Automatic Transmissions

An automatic transmission is much easier to drive than a manual transmission, because you don't have to use a clutch pedal or gearshift lever. An automatic transmission does the work all by itself. The first automatic transmission appeared in 1939. Automatic transmissions automatically change to higher and lower gears with changes in the car's speed and the load on the engine. These transmissions are also aware of how far down you have pushed the gas pedal, and shift accordingly. The system is operated by transmission fluid pressure; shift valves control the gear changes. A "governor" controls the shifting of the gears. It's linked to the output shaft and throttle valve and controls the transmission fluid supply, at different pressures, to the shift valve. Here's how it works: the output shaft turns the governor. The faster the car goes, the faster the governor turns. Oil is sent from the pump to the shift valves by centrifugal force from the governor. The shift valves move out, and send the transmission fluid to the gear shifting mechanisms in the transmission. When you slow down, the valves move in, and send the transmission fluid in the opposite direction. This action changes the gears. By routing the pressure to the clutches and brake bands, the different gears are selected.