Drive Train
Fundamentals
Trace power from flywheel to tire: clutch or torque converter, transmission (manual, automated manual, or automatic), driveshaft and U-joints, differential ring and pinion, axles, and wheels. Know clutch adjustment and free play, release bearing and pilot bearing symptoms, and the difference between a slipping clutch (rpm rises without vehicle speed) and a dragging clutch (hard shifting, gear clash). For automatics, understand converter stall, line pressure, and shift solenoids.
Differentials and drivelines
- Backlash and pinion preload are set with shims and a crush sleeve; a whining differential that changes pitch on acceleration versus deceleration points to pinion depth or bearing preload rather than gear oil level.
- U-joint phasing: slip yokes on a two-piece shaft must be in phase, and U-joint operating angles at each end should be roughly equal and opposite. Out-of-phase shafts cause vibration that cycles with vehicle speed.
- Limited-slip axles need the correct friction modifier or they chatter on turns.
- Wheel seals leaking gear oil onto brake shoes is a common inspection finding on rear axles.
Traps
Questions confuse vibration frequency: tire/wheel issues cycle at wheel speed, driveline issues at shaft speed (higher frequency), and engine issues at rpm in neutral. Also expect a question where the differential is overfilled because the tech checked the fill plug on a slope.
Worked example
Road speed rule of thumb: mph = (rpm × tire diameter in inches) ÷ (axle ratio × 336). At 1,800 rpm with 40-inch tires and a 4.10 ratio: (1,800 × 40) ÷ (4.10 × 336) = 72,000 ÷ 1,378 ≈ 52 mph.