike) to open the solenoid, then switches to a lower hold current to keep it open. If the hold current plateau is missing and the signal drops to zero after the spike, the hold-phase regulation is inoperative — this points to the driver stage (often a separate injector driver module or an ECM output stage) failing to transition to the hold phase. An open winding would prevent even the initial spike from forming. A derating strategy would still show normal waveform shape (possibly shortened duration). Oscilloscope saturation would clip the spike, not remove the hold plateau.

Question 102: What does a quick-release valve do in an air brake system?

Reduces pressure spikes during hard braking
Exhausts air locally to speed up brake release (Correct answer)
Prevents air loss if a brake line ruptures
Quickly charges the reservoirs

Correct answer: Exhausts air locally to speed up brake release

A quick-release valve exhausts air directly at the brake chamber location rather than routing it back through the treadle valve, speeding up brake release.

Question 103: During a pre-delivery inspection on a coach, you notice the air dryer purge valve is cycling every 45 seconds even at idle with no accessories running. What is the MOST likely root cause?

A faulty air dryer heater element causing moisture-induced valve chatter
Normal operation — purge cycles are governed by compressor unloader timing
A leaking service brake chamber diaphragm causing continuous air loss (Correct answer)
The governor cut-in pressure is set too high, causing rapid compressor cycling

Correct answer: A leaking service brake chamber diaphragm causing continuous air loss

A purge cycle every 45 seconds at idle with no accessories indicates abnormal air consumption. A leaking service brake chamber diaphragm creates a continuous slow bleed that forces the compressor to cycle repeatedly to maintain system pressure, triggering frequent purge events. A high governor cut-in would extend cycle time, not shorten it. Heater element faults cause freeze-up, not rapid cycling. Normal purge intervals are typically 10–20 minutes under light demand.

Question 104: While performing a road test after replacing a drive axle pinion seal on a Class 8 truck, the technician notices a cyclic vibration that increases with vehicle speed but is NOT present during engine braking. The vibration is felt through the floor, not the steering wheel. What is the MOST likely cause?

Worn front steering knuckle bearings resonating under drive torque
Excessive pinion bearing preload set too high during seal replacement
An out-of-phase driveshaft caused by incorrect installation of the companion flange (Correct answer)
An unbalanced rear wheel and tire assembly disturbed during axle work

Correct answer: An out-of-phase driveshaft caused by incorrect installation of the companion flange

A vibration that is present under drive torque but absent during engine braking (coast) is a classic symptom of a driveshaft phasing error. When the companion flange is removed and reinstalled without marking its original orientation, the U-joints can be put out of phase — the cancellation geometry is lost and a 2nd-order vibration appears at speed under load. Wheel imbalance would be present at all times regardless of torque direction. Pinion preload issues would typically manifest as noise, not a torque-sensitive vibration. Steering knuckle bearings would transmit vibration through the steering wheel, not the floor.

Question 105: A technician is diagnosing a truck with a 'cranks but does not start' condition. The starter motor turns the engine over at a normal speed, but the engine fails to fire. The batteries, starter, and fuel delivery to the injection pump have been verified as good. Which of the following electrical system faults is the MOST likely cause?

A faulty engine crankshaft position sensor. (Correct answer)
An open circuit in the glow plug relay.
High resistance in the battery ground cable.
A failed alternator diode.

Correct answer: A faulty engine crankshaft position sensor.

The Engine Control Module (ECM) relies on the crankshaft position sensor signal to determine engine speed and position. This information is critical for timing fuel injection and ignition. Without a valid signal from this sensor, the ECM will not command the fuel injectors to fire, resulting in a 'cranks but no start' condition even if fuel pressure is present.

Question 106: During a pre-trip inspection procedure check on a highway coach, a technician notices the air dryer purges immediately and repeatedly every 15–20 seconds even with the engine running at high idle and no air-consuming accessories active. The compressor cut-out pressure is verified at 120 psi. What does this symptom pattern MOST specifically indicate?

The compressor unloader valve is failing to fully unload, causing the compressor to continue pumping in a reduced-capacity cycle
A high-flow leak in the wet tank is causing the governor to cycle the compressor faster than the dryer's timed purge interval (Correct answer)
A failed purge valve solenoid is triggering timed purge cycles independently of the pressure governor
The air dryer desiccant cartridge is fully saturated and cannot hold charge between cycles

Correct answer: A high-flow leak in the wet tank is causing the governor to cycle the compressor faster than the dryer's timed purge interval

The pattern — rapid, repeated purges at 15–20 second intervals — indicates the governor is cycling rapidly because system pressure drops quickly after cut-out, forcing the compressor back on and triggering subsequent purges. A high-flow leak in the wet tank (supply side) would cause exactly this: the compressor reaches cut-out, the dryer purges, pressure drops quickly due to the leak, the compressor reloads, builds back to cut-out, and the cycle repeats. A saturated desiccant would affect moisture removal but not purge timing. An unloader valve failure would cause compressor mechanical damage symptoms, not rapid purging. A failed purge solenoid would produce purges at a fixed timer interval regardless of system pressure, not governor-linked intervals.

Question 107: A coach technician is replacing fuel filters on a common rail diesel. After installation, the correct bleeding procedure is to:

Apply shop air at 30 psi to the fuel tank vent to push fuel forward
Crank the engine for 30 seconds without starting to prime the system
Start the engine immediately and allow the high-pressure pump to self-prime
Use a hand primer pump to fill the filters before cranking (Correct answer)

Correct answer: Use a hand primer pump to fill the filters before cranking

Hand-priming the fuel filters before cranking prevents dry starts that can damage the high-pressure pump and ensures rapid, damage-free startup.

Question 108: During a scheduled PM, a technician finds that a truck's inter-axle differential lock (power divider) engages smoothly but produces a pronounced 'wind-up' shudder when turning in a tight yard maneuver with the lock engaged. The technician should document this as:

A defective inter-axle differential requiring immediate replacement before return to service
Normal and expected behavior; inter-axle locks must always be disengaged before tight maneuvering (Correct answer)
A symptom of worn spider gears inside the power divider that only manifest under torque wind-up conditions
A front-rear axle ratio mismatch caused by different tire circumferences on the drive axles requiring tire rotation

Correct answer: Normal and expected behavior; inter-axle locks must always be disengaged before tight maneuvering

Inter-axle differential locks (power dividers) are designed for straight-line, low-traction conditions only. When the lock is engaged during turning, the front and rear drive axles are forced to rotate at the same speed, but because they trace different arc lengths in a turn, torque wind-up is inevitable and produces exactly the kind of shudder described. This is normal, expected behavior and is not a defect. The technician should document that the driver was operating with the inter-axle lock engaged during maneuvering — operator training, not a repair order, is the correct response. Actual differential wear would present differently (noise or slippage under load on straight roads).

Question 109: During a brake adjustment inspection on a straight truck, a technician finds the pushrod stroke on one S-cam brake chamber is within spec at 1.5 inches, but the brake drags after release and the chamber is warm. The slack adjuster and S-cam move freely by hand with the brakes released. What is the MOST likely cause of the dragging brake?

A cracked brake drum causing uneven contact with the lining
An out-of-adjustment automatic slack adjuster holding residual pushrod extension
A weak or broken brake return spring failing to fully retract the shoes (Correct answer)
A ruptured diaphragm in the service chamber allowing air to bypass the pushrod

Correct answer: A weak or broken brake return spring failing to fully retract the shoes

If the slack adjuster and S-cam move freely by hand but the brake drags after air release, the mechanical return path is not fully retracting the shoes — pointing directly to a weak or broken brake return spring. The stroke is within spec, so the adjuster is not the culprit. A ruptured diaphragm would cause air loss, and a cracked drum would more likely cause noise and vibration rather than heat from dragging.

Question 110: What is the function of the synchronizer blocking ring in a manual transmission?

It locks the gear to the mainshaft permanently
It absorbs shock during aggressive downshifts
It prevents gear engagement until the gear and shaft speeds are matched (Correct answer)
It provides lubrication to the gear engagement teeth

Correct answer: It prevents gear engagement until the gear and shaft speeds are matched

The synchronizer blocking ring uses friction to equalize the speed of the sliding clutch and the gear being engaged, blocking engagement until synchronization is complete to prevent gear clash.

Question 111: When adjusting a manual slack adjuster on a drive axle, the technician should ensure the pushrod stroke at full application does NOT exceed:

1.0 inch (25 mm)
3.0 inches (76 mm)
1.75 inches (44 mm) for a standard 30-type chamber (Correct answer)
2.5 inches (64 mm)

Correct answer: 1.75 inches (44 mm) for a standard 30-type chamber

Federal regulations specify maximum pushrod stroke limits by chamber type; a type-30 chamber has a 1.75-inch limit before the brakes are considered out of adjustment.

Question 112: A hydraulic lift gate on a truck drops slowly under load even though the pump motor runs continuously. What is the MOST likely cause?

A blocked filter restricting return flow
An incorrectly adjusted relief valve set too high
Low hydraulic fluid level
A worn or bypassing hydraulic pump (Correct answer)

Correct answer: A worn or bypassing hydraulic pump

When the pump runs but the cylinder cannot hold load under continuous demand, internal pump wear causes fluid to bypass internally, reducing effective flow and pressure. Low fluid would also cause slow operation but typically stalls intermittently; a blocked return filter increases back-pressure rather than causing bypass; and a relief valve set too high would allow excessive pressure, not cause slow descent.

Question 113: When replacing a diesel fuel injector, why must the injector hold-down torque be tightened to specification?

To align the injector spray pattern with the piston bowl
To prevent combustion gas blow-by past the injector seat and seal (Correct answer)
To ensure the injector electrical connector seats properly
To compress the fuel return fitting for a leak-free connection

Correct answer: To prevent combustion gas blow-by past the injector seat and seal

Proper torque ensures the injector sealing washer or copper crush washer seats correctly, preventing high-pressure combustion gases from escaping past the injector.

Question 114: Which type of refrigerant oil is specified by most manufacturers for use in heavy-duty truck A/C systems charged with R-134a?

PAG (Polyalkylene Glycol) Oil (Correct answer)
Mineral Oil
POE (Polyol Ester) Oil
Alkylbenzene Oil

Correct answer: PAG (Polyalkylene Glycol) Oil

PAG (Polyalkylene Glycol) oil is the lubricant most commonly specified for use in mobile A/C systems with R-134a refrigerant. Mineral oil is not miscible with R-134a and will not circulate properly, leading to compressor failure. POE oil is often used for retrofits or in electric/hybrid vehicles but PAG is the standard for conventional R-134a systems.

Question 115: A 310T journeyperson is considering pursuing a position as an apprenticeship in-shop assessor (also called a workplace assessor or in-class assessor in some provinces) for their trade. Which of the following most accurately describes a conflict-of-interest scenario that provincial apprenticeship authorities specifically guard against in this role?

An assessor who is also the direct on-floor supervisor of the apprentice being assessed creates a conflict of interest, because the supervisor has a vested interest in the apprentice advancing quickly to reduce training overhead, which can bias competency sign-offs (Correct answer)
Assessors who previously failed their own Certificate of Qualification exam are automatically disqualified from the role, creating a systemic shortage of qualified assessors in rural areas
Assessors are prohibited from holding a Red Seal because the endorsement implies they are too senior to fairly evaluate entry-level apprentices
The conflict of interest arises when the assessor works for a unionized shop, since collective agreements require that apprentice evaluations be conducted exclusively by union stewards

Correct answer: An assessor who is also the direct on-floor supervisor of the apprentice being assessed creates a conflict of interest, because the supervisor has a vested interest in the apprentice advancing quickly to reduce training overhead, which can bias competency sign-offs

Provincial apprenticeship frameworks explicitly identify the supervisor-as-assessor scenario as a structural conflict of interest. A direct supervisor benefits operationally when an apprentice advances through levels faster (lower supervision ratios, higher-productivity classification), creating pressure — conscious or not — to sign off competencies prematurely. Most provinces require that assessors be independent of the direct supervisory relationship with the apprentice they assess. The other options are fabricated restrictions: Red Seal holders are preferred as assessors, union stewards do not conduct provincial competency assessments, and there is no rule disqualifying technicians based on exam history.

Question 116: A coach arrives with a complaint of erratic operation of the multiplex body control system — random lighting faults and intermittent door controller dropouts occur only after the vehicle has been operating for 20–30 minutes. The battery voltage reads 13.8V at idle. What diagnostic approach is MOST appropriate for this thermal-onset fault?

Flash the body controller firmware immediately, as thermal resets are caused by software memory leaks
Replace the main chassis ground strap, as thermal expansion causes intermittent ground loss in all multiplexed systems
Load test the coach batteries cold and replace if capacity is below 75% CCA rating
Monitor CAN bus voltage differential and termination resistance with the system at operating temperature using a lab-grade oscilloscope (Correct answer)

Correct answer: Monitor CAN bus voltage differential and termination resistance with the system at operating temperature using a lab-grade oscilloscope

Thermal-onset multiplex faults that affect multiple unrelated nodes (lighting and door controllers) point to a shared communication infrastructure problem — most commonly the CAN bus itself. At operating temperature, a marginal termination resistor, a chafed wire with borderline resistance, or a connector with oxidized pins can shift the bus differential signal outside acceptable voltage windows, causing random node dropouts. An oscilloscope at temperature will reveal ringing, signal amplitude collapse, or missing ACK bits. Battery load testing and ground strap replacement are valid steps but are too broad for what is clearly a data communications fault pattern. Firmware flashing without a confirmed calibration issue is inappropriate.

Question 117: When performing PM on a truck's diesel particulate filter (DPF), what maintenance task is periodically required beyond normal passive regeneration?

Forced active regeneration or physical ash cleaning of the DPF (Correct answer)
Replacing the EGR valve cooler
Replacing the engine air filter element
Cleaning or replacing fuel injectors

Correct answer: Forced active regeneration or physical ash cleaning of the DPF

Ash from engine oil additives accumulates in the DPF and cannot be burned off during normal regeneration, requiring periodic forced regen cycles or physical removal and cleaning.

Question 118: A technician is performing fuel injector balance testing on a HEUI (Hydraulically-actuated Electronically-controlled Unit Injector) system. One injector shows a significantly lower contribution than the others, but the injector passes a leak-down test. What should the technician investigate NEXT?

The injector solenoid coil resistance and the Injector Driver Module (IDM) output for that cylinder (Correct answer)
The injection actuation pressure control valve for system-wide pressure faults
The low-pressure fuel supply to the high-pressure oil pump
The air-fuel ratio sensor downstream of the affected cylinder's exhaust port

Correct answer: The injector solenoid coil resistance and the Injector Driver Module (IDM) output for that cylinder

In a HEUI system, the injector is actuated by high-pressure engine oil controlled by a solenoid. If the injector passes a leak-down test (meaning the hydraulic and fuel check components are mechanically sound), the fault lies in the electrical actuation circuit — specifically the solenoid coil resistance or the Injector Driver Module (IDM) output signal for that cylinder. A weak or missing IDM pulse would prevent full actuation, causing low fuel delivery without mechanical failure. The actuation pressure valve affects all cylinders equally, and a diesel does not use per-cylinder exhaust sensors in this context.

Question 119: If a 310T candidate fails the certification exam, what is the typical minimum waiting period before they may rewrite it?

2 weeks
60 days
6 months
30 days (Correct answer)

Correct answer: 30 days

Most provincial regulators require a minimum 30-day waiting period between exam attempts to allow the candidate time to study and prepare.

Question 120: A driver reports that the trailer ABS warning lamp on the dashboard is continuously illuminated, but the tractor ABS lamp functions normally (cycles on and off at startup). What is the MOST likely location of the fault?

The brake treadle valve position sensor.
A faulty wheel speed sensor or wiring on a trailer axle. (Correct answer)
The main ABS electronic control unit (ECU) on the tractor.
The gladhand seals between the tractor and trailer.

Correct answer: A faulty wheel speed sensor or wiring on a trailer axle.

The tractor and trailer have separate ABS systems that communicate. If the tractor's ABS light is off but the trailer's is on, the fault lies within the trailer's ABS components. A failed wheel speed sensor or damaged wiring to the sensor is the most common cause of a persistent trailer ABS fault.

Question 121: A 310T candidate is asked to explain the function of the inversion valve in a tractor protection system. Which statement MOST accurately describes the inversion valve's operation during a trailer supply line failure?

It diverts primary circuit air pressure to the trailer emergency line to maintain braking until the driver manually sets the parking brake
It closes the trailer supply line to prevent back-flow of air from the tractor primary circuit into the failed trailer circuit
It uses the loss of trailer supply line pressure as a signal to release a pilot port, allowing spring brake hold-off pressure to exhaust and setting the tractor spring brakes (Correct answer)
It inverts the signal pressure from the trailer supply line to apply the tractor spring brakes proportionally as trailer supply pressure falls

Correct answer: It uses the loss of trailer supply line pressure as a signal to release a pilot port, allowing spring brake hold-off pressure to exhaust and setting the tractor spring brakes

The inversion valve (also called the spring brake control valve in some configurations) operates on an inverse logic principle: it uses supply pressure at its signal port to hold the spring brake release line open. When the trailer supply line fails or pressure drops below approximately 45 psi, the signal pressure at the inversion valve's pilot port drops, causing the valve to shift and exhaust the spring brake hold-off air, setting the tractor spring brakes automatically. This is 'inverted' because the valve acts when pressure is lost, not when it is applied. It does not proportionally apply spring brakes, does not prevent back-flow (that is the trailer supply valve's function), and does not divert primary circuit air to the trailer.

Question 122: Which of the following BEST describes the primary advantage of a multiplexed electrical system in a modern truck?

It reduces the amount and complexity of wiring by allowing multiple electronic control modules to communicate over a shared data bus. (Correct answer)
It increases the amperage capacity of individual circuits, allowing for more powerful accessories.
It provides a dedicated, hard-wired circuit for every switch and component, improving reliability.
It eliminates the need for fuses and circuit breakers by using self-resetting electronic components.

Correct answer: It reduces the amount and complexity of wiring by allowing multiple electronic control modules to communicate over a shared data bus.

Multiplexing's main purpose is to reduce the number of wires needed in a vehicle. By allowing different electronic control units (ECUs) to send and receive messages over a common data bus (like a J1939 network), it eliminates the need for miles of dedicated wires for every single function, which simplifies the harness, reduces weight, and saves cost.

Question 123: A technician inflates a truck tire to the maximum pressure stamped on the tire sidewall instead of the pressure specified in the vehicle manufacturer's placard. What is the likely consequence?

Improved load-carrying capacity and fuel economy
Harsh ride, uneven tread wear, and potential overloading of wheel components (Correct answer)
Reduced risk of heat buildup and blowout at highway speeds
Reduced stopping distance due to increased contact patch rigidity

Correct answer: Harsh ride, uneven tread wear, and potential overloading of wheel components

The maximum pressure on the tire sidewall is a structural limit, not a recommended operating pressure. Inflating to the maximum causes the tread center to bulge, concentrating wear in the center, stiffens the sidewall producing a harsh ride, and may subject the wheel, hub, and suspension to higher peak impact loads than they are designed for. The vehicle manufacturer's placard specifies pressures matched to the vehicle's load and suspension — always follow the placard, not the tire's maximum.

Question 124: A coach arrives at the shop with the malfunction indicator lamp (MIL) on. The technician finds fault codes related to SCR catalyst contamination. Which of the following would be a direct cause of this type of failure?

Extended periods of engine idling
A faulty EGR valve
Using a non-OEM air filter
Adding diesel fuel to the DEF tank (Correct answer)

Correct answer: Adding diesel fuel to the DEF tank

The SCR catalyst is extremely sensitive to contamination. Adding any substance other than DEF, such as diesel fuel, coolant, or oil, into the DEF tank will cause irreversible damage to the catalyst substrate, leading to costly repairs and system failure.

Question 125: When inspecting a trailer kingpin, which measurement determines whether it must be replaced?

The kingpin-to-apron plate bolt torque
The length of the kingpin below the apron plate
The diameter of the kingpin shank measured at its wear zone (Correct answer)
The height of the kingpin above the trailer apron plate

Correct answer: The diameter of the kingpin shank measured at its wear zone

Kingpin shank diameter is the critical wear measurement. Standards (such as CVSA) specify that a kingpin shank worn below a minimum diameter (typically 2 inches / 50.8 mm) must be replaced because excessive wear allows the fifth-wheel jaw to rock, creating coupling slop that can cause trailer separation. Height above and length below the plate are not wear measurements. Bolt torque ensures the kingpin is retained in the apron but does not measure kingpin condition.

Question 126: A turbocharged heavy-duty diesel truck engine is diagnosed with excessive black smoke under load but normal smoke at idle. The turbocharger compressor and turbine wheels are in good condition with no shaft play. Boost pressure meets specification. Which diagnosis is MOST accurate?

The EGR valve is stuck open, diluting the intake charge under load (Correct answer)
Injector return fuel flow is excessive, causing over-fueling at high load
The turbocharger wastegate is stuck open, limiting maximum boost
The charge air cooler (intercooler) has internal leaks reducing charge air density under boost

Correct answer: The EGR valve is stuck open, diluting the intake charge under load

An EGR valve stuck open introduces exhaust gases into the intake at all times, but the effect is most pronounced under load when more fresh air is needed for complete combustion. This dilutes the oxygen content of the intake charge, resulting in a rich condition and black smoke under load. Boost pressure meeting specification rules out the wastegate and intercooler as primary causes. Excessive injector return flow would cause low power and fuel economy issues but typically presents differently. A failed intercooler would reduce charge density but would also reduce measured boost pressure downstream.

Question 127: A truck's CAN bus network is experiencing intermittent communication faults. Scope testing reveals the CAN High line sits at 2.5V at rest but only rises to 3.1V during transmission instead of the expected 3.5V. The CAN Low line drops normally to 1.5V. What is the MOST likely cause?

A 60-ohm termination resistor has been installed in parallel with an existing resistor, dropping total termination resistance to 30 ohms (Correct answer)
A node on the network has a shorted CAN High-to-ground fault
One of the two 120-ohm termination resistors is open-circuited, doubling the termination impedance
The ECM reference voltage is 0.4V below specification

Correct answer: A 60-ohm termination resistor has been installed in parallel with an existing resistor, dropping total termination resistance to 30 ohms

A CAN bus requires exactly 120 ohms at each end for a total of 60 ohms. When an extra 60-ohm resistor (or a second 120-ohm in parallel) is added, total termination resistance drops to 30 ohms. This excessive termination load dampens the differential signal amplitude — CAN High cannot reach 3.5V because the reduced impedance pulls it down. An open terminator would cause ringing and signal overshoot, not a uniformly reduced high-side amplitude.

Question 128: In S-cam foundation brakes, what converts the rotational force of the cam into outward shoe movement?

Anchor pin
Brake spider
Roller followers on the brake shoes (Correct answer)
Return spring

Correct answer: Roller followers on the brake shoes

Rollers at the toe of each brake shoe ride the lobes of the S-cam; as the cam rotates, the rollers are pushed apart, forcing the shoes outward against the drum.

Question 129: A coach equipped with an ABS/ATC system exhibits the following symptom: the ATC light illuminates and the system reduces engine torque momentarily at speeds above 55 mph on dry pavement on straight roads, with no wheel slip present. Wheel speed sensor outputs are all within 2 rpm of each other. What should the technician suspect FIRST?

A sticking throttle position sensor causing the ECU to misread driver intent as wheel slip
An ABS ECU with corrupted non-volatile memory causing false ATC activation thresholds
Air contamination in the ATC modulator solenoid causing intermittent valve chatter
A cracked or missing tone ring tooth creating a false drop-out signal at higher rotational speeds (Correct answer)

Correct answer: A cracked or missing tone ring tooth creating a false drop-out signal at higher rotational speeds

A cracked or missing tone ring tooth produces a momentary dropout in wheel speed signal. At low speeds this dropout may be too brief to trigger ATC logic, but at higher speeds the wheel rotates faster, making the dropout longer in duration (more time between pulses) relative to the adjacent teeth signals — crossing the ECU's slip detection threshold. All four sensors reading within 2 rpm is a static measurement; the fault is dynamic and speed-dependent. A corrupted ECU would typically set multiple fault codes and affect lower speeds too. ATC modulator contamination and TPS issues would present very differently.

Question 130: Landing gear on a semi-trailer is designed to support the trailer when uncoupled. What type of lubrication is typically specified for landing gear gearbox maintenance?

Engine oil SAE 15W-40
Automatic transmission fluid (ATF)
Gear oil such as SAE 90 or equivalent (Correct answer)
Chassis grease (NLGI #2)

Correct answer: Gear oil such as SAE 90 or equivalent

Landing gear gearboxes are small enclosed gear sets that require gear oil (typically SAE 90 or a manufacturer-specified GL-4/GL-5 equivalent) to lubricate the worm gear and bevel gears under high load. Engine oil is not formulated for gear mesh pressures. NLGI #2 grease is used for external fittings such as the leg tube, not the enclosed gearbox. ATF is designed for automatic transmissions and is not specified for landing gear gearboxes.

Question 131: A heavy-duty truck with a front-steer rear-axle (tag/pusher) configuration has uneven rear tire wear — the right side wears faster on the outer shoulder. Toe and camber on the rear steer axle are within specification. What suspension or steering measurement is MOST likely out of specification?

Rear steer axle thrust angle relative to vehicle centerline (Correct answer)
Insufficient rear axle pinion angle causing axle windup
Front axle caster differential between left and right sides
Total front axle toe set to the minimum of the acceptable range

Correct answer: Rear steer axle thrust angle relative to vehicle centerline

When toe and camber are correct but shoulder wear is present on a steerable rear axle, the thrust angle is the primary suspect. A thrust angle error means the rear axle is not pointing along the vehicle centerline — it is effectively 'aimed' to one side. This forces the tires to operate at a slip angle, causing lateral scrubbing and shoulder wear on the side experiencing the higher slip angle. Front caster differential and front toe affect front tire wear primarily, and pinion angle relates to driveline vibration, not lateral tire wear.

Question 132: A coach steering system uses an electric power steering (EPS) assist. The driver reports steering assist is present at low speeds but disappears at highway speeds. This is:

A fault — EPS should provide full assist at all speeds
A sign of a failing EPS motor
Normal operation — EPS reduces assist as speed increases for better road feel (Correct answer)
Caused by a faulty vehicle speed sensor sending no signal

Correct answer: Normal operation — EPS reduces assist as speed increases for better road feel

EPS systems are speed-sensitive by design, providing maximum assist at low speeds and progressively reducing assist at higher speeds to improve stability and road feel.

Question 133: A coach technician is diagnosing a steering pull that only occurs during moderate to hard braking. Steering geometry, tire pressure, and brake adjustment are all within spec. What is the MOST probable cause?

Excessive positive camber on one side overpowering the self-centering torque
Worn drag link center socket allowing horizontal lash under longitudinal load (Correct answer)
Loose steering gear mounting bolts deflecting the gear under braking G-force
Unequal front brake torque creating asymmetric deceleration loading on the steering linkage

Correct answer: Worn drag link center socket allowing horizontal lash under longitudinal load

A worn drag link center socket (the internal socket at mid-span or at the steering gear arm end) can have acceptable static lash but exhibit significant lash under the longitudinal deceleration forces generated during braking. As weight shifts forward and the steering geometry loads up, the worn socket allows the drag link to deflect, steering the front axle and producing a pull that disappears when braking ends. Brake torque asymmetry would show up in brake adjustment checks. Gear mount bolts typically cause wander or play, not a load-specific pull. Camber affects steady-state pull, not a braking-triggered pull.

Question 134: A Mack mDRIVE automated manual transmission (AMT) is performing unexpected downshifts on a 3% grade at steady-state cruise, with no driver input. The TCU fault log shows no active codes, but historical data reveals intermittent CAN Bus timeout events from the engine ECM. What is the MOST likely mechanism causing the uncommanded downshifts?

CAN Bus timeout causes the TCU to enter limp-home mode, which is calibrated to hold 4th gear regardless of road speed
The AMT actuator motor encoder is miscounting gear position, causing the TCU to re-select ratio
Loss of engine torque broadcast data forces the TCU to use a default low-torque map, which triggers downshift logic when the predicted road load exceeds the estimated engine output (Correct answer)
The grade sensor (longitudinal accelerometer) is saturating during CAN dropouts, causing the shift map to select a lower gear for perceived steep grade

Correct answer: Loss of engine torque broadcast data forces the TCU to use a default low-torque map, which triggers downshift logic when the predicted road load exceeds the estimated engine output

AMT shift logic in modern systems like mDRIVE relies on real-time engine torque data broadcast over CAN to calculate available tractive effort vs. required road load. When the engine ECM CAN message times out, the TCU substitutes a conservative default torque value (typically idle or low-load torque). On a 3% grade, the predicted road load exceeds the default torque estimate, so the TCU's shift algorithm determines the current gear cannot sustain speed and commands a downshift — even though the actual engine is producing adequate torque. This creates phantom downshifts that disappear if the grade decreases or if CAN communication is re-established. No active fault codes appear because CAN timeouts are transient and self-clearing.

Question 135: A coach experiences excessive body lean in corners. The air suspension ride height is correct. Which component should be inspected FIRST?

Air spring bellows
Shock absorbers
Stabilizer bar and end links (Correct answer)
Axle U-bolts

Correct answer: Stabilizer bar and end links

The stabilizer (sway) bar resists body roll; worn or disconnected end links allow excessive lean even when ride height is correct.

Question 136: A technician is diagnosing a truck with a complaint of slow air pressure build-up. The test shows it takes over three minutes for the pressure to rise from 85 to 100 PSI at governed RPM. Which of the following is the LEAST likely cause of this issue?

A leaking unloader valve on the compressor
Excessive carbon buildup in the discharge line
A slipping compressor drive belt
A faulty low-pressure warning switch (Correct answer)

Correct answer: A faulty low-pressure warning switch

The low-pressure warning switch is a sensor that activates a light or buzzer when system pressure drops below a safe level (typically 60 PSI); it has no role in the generation or build-up of air pressure. A slipping belt, a restricted discharge line, or a leaking unloader valve would all directly impede the compressor's ability to efficiently build pressure.

Question 137: A hybrid truck's high-voltage battery management system (BMS) triggers a fault and limits power output. Which parameter does the BMS monitor to protect the battery pack?

Cell voltage, state of charge (SOC), and cell temperature (Correct answer)
Engine coolant temperature and oil pressure
Transmission fluid temperature and gear position
12 V auxiliary battery voltage and alternator output

Correct answer: Cell voltage, state of charge (SOC), and cell temperature

The Battery Management System (BMS) continuously monitors individual cell voltage (to detect over- or under-voltage), state of charge (SOC) to prevent over-discharge or overcharge, and cell temperature (to prevent thermal runaway). When any parameter exceeds safe limits, the BMS reduces or cuts power to protect the pack. Engine oil pressure, transmission temperature, and 12 V charging are managed by separate control modules and are not primary BMS inputs.

310T Truck and Coach Technician Exam

The 310T Truck and Coach Technician certification exam (Red Seal / Skilled Trades Ontario) validates competency in diagnosing, repairing, and servicing commercial trucks, buses, and coaches.

135

Questions

240 min

Time Limit

70%

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