Evaluation and Testing Process Flashcards

A technician is performing a dynamic road test on a coach and notices intermittent transmission shudder only when the torque converter clutch (TCC) applies at highway speed under light throttle. The fault is NOT present on the diagnostic scan tool as an active or pending code. Which evaluation step should the technician perform NEXT?

Answer: Record freeze-frame data during a test drive using a bidirectional scan tool, monitoring TCC slip RPM, TCC duty cycle, and transmission fluid temperature simultaneously

Intermittent faults with no stored codes require capturing live data during the fault condition. Monitoring TCC slip RPM, duty cycle, and fluid temperature simultaneously allows the technician to correlate the shudder event with specific operating parameters (e.g., fluid temp threshold, slip percentage out of spec). Replacing the converter without data is premature and costly. A fluid flush before diagnosis may destroy evidence of contamination. A stall test evaluates converter stall speed but does not diagnose intermittent lockup shudder.

During the 310T certification evaluation, a candidate is asked to assess a truck's air brake system that passed its last pre-trip inspection but now exhibits a governor cut-out pressure of 920 kPa (133 psi) and a cut-in pressure of 690 kPa (100 psi). The legal maximum cut-out is 965 kPa (140 psi) and minimum cut-in differential is 55 kPa (8 psi). What is the CORRECT evaluation conclusion?

Answer: The system is within specification; no adjustment is required

Cut-out at 920 kPa (133 psi) is below the 965 kPa (140 psi) maximum — acceptable. Cut-in at 690 kPa (100 psi) with cut-out at 920 kPa gives a differential of 230 kPa (33 psi), which exceeds the MINIMUM required differential of 55 kPa (8 psi) — meaning the compressor cycles adequately before cut-in. There is no legislated MAXIMUM differential in standard NSC regulations. Both values are within operating range. The system is serviceable as tested.

A Class 8 truck arrives with a complaint of excessive engine oil consumption (1L per 1,000 km) but no visible external leaks, blue smoke, or coolant contamination. A cylinder contribution test shows all cylinders contributing evenly. Which ADVANCED evaluation test would BEST differentiate between worn valve stem seals and worn piston rings as the source?

Answer: Perform a running leak-down test at idle immediately after a cold start, then repeat after the engine reaches operating temperature, comparing percentage of leakage across both conditions

Valve stem seals typically leak most when cold (seals are hard and shrunk) and improve as the engine warms and seals expand. Piston rings tend to leak more consistently or worsen under load. A running leak-down test (pressurizing the cylinder while running) compared cold vs. hot isolates this differential behavior — higher cold leakage that normalizes when hot points to valve stem seals. A static compression test won't detect seal-vs-ring differences under these conditions. Visual inspection of seals requires significant disassembly without confirming the root cause. Additives are not a diagnostic tool.

A technician evaluating a coach chassis finds the front axle right-side wheel bearing has a measured end-play of 0.15 mm (0.006 in) on a non-unitized, adjustable tapered roller bearing. The OEM specification calls for 0.025–0.127 mm (0.001–0.005 in) end-play. What is the MOST correct action?

Answer: Re-adjust the bearing to achieve end-play within the 0.025–0.127 mm specification, then recheck with a dial indicator mounted rigidly to the hub

0.15 mm exceeds the OEM maximum of 0.127 mm, meaning the bearing is too loose — this increases the risk of roller skewing, heat generation, and premature failure. The correct response is to readjust using the OEM procedure to bring end-play within 0.025–0.127 mm, then verify with a properly mounted dial indicator (rigid base, plunger parallel to spindle axis). Simply eliminating end-play and backing off risks introducing preload, which is equally damaging. Replacement is premature without evidence of actual race or roller damage — excess end-play alone is a setup issue, not necessarily a wear failure.

During a 310T practical evaluation, a candidate is testing a truck's ABS system using a scan tool. After commanding a specific wheel speed sensor signal fault simulation, the ABS module sets a code and disables ABS for that corner. Upon clearing the code, the ABS warning lamp extinguishes but the wheel sensor's live data still shows erratic signal dropouts at low speed. What does this evaluation finding indicate about the technician's diagnostic approach?

Answer: Clearing the code without correcting the root cause of the erratic sensor signal is an incomplete repair — the underlying signal fault (wiring, tone ring, or sensor air gap) must be identified and corrected before the repair is verified

A correct evaluation process requires not only clearing codes but verifying that the root cause has been resolved — observable by clean, consistent live sensor data. Live data showing continued erratic dropouts means the fault condition still exists (possible causes: damaged tone ring teeth, excessive sensor-to-ring air gap, chafed wiring, or loose connector). Clearing codes without correcting the underlying condition is a failed repair verification. The module's behavior (not immediately resetting) is normal — intermittent faults may require specific conditions to retrigger. Attributing erratic signals to EMI without evidence is speculative.

A coach technician is evaluating a Cummins ISL9 that exhibits a 15-minute cranking-without-start condition when the engine is hot-soaked for 45+ minutes after a normal operating run. Cold starts are normal. No fuel pressure codes are stored. Cranking RPM is adequate. Which evaluation sequence CORRECTLY prioritizes the most likely hot-soak failure mode for this engine family?

Answer: Measure residual fuel rail pressure immediately after a hot-soak period before cranking begins, to detect injector return leak-down or check valve failure in the high-pressure fuel pump

Hot-soak no-start on common-rail diesel engines (including the ISL9) frequently results from rail pressure bleed-down during the heat soak period due to leaking injector return circuits, failed high-pressure pump check valves, or leaking pressure limiting valves. Measuring residual rail pressure immediately after soak (before cranking) — and comparing to the minimum required prime pressure — isolates whether the system can build pressure fast enough for starting. A healthy system holds significant residual rail pressure. Starter performance is irrelevant since cranking RPM is confirmed adequate. ECM replacement without evidence is not a diagnostic step. Vapor lock is not a typical failure mode for common-rail diesel injection systems.