What are The Parts of A Transmission?

A transmission is a formidable puzzle for most drivers, made up of many intricate connections between a multitude of transmission parts. The complexity of these parts and how they fit together is intimidating, especially when faced with transmission repairs and failing to understand what is happening or why.

While we must trust our transmission mechanics to take care of the details, a little knowledge of the possibly faulty transmission parts can help put you at ease during the transmission repair process.

The first step in having a basic understanding of the transmission parts is understanding the purpose of the transmission as a whole. Your automatic transmission is responsible for transferring the power of your engine to the driveshaft and wheels so your car can move within its optimal speed range per minute (RPM). The transmission maintains this optimal range by shifting the transmission as you go faster or slower.

Related Article: What is Gearbox?

What Are the Parts of a Transmission?

The main transmission parts that need to work together are:

  • Transmission Casing
  • Planetary Gear Sets
  • Hydraulic System
  • Torque Converter
  • Computer
Automatic Transmission

Transmission Casing

A transmission casing houses all the parts of the transmission. It sort of looks like a bell, so you’ll often hear it referred to as a “bell casing.” The transmission casing is typically made of aluminum. Besides protecting all the moving gears of the transmission, the bell casing on modern cars has various sensors that track input rotational speed from the engine and output rotational speed to the rest of the car.

Torque Converter

When driving a manual transmission, the driver must engage the clutch or shift to neutral when the vehicle comes to a standstill, such as at a red light, or the engine stalls. The torque converter in an automatic transmission allows the engine to keep running when the vehicle is stopped and still in gear. Torque is defined as a force that causes rotation.

The torque converter uses the pressure of the transmission fluid to control the rotation of its parts. When stopped at this red light, one half of the torque converter is spinning while the other is stationary. As you accelerate, the fluid pressure forces the other half to spin in conjunction with the first half, causing the vehicle to move forward.

The torque converter sits between the engine and the transmission. It’s a donut-looking thing that sits inside the big opening of the transmission’s bell case. It has two primary functions in terms of transmitting torque:

  • Transfers power from the engine to the transmission input shaft
  • Multiplies engine torque output

It performs these two functions thanks to hydraulic power provided by the transmission fluid inside your transmission.

To understand how this works, we need to know how the different parts of a torque converter work.

There are four main parts of a torque converter in most modern vehicles: 1) the pump, 2) the stator, 3) the turbine, and 4) the torque converter clutch.

1. Pump (aka impeller).

The pump looks like a fan. It has a bunch of blades radiating from its center. The pump is mounted directly to the torque converter housing which in turn is bolted directly to the engine’s flywheel.

Consequently, the pump spins at the same speed as the engine’s crankshaft. (You’ll need to remember that when we walk through how the torque converter works.) The pump “pumps” transmission fluid outwards from the center towards the . . .

2. Turbine.

The turbine sits inside the converter housing. Like the pump, it looks like a fan. The turbine connects directly to the input shaft of the transmission. It’s not connected to the pump so it can move at a different speed than the pump. This is an important point. This is what allows the engine to turn at a different speed than the rest of the drivetrain.

The turbine can spin thanks to the transmission fluid that gets sent from the pump. The turbine’s blades are designed in a way so that the fluid it receives is moved towards the center of the turbine and back towards the pump.

3. Stator (aka Reactor).

The stator sits between the pump and turbine. It looks like a fan blade or airplane propeller (do you see a pattern here?). The stator does two things: 1) sends transmission fluid from the turbine back to the pump more efficiently, and 2) multiplies torque coming from the engine to help get the car moving, but then sends less torque once the car is going at a good clip.

It accomplishes this thanks to some clever engineering. First, the blades on the reactor are designed in a way so that when the transmission fluid leaving the turbine hits the stator’s blades, the fluid is diverted in the same direction as the pump’s rotation.

Second, the stator is connected to a fixed shaft on the transmission via a one-way clutch. This means that the stator can only move in one direction. This ensures that fluid from the turbine is directed in one direction. The stator will only start spinning when the fluid speed from the turbine reaches a certain level.

These two design elements of the stator make the work of the pump easier and generate more fluid pressure. This, in turn, creates an amplified torque at the turbine and because the turbine is connected to the transmission, more torque can be sent to the transmission and the rest of the car. Whew.

4. Torque converter clutch.

Thanks to how fluid dynamics work, power is lost as the transmission fluid goes from the pump to the turbine. This results in the turbine spinning at a slightly slower speed than the pump. This isn’t a problem when the car is getting going (in fact that speed difference is what allows the turbine to deliver more torque to the transmission), but once it’s cruising, that difference results in some energy inefficiencies.

To negate that energy loss, most modern torque convertors have a torque converter clutch that’s connected to the turbine. When the car reaches a certain speed (usually 45-50 mph), the torque converter clutch engages and causes the turbine to spin at the same speed as the pump. A computer controls when the converter clutch is engaged.

Planetary Gear Sets

Manual transmissions use a clutch to connect the engine and transmission. They require the driver to shift gears, which means that the gears are actually moved in a somewhat linear, sliding transmission to engage the coordinating gears necessary to maintain the correct speed ratio. Automatic transmissions hold the transmission gears in one place in a more circular structure. This is no different from a small solar system, hence the name Planetary Gear Sets.

Through the combination of an outer ring gear, a central “sun” gear, and two or more smaller “planet” gears, all of which are constantly intermeshed, the gear shifts from the driver. Similar to the solar system, the sun gear is in the center and remains stationary, and the smaller planet gears mesh with the gear and ring gear to keep the car running smoothly.

The ring gear is connected to the input shaft that supplies power to the motor. The planetary gears are located in a housing or carrier that is connected to the output shaft that transmits the power to the drive train and the wheels. The planet gears are also connected to a clutch pack. The sun gear is connected to a drum which is connected to the other half of the clutch pack.

Transmission clutch packs consist of a series of washers, half of which are wedged on the outer edge and half with wedges on the inner edge. These alternating disks fit together to lock and rotate together. They do this with hydraulic functions.

Brake Bands and Clutches

Brake bands are made of metal lined with organic friction material. The brake bands can tighten to hold the ring or sun gear stationary or loosen to let them spin. Whether a brake band tightens or loosens is controlled by a hydraulic system.

A series of clutches also connect to the different parts of a planetary gear system. Transmission clutches in automatic transmissions are made up of multiple metal and friction discs (which is why they’re sometimes referred to as a “multi-disc clutch assembly”). When the discs are pressed together, it causes the clutch to engage.

A clutch can cause a planetary gear part to become an input gear or it could cause it to become stationary. It just depends on how it’s connected to the planetary gear. Whether a clutch engages or not is driven by a combination of mechanical, hydraulic, and electrical design. And it all happens automatically.

Hydraulic System

All transmission parts are constantly immersed in gear oil. This fluid is manipulated to create pressure that compresses the transmission clutch pack at the right time. A complex system of pipes moves the fluid around the transmission and torque converter to create this pressure.

The transmission hydraulic system has three main purposes: to help control the shifting process of the transmission, lubrication of the transmission parts to prevent frictional damage and cooling of the transmission. The fluid pressure in the transmission must be maintained at all times to avoid damage.

The tubes that carry the transmission fluid have two large outer seals at the front and rear. The seal on the front protects the connection to the torque converter and the rear seal contains the fluid where the transmission meets the output shaft.

Seals are made of neoprene. Within the transmission exists another type of seal, called a gasket, which connects and protects two stationary transmission parts. Gaskets may be made of a variety of materials, such as rubber or silicone. Seals and gaskets can harden over time, which might cause leaks and a drop in transmission fluid pressure, both of which can lead to damage to the transmission.


In most automobiles today, a computer controls the transmission function so that all of the vehicle systems can work together to achieve optimal fuel economy and performance. Up to 30 sensors read all of the various factors such as vehicle speed, engine temperature, engine RPM, etc. that control the shifting of the transmission to ensure that the optimal shift points are used.

The many transmission parts in your vehicle may remain a mystery, but understanding some of the basics can help you have a more informed conversation with your transmission mechanics before leaving it in their capable hands.