Automatic Transmission

A conventional automatic uses planetary gear sets, multi-plate clutches and brakes, and a torque converter that couples it to the engine. The transmission control module chooses gears from road speed, load and pedal position and operates them through solenoid valves.

The term is also used loosely for other self-shifting designs: dual-clutch gearboxes, continuously variable transmissions (CVT) and automated manuals. They work differently, and their maintenance and failure patterns are different too. The term covers several designs: the classic torque-converter automatic, the continuously variable transmission (CVT) and the dual-clutch transmission (DCT). They all shift without a clutch pedal, but work very differently inside.

Pressurised fluid from a pump is routed through the valve body to engage the right clutches for each gear. Sensors report input and output speed so the control unit can time shifts and detect slipping. Modern automatics adapt their shift points and clutch pressures to the driver and to wear over time. After a battery disconnection or repair, some need a relearn procedure to shift smoothly again.

Low or worn fluid, faulty solenoids or speed sensors cause harsh, late or slipping shifts and often put the gearbox into limp mode. Fluid checks and changes follow the manufacturer’s procedure, which varies a lot between transmissions. Harsh shifts, slipping, delayed engagement or a transmission warning light are reasons to check the fluid level and read fault codes early. Many problems start small and become far more expensive if the car keeps being driven.