Evaluation and Testing Process Flashcards

During a pre-delivery inspection of a coach, a technician performs a cylinder contribution test by disabling each cylinder sequentially while monitoring the engine's RPM drop. Cylinders 1 through 5 each cause a 45–50 RPM drop, but cylinder 6 causes only an 8 RPM drop. The fuel injector on cylinder 6 was recently replaced. Which diagnostic step should the technician perform FIRST before condemning the new injector?

Answer: Perform a relative compression test on cylinder 6 to rule out a mechanical contributing factor

A low contribution reading can result from low compression (worn rings, valve seating issue, head gasket leak) rather than a faulty injector. Because the injector was recently replaced and the mechanical condition of that cylinder is unknown, a relative compression test should be performed first to eliminate a mechanical root cause before assuming the new injector is defective. If compression is normal, then injector swapping or electrical diagnosis follows.

A 310T candidate is evaluating a truck that has intermittently set a DTC for an inlet air temperature (IAT) sensor rationality fault. The DTC only sets at highway speeds under load. Static testing at idle shows the IAT sensor reading 22°C with a 4.7V signal, ambient temperature is 21°C, and reference voltage is 5.0V. What is the MOST likely condition producing the rationality fault under load?

Answer: A failing intercooler causing elevated post-compression air temperatures exceeding expected IAT values

A rationality DTC for IAT under load means the PCM/ECM compared the IAT sensor reading against a calculated or modeled expected value and found them divergent. If the intercooler is degraded (blocked internally or has reduced efficiency), post-charge-air temperatures will be significantly higher than the ECM expects given the ambient IAT and boost level, triggering a rationality fault. Static readings at idle appear normal because minimal heat rejection is required. The air filter restriction would lower MAP, not elevate IAT irrationally, and an open circuit would set a range/circuit fault, not rationality.

While performing an ABS functional evaluation on a Class 8 truck, the technician activates each wheel-speed sensor by rotating the corresponding wheel by hand and observes the scan tool's live data. Three sensors generate an AC signal that increases in frequency with wheel speed. The fourth sensor (right rear drive axle) generates a pulsing DC signal that also increases with speed. What does this finding indicate?

Answer: The right rear sensor is an active (Hall-effect) sensor connected to a different ABS control module input, which is normal on some tandem configurations

Modern Class 8 trucks with tandem drive axles sometimes use a mix of passive (VR) and active (Hall-effect) wheel-speed sensors, particularly if an axle was replaced or the ABS module supports both input types. Active sensors output a digital (pulsing DC) square wave, while passive sensors produce an analog AC sine wave. If the system is functioning correctly and that channel is designed for an active sensor, this is not a fault. Mismatched types should not be 'corrected' by replacing a functioning active sensor with a passive type without confirming the module's input specification. Tone ring runout produces amplitude variation in a VR signal, not a DC signal, and a shorted bias resistor would clamp the signal, not produce a clean frequency-proportional pulse.

A coach returns with a customer complaint of erratic shifting only when the transmission fluid is at full operating temperature. Cold performance is normal. The technician performs a stall test: stall RPM is 150 RPM below specification. Line pressure at idle is 95 psi (spec: 60–100 psi) and at stall is 185 psi (spec: 220–260 psi). Fluid is at correct level and appears normal. What is the MOST accurate interpretation of these results?

Answer: The pump is worn, producing adequate idle pressure but insufficient volume under high-demand stall conditions

Idle line pressure is within spec (95 psi vs. 60–100 psi), which indicates the pump can produce sufficient pressure at low demand. However, stall line pressure is significantly below spec (185 vs. 220–260 psi) and stall RPM is also low, indicating the pump cannot sustain volume and pressure under high-demand conditions — a classic signature of a worn hydraulic pump. A freewheeling stator would produce below-spec stall RPM but would not directly lower line pressure in this manner; it also would not affect hot-vs-cold shift quality differentially. A stuck-open pressure regulator would cause low pressure at both idle and stall. Internal shaft seal leakage would typically present with low pressure at idle as well.

A 310T technician is performing an in-chassis diesel engine evaluation and suspects camshaft wear. The engine has 900,000 km on it and shows low power complaints. A relative cylinder compression test shows all cylinders within 8% of each other. Injector contribution is also balanced. When performing an intake manifold vacuum test at governed RPM, the vacuum reads −4 in-Hg (spec: −16 to −22 in-Hg). What component failure BEST explains the combination of balanced compression, balanced injection, and low intake manifold vacuum?

Answer: Worn intake lobe profiles across all camshafts causing symmetrical but reduced valve lift on all cylinders

Low intake manifold vacuum at governed RPM with balanced compression and balanced injection points to a systemic reduction in intake valve opening — consistent with worn camshaft lobes. Because all intake cam lobes are worn uniformly, the relative compression test (which compares cylinders against each other) still shows balance; absolute compression values may be lower but the test masks this. Reduced valve lift means the engine cannot draw adequate airflow, showing as low vacuum. A DPF restriction would elevate exhaust backpressure and could mask low compression, but would also likely show performance codes and elevated EGTs. A stuck-open EGR would dilute intake charge but not reduce manifold vacuum this severely. An intercooler rupture would reduce boost (positive pressure side) but would not cause low intake vacuum.

During a brake certification test on a tandem-axle truck, a technician uses a roller brake tester and records the following results: steer axle left 4,200 N / right 4,150 N; drive axle 1 left 8,100 N / right 5,200 N; drive axle 2 left 8,050 N / right 8,000 N. The air pressure is stable at 690 kPa during testing. Which fault interpretation and corrective action sequence is MOST appropriate?

Answer: Drive axle 1 shows an axle imbalance exceeding the allowable side-to-side percentage; inspect for a seized chamber, glazed lining, or restricted brake line on the right side before replacing components

Drive axle 1 shows 8,100 N on the left vs. 5,200 N on the right — an imbalance of approximately 36%, which exceeds the typically allowable 30% side-to-side axle imbalance threshold under Canadian CVSE and provincial inspection standards for 310T. The correct response is to investigate the cause (seized brake chamber, glazed or oil-contaminated linings, restricted air line) before condemning parts, because replacing linings on a seized chamber will not resolve the imbalance. Drive axle 2 (50 N difference) is well within limits. Absolute force on the right side of drive axle 1 is not independently below a minimum threshold — the issue is imbalance, not absolute deficiency. Total system efficiency cannot be calculated from these figures alone without GVWR data.