Transmission and Final Drive Systems
Transmission concepts
Understand how manual gearboxes use input shaft, countershaft, synchros and selector forks, and why clutch release, cable or hydraulic actuation and pedal free play matter. Automatic transmissions use a torque converter, planetary gear sets, clutches and bands, valve body and hydraulic pressure; modern units add electronic solenoids and adaptive learning. Dual-clutch and CVT designs have distinct service and driving precautions. Final drive converts gearbox output through ring and pinion teeth; differentials allow wheel-speed difference, while limited-slip and locking units change traction behaviour. Transfer cases and AWD couplings distribute torque.
Traps
Do not use the wrong fluid because it “looks the same”; friction modifiers and CVT fluids are application-specific. A whine that rises with road speed may be final drive, tyre or wheel bearing, not gearbox. An automatic that slips when hot may have low pressure or degraded fluid, not simply worn clutches. Towing and dyno testing can damage some automatic or AWD systems if the correct procedure is not followed. When calculating ratios, use driven teeth divided by drive teeth, and distinguish individual gear ratio from overall ratio.
Worked example
A ring gear has 41 teeth and the pinion has 10. Final drive ratio is 41 ÷ 10 = 4.1:1. If top gear is 0.8:1, overall ratio is 4.1 × 0.8 = 3.28:1. Rule of thumb: higher numerical ratio means more torque multiplication and higher engine speed for a given road speed.
Exam approach
Match fluid and procedure to the exact transmission family. Diagnose with pressure, scan data and road-test symptoms before disassembly.