ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 3 — Questions and Answers
Question 1: A Class 8 truck has a complaint of poor fuel economy and excessive black smoke under load. A scan tool shows turbocharger boost is within specification. What should the technician inspect next?
- The fuel tank for water contamination
- The EGR system for improper flow, and the fuel injectors for correct spray pattern and timing — over-fueling or EGR malfunction are common causes of black smoke with correct boost (Correct answer)
- The transmission clutch pack wear
- The rear differential oil level
Correct answer: The EGR system for improper flow, and the fuel injectors for correct spray pattern and timing — over-fueling or EGR malfunction are common causes of black smoke with correct boost
Black smoke with correct boost indicates excessive fuel relative to the air available for combustion. With boost confirmed normal, the issue is typically excessive fuel delivery from worn or stuck-open injectors, incorrect injection timing, or a malfunctioning EGR system (if EGR is stuck closed, NOx increases; if stuck open at the wrong time, combustion quality drops and smoke may increase). Fuel system diagnosis including injector balance rates and EGR flow testing are the next steps.
Question 2: What is the purpose of a driveshaft slip yoke on a heavy truck?
- To provide a flexible joint to accommodate engine torque peaks
- To allow the driveshaft to change length as the rear axle moves up and down on the suspension, preventing binding or separation during suspension travel (Correct answer)
- To connect the transmission output to the driveshaft permanently
- To reduce driveshaft vibration at highway speeds
Correct answer: To allow the driveshaft to change length as the rear axle moves up and down on the suspension, preventing binding or separation during suspension travel
As the rear axle moves up and down over road irregularities, the distance between the transmission output and the axle pinion changes. The slip yoke (a sliding splined coupling) allows the driveshaft to increase or decrease in effective length to accommodate this movement without binding or placing stress on the U-joints. Without the slip yoke, suspension movement would either stretch or compress the driveshaft, damaging the drivetrain.
Question 3: During a pre-trip inspection on an air brake-equipped truck, what is the FIRST test the driver should perform regarding air system integrity?
- Release the parking brakes at maximum pressure and drive forward
- With the air system fully charged, shut off the engine and monitor the air gauge — pressure should not drop more than 2 PSI per minute with no brake application (3 PSI with brakes applied) (Correct answer)
- Apply the service brakes 10 times rapidly to test the compressor
- Check the air tank condensate drains are closed only
Correct answer: With the air system fully charged, shut off the engine and monitor the air gauge — pressure should not drop more than 2 PSI per minute with no brake application (3 PSI with brakes applied)
FMCSA-required pre-trip brake checks include testing for air system leaks. The driver charges the system to governed pressure, shuts off the engine, and observes the air gauges. A leak rate of more than 2 PSI per minute with brakes released (or 3 PSI per minute with brakes held applied) indicates an air system leak that must be repaired before the vehicle is operated. This test detects leaking compressors, fittings, valves, or brake chamber diaphragms.
Question 4: A truck technician notices a wet and oily residue around the air compressor discharge port and in the air dryer. What does this indicate?
- Normal condensation accumulation in the air system
- The air compressor is passing engine oil into the air system — this indicates worn compressor rings or valve seals, contaminating the air dryer, valves, and brake components (Correct answer)
- The air dryer desiccant is saturated and needs replacement only
- The engine oil level is too high and is overflowing
Correct answer: The air compressor is passing engine oil into the air system — this indicates worn compressor rings or valve seals, contaminating the air dryer, valves, and brake components
Oil in the air system is a serious problem on a heavy truck. The air compressor uses engine oil for lubrication, and worn piston rings or valve seals allow oil to be pumped into the air lines. Oil saturates and ruins the air dryer desiccant, contaminates and degrades rubber components in brake valves, air chambers, and ABS modulators, and can prevent brakes from releasing properly. The compressor must be rebuilt or replaced and the entire air system drained and cleaned.
Question 5: What does an "over/under" or "unbalanced" axle weight condition mean on a multi-axle truck, and how is it corrected?
- The truck is loaded asymmetrically from left to right
- The weight distribution between the steer, drive, and trailer axles does not meet legal load limits for one or more axles — corrected by sliding the fifth wheel or trailer tandem axles forward or rearward to redistribute weight between axles (Correct answer)
- The suspension air bags are inflated to different pressures
- The frame is bent causing unequal weight on axles
Correct answer: The weight distribution between the steer, drive, and trailer axles does not meet legal load limits for one or more axles — corrected by sliding the fifth wheel or trailer tandem axles forward or rearward to redistribute weight between axles
Federal and state regulations set maximum weight limits per axle (typically 12,000 lbs for steers, 34,000 lbs for drive tandems, 34,000 lbs for trailer tandems in a standard 80,000 lb GVW combination). If the steer axle is overweight, sliding the fifth wheel rearward shifts weight to the drives. If the drives are overweight, sliding the trailer tandems forward shifts weight rearward. The sliding fifth wheel and trailer slider allow weight redistribution without reloading cargo.
Question 6: What is "wheel bearing end play" and what is the consequence of excessive end play on a heavy truck steering axle?
- The clearance between brake pads and rotor — excess play causes brake fade
- The axial (in/out) movement of the wheel hub on the spindle — excessive end play causes bearing wear, tire wobble, irregular tire wear, and can lead to wheel-off conditions if bearings fail (Correct answer)
- The amount the steering wheel can turn before the wheels begin to move
- The clearance between the U-bolt and axle housing
Correct answer: The axial (in/out) movement of the wheel hub on the spindle — excessive end play causes bearing wear, tire wobble, irregular tire wear, and can lead to wheel-off conditions if bearings fail
Wheel bearing end play refers to how far the wheel hub can be moved in and out along the spindle axis. FMCSA-regulated specifications allow a small amount of end play (typically 0.001–0.010 inches for tapered roller bearings). Excessive end play indicates bearing wear or improper adjustment. On a steering axle, excess play causes tire wobble, wander, uneven tire wear, and stress on hub components. Severely worn bearings can fail completely, causing wheel separation — a catastrophic and dangerous failure on heavy trucks.
ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 3
A Class 8 truck has a complaint of poor fuel economy and excessive black smoke under load.
A scan tool shows turbocharger boost is within specification.
What should the technician inspect next?