Preventive Maintenance and Inspection Flashcards

During a Class 6 truck PM inspection, you notice the chassis lubrication specification calls for NLGI #2 grease, but the previous technician used NLGI #0. The vehicle has been operating for 8,000 km since the last service. What is the MOST appropriate corrective action?

Answer: Purge all affected fittings completely until only NLGI #2 grease emerges, then document the correction

Mixing NLGI grades can cause incompatibility issues — NLGI #0 is significantly softer and may not provide adequate film strength in high-load joints. The correct action is to purge each fitting by pumping in the correct NLGI #2 grease until the old grease is fully displaced from the relief point or boot, confirming visual change, then documenting the correction. Simply topping up risks leaving incompatible grease in load-bearing areas; solvent flushing is excessive and risks contaminating sealed cavities.

A coach's air dryer purge valve is discharging a continuous small stream of oily air between purge cycles rather than only during the timed purge event. Which PM-related root cause should be investigated FIRST?

Answer: Check valve inside the air dryer leaking, allowing downstream system pressure to back-bleed through the purge orifice

A continuous small stream between purge cycles (not during them) is the classic symptom of a failed internal check valve in the air dryer. This valve is designed to hold downstream reservoir pressure and prevent back-flow; when it leaks, pressurized air (along with any moisture or oil it carries) continuously bleeds back through the purge orifice. A saturated desiccant bed causes wet downstream air but not continuous purge-orifice discharge. A failed unloader causes compressor oil carry-over but that discharge tracks compressor cycling, not a constant stream. A clogged inlet filter reduces system fill rate but does not produce continuous discharge.

According to CAN/CGSB standards and NSC Schedule 1 inspection criteria, what is the MINIMUM remaining brake lining thickness (measured at the thinnest point excluding any taper) before a drum brake shoe on a steering axle of a Class 8 truck is considered defective and must be removed from service?

Answer: 3.2 mm (1/8 inch)

Under NSC Schedule 1 / National Safety Code standards adopted across Canadian jurisdictions, a brake lining with 3.2 mm (1/8 inch) or less of material remaining at its thinnest point (excluding tapered ends) on a drum brake is considered defective for any axle position, including steering axles. Some technicians confuse the 6.4 mm threshold, which is a common manufacturer recommendation for replacement scheduling during PM (to avoid emergency removal), with the legal out-of-service threshold. The 3.2 mm limit is the regulatory defect standard.

While performing a PM on a heavy coach with a Detroit Diesel Series 60 engine, you find that the engine oil analysis report shows elevated silicon (Si) levels at 42 ppm — up from a baseline of 8 ppm — along with normal wear metals. The oil and air filtration were changed on schedule. What is the MOST likely cause and the correct PM follow-up action?

Answer: Air induction system breach or filter seating failure allowing ambient dust/dirt ingestion

Elevated silicon with normal wear metals and no coolant chemistry markers (e.g., no elevated sodium/potassium/boron) is the classic oil analysis signature of air induction contamination — dirt and silica entering past a compromised air filter, collapsed filter media, cracked intake ducting, or a poorly seated filter element. Coolant intrusion would show elevated Na/K and glycol alongside Si. Piston silicon content doesn't appear selectively in oil analysis. Silicone gasket sealant contamination is rare, localized, and would show as a one-time spike rather than a trend from baseline. The correct follow-up is a thorough air induction inspection: check filter element seating, intake ducting integrity, and air cleaner housing for cracks.

A 310T technician is performing a PM inspection on a highway coach and measures front axle wheel bearing end-play using a dial indicator. The reading is 0.18 mm (0.007 inch) on a non-ABS hub assembly with tapered roller bearings. What is the correct PM disposition?

Answer: Acceptable — within the standard 0.025–0.254 mm (0.001–0.010 inch) allowable end-play specification for pre-set tapered roller bearings

For manually-adjusted tapered roller wheel bearings (non-unitized/pre-set hubs), the standard accepted end-play range per TMC RP618 and most OEM specifications is 0.025 mm to 0.254 mm (0.001–0.010 inch). A reading of 0.18 mm falls within this range and is acceptable. Setting tapered roller bearings to zero or pre-load causes excessive heat generation and premature failure. The 0.127 mm threshold (Answer C) is not a standard rejection limit for this bearing type. Flagging for future adjustment (Answer D) is unnecessary since 0.18 mm is mid-range acceptable.

During a scheduled PM on a coach equipped with a Meritor RT-46-160 tandem rear axle, you find the interaxle differential lock (power divider) oil level is correct, but the oil appears dark grey-black with a metallic sheen and has a burnt odour. Gear tooth inspection through the fill port shows no visible chipping. What is the MOST accurate PM assessment?

Answer: Indicates prolonged interaxle differential lock engagement causing clutch pack wear debris and oil degradation; change oil and inspect lock engagement mechanism

Dark grey-black oil with metallic sheen and burnt odour in the power divider (interaxle differential) of a Meritor tandem axle — without macroscopic gear tooth damage — is the classic signature of clutch pack wear from extended or repeated lock engagement. The interaxle differential lock contains a multi-disc clutch pack that produces fine ferrous particles and degrades the oil when the lock is engaged during high-speed or paved-road operation (driver misuse) or when the engagement mechanism sticks in the locked position. The correct PM response is to drain and replace the power divider oil, inspect the locking collar and actuator for proper release, and educate the driver on correct lock-engagement procedures. Simple darkening from EP additives doesn't produce metallic sheen and odour together. Water contamination produces milky/foamy oil, not dark metallic oil.