ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 3 Flashcards

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?

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.

What is the purpose of a driveshaft slip yoke on a heavy truck?

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.

During a pre-trip inspection on an air brake-equipped truck, what is the FIRST test the driver should perform regarding air system integrity?

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.

A truck technician notices a wet and oily residue around the air compressor discharge port and in the air dryer. What does this indicate?

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.

What does an "over/under" or "unbalanced" axle weight condition mean on a multi-axle truck, and how is it corrected?

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.

What is "wheel bearing end play" and what is the consequence of excessive end play on a heavy truck steering axle?

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.