What Are the Parts of a Transmission?

An automatic transmission has a lot of parts to it, and it’s not an easy thing to understand at first. The main components of an automatic transmission include the torque converter, planetary gearset, pump, clutches, bands, sensors, valve body, and last but not least is the transmission fluid, otherwise known as ATF. All the parts of a transmission are crucial, and work together to deliver torque to the wheels so you can get from point A to point B. It’s a remarkable piece of machinery, and to better understand how your car works we’ll explain all the parts of a transmission here.

Parts of a Transmission

parts of a transmission - transmission pump - transmission repair guyPump – Located between the torque converter and the planetary gear set, the pump is what draws transmission fluid in and pressurizes it for the torque converter and transmission. An automatic transmission relies heavily on fluid pressure in order for its components to function. Think of the pump as the heart of the transmission, which provides all the necessary fluid to work.

Planetary gear set – The planetary gear set consists of a sun gear, planet gears, and a ring gear, and is responsible for all of the gear ratios that your car uses. This is probably the most important part of your car, and everything else in the transmission depends on the planetary gear set. How it works is that the ring gear connects to the planet gears, with the sun gear in dead center. The gear ratio is determined by how the planet gears are locked or unlocked, and the planet gears revolve around the sun gear. The rotational force produced is then sent to the output shaft, which is connected to the wheels.

Torque converter – The torque converter is a hydraulic fluid coupling that connects the engine to the transmission. Consisting of a stator, impellor, and turbine, the torque converter is able to produce torque that the transmission can use by way of fluid pressure. The torque produced is exactly what allows your car to accelerate after stopping, but it also allows the transmission to stay in gear while the car is stationary. This is accomplished by the way the torque converter is connected to the engine and transmission. The torque converter is able to spin however fast the engine is going while also spinning independently from the transmission. The stator is the part of the torque converter that allows you to accelerate again after a complete stop. If the torque converter malfunctions, you will often experience erratic behavior such as shuddering and slipping.

Clutches & Bands – Clutches and bands are used to help the transmission shift gears by allowing gears to rotate, engage, or disengage. With clutches, pressurized fluid is applied to the pack which causes the pistons to engage, and the power is then sent to the wheels. Bands are wrapped around the gear train, and will either tighten or loosen depending on whether gears should be engaged or disengaged.

Sensors – Sensors are the control units of the transmission, and calculate the speed of the engine and wheels in order to decide which gear should be used. Transmissions use both an input speed sensor as well as an output speed sensor to accomplish this, and there is also a park and neutral switch for safety.

parts of a transmission - transmission valve body - transmission repair guyValve body – The valve body is a hydraulic control center that regulates incoming transmission fluid and uses it to run a network of spring loaded valves, check balls, and servo pistons. The valves determine what gear ratio is used by sending fluid to the clutches and bands, and the pressure of the transmission fluid determines which valves are opened or closed. Fluid pressure will change depending on the engine’s speed. Nowadays, many modern automatic transmission rely on the engine control unit or the transmission control unit to regulate the valves.

Transmission fluid (ATF) – Last but not least is the automatic transmission fluid, which is CRUCIAL. Transmission fluid not only provides fluid pressure, it also has lubricating and cooling capabilities so that your transmission doesn’t overheat. ATF is made from a variety of synthetic liquids and oils with added chemical properties such as detergents, rust preventatives, and lubricants. ATF is the only part of the transmission that needs routine maintenance, as age causes it to collect contaminants as well as lose effectiveness. We recommend checking the fluid once a month and changing it every 30,000 to 60,000 miles for optimal performance.

parts of a transmission infographic - transmission repair guy

Infographic by Miramar Transmission