Guide to Automatic Transmission Parts: Names, Functions & Diagram

An automatic transmission is a multi-speed transmission used in internal combustion engine-based motor vehicles that do not require driver input to change forward gears under everyday driving conditions. Typically, automatic transmission parts are torque converters, planetary gear sets, hydraulic systems, clutches, brakes, output shafts, valve body & computers. It has only forward, neutral, and reverse options while driving. Transmission is the most complicated part to understand when it comes to repairing. So we have a better understanding of the transmission parts and their functions. With the automatic transmission, we can drive the vehicle safely and comfortably. As there are fewer gears, we can drive the vehicle smoothly.

When we are buying a new car, we have all the information about parts of automatic transmission.

Transmission Parts Diagram

Parts of Automatic Transmission, Names, Functions & Diagram

Automatic Transmission Parts Names

  • Torque Converter
  • Oil Pump
  • Planetary Gear Sets
  • Hydraulic System
  • Clutches
  • Brake Bands
  • Output Shaft
  • Oil Pan
  • Valve Body
  • Computer

Automatic Transmission Parts & Functions

Torque Converter

The torque converter is a type of fluid coupling that transfers the rotating power from the engine to the vehicle wheels and increases the torque to drive at speed. It presents between the engine and the transmission.

When our car is a red signal, half of the torque converter rotates, and the other half does not spin. When the signal greens, we press the pedal and accelerate the vehicle.

It causes the engine to speed up and pumps more fluid in the torque converter to rotate the first half with speed. Hence due to the high rotation of the converter’s first half, the fluid creates pressure on the second half of the converter. It causes the vehicle moves forward on the road.

Let us look at the torque converter parts and how it works.

  • Impeller (also called pump)
  • Turbine
  • Stator
  • Lock-up clutch

Impeller (also called pump)

The impeller is a type of centrifugal pump. It has a curved blade connected to the housing and radiates outward from the center. It is connected to the flywheel of the engine. 

When the impeller spins, fluid flows in an outward direction due to centrifugal force caused due to rotation of the impeller. The same principle happens in the washing machine. The faster the impeller rotates, high the centrifugal force develops and causes more fluid flows outward. 


The turbine is on the opposite side of the impeller. It directly connects to the transmission shaft of the transmission system. As it is not connected to the impeller, both have different speeds. 

When the fluid passes over the turbine blades, it rotates the turbine. It causes the car to move forward. In the initial phase, the turbine blades rotate slower than the impeller blades. As we accelerate the speed through the vehicle pedal, turbine blades rotate faster and match the speed of the impeller.

The blades of the turbine are curved in shape. When the blade rotates, the fluid reaches the turbine and passes to the impeller through the turbine center. Due to this directional change, the turbine rotates. This cycle repeats every time. 


The stator is the middle part of the impeller and turbine. It sends transmission fluid back to the impeller in the right direction and multiplies the torque.

The one-way clutch connects the stator to the fixed shaft of the transmission. So, the stator spins in its direction identical to the turbine. It doesn’t rotate in the direction of the fluid.

The fluid at the center of the turbine moves in opposition to the direction when it enters. It causes the fluid to change its path of flow. 

When it enters the stator, stator blades help to switch the direction of fluid similar to the impeller rotational direction and then enter into the impeller. 

Lock-up clutch

When we start the vehicle, the impeller rotates due to the engine’s rotational speed, and the turbine is stationary. As we accelerate the pedal, the turbine begins spinning due to the outward motion of the fluid in the impeller. The outward movement of the fluid occurs due to the high speed of the impeller. But still, the turbine speed is less than the impeller speed. 

When a vehicle crosses a certain speed (more than 45mph) to reduce the speed differences, the lock-up clutch engages, and the turbine and impeller rotate at the same speed. 

Oil Pump

Oil pumps help to supply fluid from the oil pan to the valve body. Further valve bodies provide this fluid to the various parts of the automatic transmission system.

Planetary Gear Sets

In the manual transmission system, the clutch system is to connect and disconnects the engine to the transmission. For that, we have to shift gears in the gearbox by pushing the clutch pedal.

But the automatic transmission system uses the planetary gear system. It is more similar to the solar system.

It uses a ring gear that has internal teeth. Middle of the ring gear, the sun gear rotates, and two or more planetary gears constantly intermesh with the sun gear. Ring gear encloses sun gear and planetary gear.

Ring gear connects with the engine shaft, and planetary gears connect with the transmission shaft, which carries power to drive the vehicle. Sun gear connects with the drum, which connects to half of the clutch pack, and planetary gears connect with the other half of the clutch pack.

These transmission clutches plates are spline in shape, some are from the inside, and some are from the outside. This disc locks together while motion transmissions.

Hydraulic System

As you know, all the transmission parts are submerged inside the transmission fluid. This system has three main functions. It prevents frictional damage, controls the transmission gear shifting, and cools the system.

The hydraulic system helps to engage the clutch plates at the right time. It builds the correct pressure inside the clutch system for engaging together.

With time seals and gaskets becomes harder and develop internal cracks. It may cause hydraulic fluid leakage and the breakdown of the system. So, the hydraulic system lubricates these seals and gaskets.


In a manual transmission, we must manually engage the clutch to transmit the motion. But in an automatic transmission, clutch packs maintain the desired output ratio.

It uses multiple metals and friction discs. When the valve body sends oil pressure to the clutch packs, it engages some plates with each other to get the desired output ratio.

With the help of sensors, we can create an oil pressure on the discs to engage them and prevent the rotation of the planetary gear.

When the planetary gear stops spinning, it directly starts rotating the transmission shaft. Hence, the vehicle starts running.

Brake Bands

The brake band helps to hold planetary gears temporarily. It allows the engine RPM to get in line with the gear ratio before release.

Output Shaft

The output shaft is the shaft coming out from the transmission system. After finishing all magic inside the transmission system, it finally delivers the output speed to the vehicle.
This output shaft transfers the rotary motion of the transmission system to the drive shaft through a yoke-type of the universal joint. It is spline in nature.

Oil Pan

It is an oil storage present at the bottom of the vehicle. Depending upon the need oil pump pulls the oil and sends it to the automatic transmission parts.

It also uses the filter to clean the oil before supplying it to the different parts of the automatic transmission system.

Valve Body

The valve body directs the oil pressure through the pump to all parts of the automatic transmission system. It tells the oil or gives the direction of flow.

Different types of sensors and transmission control module (TCM) regulates the oil flow direction inside the valve body as per the requirement.


We can say it is the brain of the vehicle. The computer controls most of the functions to get good efficiency. Many sensors sense the data like engine speed, temperature, and more.

Based on these data, the computer controls the transmission gear shifting to ensure the use of the optimal shift point.

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