Practical Skills and Training Flashcards

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?

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.

A 310T technician is replacing clutch brake on a truck with a two-piece clutch brake design. After installation and road test, the driver reports the transmission grinds when shifting into first gear from a stop. What adjustment error is MOST likely responsible?

Answer: Excessive free pedal travel leaving insufficient clutch brake squeeze distance

Clutch brake function requires the pedal to travel into the last 1/2 inch of travel (from floor) to squeeze the clutch brake and stop transmission input shaft rotation. If free pedal is excessive, the total pedal stroke runs out before reaching the clutch brake squeeze zone — the shaft keeps spinning and gears grind on first-gear engagement. Insufficient free pedal causes clutch slip/heat, not grinding. Two-piece clutch brakes are symmetric. Pre-loading the release bearing is not part of clutch brake adjustment.

While performing a chassis dynamometer test on a turbocharged diesel truck, you observe that boost pressure reaches spec at full throttle but EGT (exhaust gas temperature) is elevated by 120°C above the acceptable limit. Fuel trim and injector balance rates are within range. Which diagnosis is MOST consistent with these findings?

Answer: A partially restricted diesel particulate filter increasing exhaust backpressure

Elevated EGT with normal boost pressure and injector balance rates points to an exhaust restriction. A partially blocked DPF raises backpressure, forcing exhaust gases to retain heat longer and reducing scavenging efficiency — both elevate EGT significantly. A worn compressor wheel would reduce boost, not leave it at spec. A lean condition would show abnormal fuel trim. An intercooler leak reduces boost pressure and causes white smoke, not elevated EGT with normal boost.

A coach with a Tag axle (pusher configuration) passes all brake balance tests on level ground but exhibits severe rear-end sway during hard stops on a 6% downgrade. The primary and secondary air circuits test within spec. What is the MOST probable cause?

Answer: The Tag axle lift solenoid is partially energized, reducing vertical load and adhesion during braking

On a downgrade under hard braking, weight transfers forward dramatically. If the Tag axle lift solenoid is partially energized (even inadvertently), it reduces the downward force on the Tag axle, lowering its tire adhesion precisely when rear braking force is needed most — causing sway. Proportioning valves are preset at the factory and do not self-adjust for grade. Worn end links cause body roll, not braking-induced sway specific to grades. A chipped reluctor wheel causes ABS pulse dump but not sway correlated to grade.

A technician is diagnosing intermittent stall on a CNG-powered transit bus. The engine restarts immediately after stall and runs normally for 20–40 minutes before stalling again. Fuel rail pressure holds steady on the scan tool. Which component failure BEST explains this pattern?

Answer: An intermittent crankshaft position sensor fault that self-clears as the connector cools

The 20–40 minute warm-up period before stall, with immediate restart, is a classic thermal intermittent pattern. A crankshaft position sensor or its connector that develops an open circuit as it heats (due to a hairline connector crack or corroded terminal) will kill the engine signal, stall the engine, and then re-establish continuity as the connector slightly cools — allowing immediate restart. CNG regulator diaphragm failure would cause hard start after stall, not immediate restart. A clogged filter would correlate with load, not time. CNG PRVs vent to atmosphere and do not cause stall.

During a scheduled PM on a Class 8 truck with an automated manual transmission (AMT), the technician finds that the clutch actuator motor current draw peaks at 28 A during engagement — well above the 18 A specification. The clutch pack and flywheel show no abnormal wear. What is the MOST likely cause of the elevated current draw?

Answer: Contaminated clutch actuator grease increasing mechanical resistance in the ball-screw mechanism

AMT clutch actuators use a precision ball-screw mechanism driven by an electric motor. The ball-screw requires specific low-viscosity grease; over time, contamination (moisture, metal particles) or grease breakdown increases friction in the threads, directly raising mechanical resistance. The motor draws higher current to overcome this resistance even though the clutch components themselves are fine. An AMT module fault would typically set a DTC and affect shift logic. Tight free travel affects slip, not actuator current. Return springs in these units are calibrated at assembly and rarely weaken without visible damage.