Location

The location of the automatic transmission differs depending on the location of the vehicle’s final drive. On cars with front-wheel drive, the transmission is offset, usually positioned underneath the engine, and connects directly to the final drive creating what is commonly referred to as a transaxle. It is the final drive that turns the wheels and propels the vehicle forward or backward, depending on which gear the transmission is set at the moment.

On most rear-wheel drive vehicles, the transmission connects to the drive shaft which, in turn, connects to the final drive. Because the transmission does not connect directly to the final drive it is not generally identified as as a transaxle.

On some rear-wheel drive vehicles, the transmission is located in the rear and connects directly to the final drive by way of a drive shaft connected to the car’s torque converter. This configuration is sometimes necessary in high-performance vehicles to evenly distribute the vehicle’s weight.

Operation

The function of the transmission is to synchronize the speed of the vehicle’s wheels with the speed of its engine and then send engine power to the wheels so they can function properly. If the engine is the vehicle’s central nervous system, the transmission is its brain, sending power at proper rates to the moving parts.

The automatic transmission’s other primary function is to ensure that the engine maintains a steady cycle of Revolutions Per Minute (RPM). Any fluctuation in RPM could cause problems from rough acceleration to total engine failure making the importance of the relationship between engine and transmission clear.

Common Parts of an Automatic Transmission

Later model vehicles with automatic transmission are made up of several common components. These include:

  1. Planetary Gear Set — so named because it consists of a series of smaller gears (planets) that work with a central gear (sun) and an outside ring gear.
  2. Hydraulic System — uses pressurized fluid delivered via oil pump through the valve body to control the planetary gears.
  3. Seals and Gaskets — control the flow of the hydraulic fluid and prevent leaking
  4. Torque Converter — functions like a clutch to allow the vehicle to slow or stop while still in gear
  5. Governor and Modulator (or Throttle Cable Monitor) — Monitors the speed of the vehicle and determines when to shift gears

Summary of Essential Functions

The functions of the automatic transmission can be most simply divided into three parts: the torque converter, gear unit and hydraulic converter. The torque converter transfers the vehicle engine’s output power to the gear unit through the use of pressurized hydraulic fluid. The gear unit deals with the necessary gear combinations required to keep the vehicle moving smoothly and steadily at various speeds as well as in reverse. The Hydraulic System (which includes the governor, modulator, and valve body) converts the engine speed into a series of signals which control shifting and regulate torque.Gearbox Operation