Drive Train Diagnosis and Repair
What the candidate must know
Transit-bus drivetrain diagnosis starts with understanding how torque flows from the engine/flywheel through the clutch or torque converter, transmission, driveshaft, differential, and axles. Candidates should be able to explain the function of each component, read a symptom to a likely cause, and describe the correct test sequence. Know the difference between a vibration that changes with road speed (driveline balance, U-joint, tire/wheel) and one that changes with engine speed (converter, flexplate, engine accessory). Know how to check U-joints for looseness and phasing, how to inspect a driveshaft for dents or missing balance weights, and how to evaluate differential backlash and pinion preload with the correct tools.
Traps in questions
Common traps include confusing road-speed vs. engine-speed symptoms, assuming a leak is the cause rather than a symptom, and overlooking the parking brake or retarder when diagnosing drag. Another trap is choosing an adjustment when the correct action is replacement, or vice versa. Read whether the stem says "first step" or "most likely cause."
Worked example / rule of thumb
A bus has a vibration that appears only above 45 mph and worsens when the throttle is released. Road-speed dependent, load-sensitive vibration points to driveline balance or a worn U-joint, not the torque converter. Rule of thumb: engine-speed vibration is present in neutral; road-speed vibration follows vehicle speed regardless of gear.
Diagnostic sequence
Verify the complaint, road-test, inspect mounts and fasteners, then isolate by component. Document measurements against specification before condemning parts.