Preventive Maintenance and Inspection Flashcards

During a Class 6 truck pre-trip inspection, you notice the air dryer purge valve is discharging continuously even when the compressor is in the unloaded state. Which condition MOST likely explains this fault?

Answer: A leaking wet tank check valve allowing reverse airflow through the purge orifice

A leaking wet tank check valve allows compressed air to flow backward from the wet tank through the dryer body and exit continuously through the purge valve orifice. Because the check valve is meant to prevent reverse flow, its failure mimics a constant-purge condition even when the compressor has cycled to unloaded. A stuck-open purge solenoid would also produce continuous discharge but would be heard during loaded cycles as well; a faulty unloader valve would keep the compressor loaded, not cause purging while unloaded.

A 310T technician is performing a scheduled PM on a coach equipped with a Allison 3000 series automatic transmission. The transmission fluid analysis report shows a Brookfield viscosity 18% below the lower specification limit at -20 °C. Which maintenance action is MOST appropriate?

Answer: Drain and refill with the OEM-specified TES-295 fluid; the low cold-viscosity indicates oxidation breakdown

A viscosity 18% below the low-temperature specification indicates significant base-oil shear degradation or contamination — both serious concerns in an Allison TES-295-rated transmission. The correct action is to drain and refill with fresh OEM-specified TES-295 fluid. Substituting a higher-viscosity grade violates Allison's fluid specification and can impair cold-start shift quality. Replacing only the filter ignores the fluid condition. A full flush is not warranted unless contamination is confirmed; shear-degraded fluid does not typically cause torque converter glazing on its own.

While inspecting the steering system of a tandem-axle truck during a PM interval, a technician measures 6.5° of front axle kingpin inclination (KPI) but only 1.5° of steering axis inclination (SAI) as calculated from a wheel alignment printout. What is the CORRECT interpretation of this discrepancy?

Answer: The difference indicates a bent steering knuckle or axle beam because KPI and SAI should be equal on a solid front axle

On a solid beam front axle, kingpin inclination (KPI) and steering axis inclination (SAI) are the same measurement — they both describe the angle of the steering axis relative to true vertical when viewed from the front. An alignment system reporting both as separate values with a large discrepancy signals a data entry error, bent knuckle, or deformed axle beam, because the two values cannot differ on a rigid axle. The technician must inspect for physical damage before any alignment adjustments are made. Caster and camber are separate angles and do not mathematically produce the observed 5° gap in SAI vs. KPI.

A coach technician is completing a PM on a vehicle with a full-air hydraulic brake booster (Hydro-Max type). During the functional check, the backup electric pump activates correctly when engine vacuum drops, but brake pedal effort increases substantially after three full pedal applications with the engine off. What does this MOST precisely indicate?

Answer: The hydraulic accumulator has lost its nitrogen pre-charge and is not storing adequate reserve pressure

A Hydro-Max booster relies on a nitrogen-precharged hydraulic accumulator to provide reserve braking assist when the engine (and thus the power steering pump) is off. The standard functional check is exactly this: with the engine off, apply the brake three to five times. Maintained low pedal effort across all applications indicates a healthy accumulator; rapid effort increase after only three applications means the accumulator is not holding reserve pressure — almost always due to loss of the nitrogen pre-charge bladder or separator. The electric backup pump activating correctly rules out pump or electrical faults; the pump runs but cannot sustain pressure because the accumulator cannot store it.

During a scheduled PM, a technician finds that a truck's inter-axle differential lock (power divider) engages smoothly but produces a pronounced 'wind-up' shudder when turning in a tight yard maneuver with the lock engaged. The technician should document this as:

Answer: Normal and expected behavior; inter-axle locks must always be disengaged before tight maneuvering

Inter-axle differential locks (power dividers) are designed for straight-line, low-traction conditions only. When the lock is engaged during turning, the front and rear drive axles are forced to rotate at the same speed, but because they trace different arc lengths in a turn, torque wind-up is inevitable and produces exactly the kind of shudder described. This is normal, expected behavior and is not a defect. The technician should document that the driver was operating with the inter-axle lock engaged during maneuvering — operator training, not a repair order, is the correct response. Actual differential wear would present differently (noise or slippage under load on straight roads).

A technician performing an annual PM on a transit coach finds that the Voith retarder fluid temperature warning illuminated briefly during the previous service week. The retarder fluid level is correct and no external leaks are found. The MOST valuable next diagnostic step during this PM is to:

Answer: Check the retarder bypass thermostat for correct opening temperature, as a stuck-closed thermostat prevents coolant flow through the heat exchanger

The Voith retarder dissipates heat by circulating retarder fluid through a heat exchanger cooled by engine coolant. If the bypass thermostat in that circuit sticks closed, coolant flow through the heat exchanger is blocked, and retarder fluid temperatures rise rapidly even under moderate retarder use — exactly the scenario described. Correct PM procedure after a retarder overheat warning with normal fluid level and no external leaks is to verify that the thermostat opens at its rated temperature (typically 70–80 °C). An internal heat exchanger leak would usually manifest as coolant contamination in the retarder fluid or vice versa. A sticking control solenoid would cause retarder application abnormalities, not isolated thermal events.