Service and Repair Transmission, Drivetrain, and Final Drive Systems
What this domain tests
This domain covers clutches, manual and automatic transmissions, CVTs, driveshafts, CV joints, differentials, and final drive ratios. Expect fluid checks, mechanical diagnosis, and ratio calculations.
Core rules and concepts
- Check clutch pedal free play and release-bearing operation. A clutch that slips under load or drags when disengaged points to different faults.
- Use the specified transmission fluid. Automatic transmission fluid level is often checked at a specified temperature with the engine running and the selector in Park or Neutral.
- Inspect CV joint boots for tears and grease loss; a clicking noise on turns suggests an outer CV joint.
- Differential work requires backlash and pinion preload checks. Final drive ratio = number of ring-gear teeth ÷ number of pinion teeth.
- Mismatched tire sizes on an all-wheel-drive vehicle can damage the drivetrain.
Worked example
A final drive has 41 ring-gear teeth and 11 pinion teeth. The ratio is 41 ÷ 11 = 3.727:1, often written 3.73:1. If the engine turns 3000 rpm in a 1:1 gear, the wheel turns 3000 ÷ 3.727 = about 805 rpm. The reasoning is that the final drive reduces engine speed and multiplies torque.
Traps that cost marks
Using the wrong automatic transmission fluid; checking fluid level at the wrong temperature; overtorquing the pinion nut; ignoring a torn CV boot; and replacing a transmission before checking linkage, fluid, and electrical controls.
Key points
- Use the specified fluid and check procedure.
- Clutch slip and drag have different causes.
- A clicking CV joint often means a torn boot.
- Final drive ratio = ring teeth ÷ pinion teeth.
- Keep tire sizes matched on driven axles.