Transmission and power transmission — from motor to wheel
Connection gearbox differential and drive axles in context
A motor can deliver plenty of power but it's useless if it can't transfer the power in a controlled way to the wheels. That's the transmission's job: to disconnect and reconnect the power to shift between ratios and to distribute torque to wheels that turn at different speeds. The vocational education regulation for car mechanics requires that you can troubleshoot repair and maintain transmissions — including clutch drive shafts and gearboxes with and without automatic shift — so it's an area you need to know in depth.
§The clutch — the controlled slip
In cars with manual gearbox there is typically a dry single-plate clutch between engine and gearbox. The clutch plate with friction lining is pressed against the flywheel by the pressure plate and when you press the pedal down the release bearing pulls the pressure plate's spring washer back so the force transfer is interrupted. Wear shows as a clutch that 'slips': engine revs rise without the car accelerating correspondingly — most obvious in high gears uphill. Noise that changes when the pedal is pressed on the other hand often points to the release bearing.
§Manual automatic dual coupling and stepless
| Type | Principle | Typical characteristic |
|---|---|---|
| Manual gearbox | Operator selects gear, synchronizer rings equalize speed | Robust; worn synchronizers cause grinding when shifting |
| Conventional automatic | Moment converter + planetary gear, controlled hydraulically/electronically | Runs on ATF oil, which both lubricates and transfers power |
| Double clutch (e.g. DSG) | Two clutches — next gear is pre-selected | Shifts almost without interruption in pulling force |
| CVT (stepless) | Pulleys or chain with variable diameter | Stepless transmission without fixed gears |
Common to them all is that oil is part of the construction. Gear oil and ATF come in many specifications with different viscosity and friction properties, and manufacturers prescribe exactly which oil may be used. If you add the wrong oil to an automatic transmission or dual-clutch transmission, clutches and synchronizers can be damaged — so looking in the workshop manual is not optional, it is the method.
§Differential and drive axles
When the car turns the outer wheel travels further than the inner one. The differential allows the wheels to rotate at different speeds while the torque is still distributed. From the differential the drive shafts carry the power out to the wheels and because the wheels both spring up and down and turn constant velocity joints (CV joints) sit at each end of the shaft. The joints are packed with grease and protected by rubber sleeves.
Typical symptoms you must be able to recognize
- 01Clicking sound at full steering wheel deflection during acceleration — worn outer CV joint.
- 02Vibrations during acceleration in a certain speed range — inner bearing or skewed axle.
- 03The axle that follows speed not engine revolutions — often bearing or differential.
- 04Grind when shifting to one specific gear — worn synchronizer ring.
- 05Jerky, hard shifts in automatic gear — first check oil level and oil condition, read fault codes.
The systematics are the same as with all other troubleshooting: listen to the customer's description, recreate the symptom in a test drive, and narrow down the fault before you take anything apart. When working with couplings and shafts, workplace environment rules also apply — gearboxes are heavy and must be handled with gearbox lifts, and old grease and gear oil are environmentally hazardous waste that must be collected and disposed of correctly.
“Transmission is car's intermediary between power and control — understand it you understand why car drives as it does”
— Professional rule of thumb from the workshop.