Automatic Transmission Parts And Functions (Diagram)

How many parts are in an automatic transmission and what are their functions? Here is the ultimate guide to automatic transmissions, including a diagram.

While the engine is making all the power necessary to move your car, the transmission puts that power (transmits) to the car’s wheels and makes it move.

Engineers love to make things as simple, or more accurately said, as convenient as possible for the users of their product. The automatic transmission is far from simple but extremely convenient.

An automatic transmission consists of many precisely engineered parts that all have their respective functions.

In this guide, we will go through the most significant parts, such as the torque converter, planetary gears, brake bands, clutches, and more.

All the functions of the respective parts will also be explained concisely and understandably.

The focus will mostly be on the hydraulic automatic transmission, which is the most common one.

Gear up and find out everything about automatic transmissions below!

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Table of ContentsShow

Why Is A Transmission Needed?

automotive transmission

Before we dig into the exact details and parts of an automatic transmission, it is important to note that an automatic transmission is incredibly complex.

As a matter of fact, it even makes rocket science look like a walk in the park, and here is why:

The automatic transmission does not only consist of mechanical parts that rotate and transfer power from the engine to the wheels. It uses fluid, gears, pumps, and much more.

If it only consisted of a shaft, there would not be a need for transmission at all, as the engine could do the job perfectly.

The transmission, however, makes it possible to keep the engine running even when the wheels are stationary. If it did not, the engine would stall due to many reasons.

On the other hand, when the car must accelerate, the power output of the engine must be controlled and carefully applied.

Without a transmission, the engine’s RPMs would directly transfer to the wheels, making them spin incredibly fast and not grip anywhere.

The gears are there to multiply or divide the torque, making it possible to drive 85mph with a higher gear but the same engine speed as if you drove the car at 20mph in a lower gear.

In a manual, the driver manually changes the gears, which are directly linked with the gear stick. In the case of an automatic transmission, it is much more complicated as a planetary gear set is used.

Parts Of An Automatic Transmission

The automatic transmission consists of hundreds of parts if we are to be picky. Most of those parts make up a few general components that make the automatic transmission work smoothly.

These components (or parts) include a torque converter, planetary gears, brake bands, clutch plates, and output shafts, each of which is described in this guide.

Due to the complexity, an automatic transmission is very expensive to rebuild.

An automatic transmission diagram helps demonstrate the shared complexity of the engineering but illustrates the parts of the transmission very well.

Car automatic transmission
Automatic car transmission

The most important parts are described in from the torque converter, located closest to the engine, to the output shaft, which transfers the output of the transmission.

Torque Converter

The torque converter is an art in itself. Put simply, it consists of four parts:

  1. Impeller (also called pump)
  2. Turbine
  3. Stator (also called reactor)
  4. Lock-up clutch

The easiest part is to name them, as all these mechanisms are impressively engineered.

Together they make up the torque converter, which is analogous to the mechanical clutch of a manual transmission.

In other words, it is the link between the engine output and the rest of the transmission.

The torque converter works with the help of mechanisms dependent on transmission fluid.

To best understand the mechanism, it is best to cover each part of the torque converter separately. There is a lot of engineering involved, but we will make sure to keep it as simple as possible.

The Impeller (Pump)

Impeller for an automatic transmission

The impeller is connected to the crankshaft via the flywheel, which rotates the impeller. This, in turn, means that it rotates at the same speed as the crankshaft.

As the impeller rotates, the fluid in the torque converter gets “pumped” toward the turbine; between these two components, the stator is located, which we will cover shortly.

Keep in mind that the impeller is directly related to the engine output and propels the automatic transmission fluid to transfer the energy. It is not mechanically connected to the turbine.


Turbine of an automatic transmission
The turbine of an automatic transmission

The turbine is on the other end of the torque converter, which means it is the “receiver” of the fluid pumped by the impeller and then connects directly to the transmission’s input shaft.

As previously mentioned, it is not connected to the impeller, which is directly affected by the engine output.

This means that the speed of the turbine, which affects and is affected by the wheel speed, is independent of the impeller and, therefore, the engine speed.

This is crucial for keeping the engine going even when the wheels are stationary. If this mechanism did not exist, the car would stall.

Stator (Reactor)


The stator lies in between the turbine and the impeller and turbine. It is responsible for “torque converting” and is not always engaged.

The stator’s engagement is dependent on the speed of the turbine, or more precisely, the speed of the fluid from the turbine. This is key to its torque-converting capability.

It does not stop there, though; the stator is also responsible for sending the transmission fluid back to the turbine, making it somewhat of a closed-loop system.

So how does the stator convert the torque, and how does it work?

The stator is connected to a shaft, which is connected to the transmission. It will only go in one direction, the same direction as the turbine.

Due to the design of the stator and fluid dynamics, the stator will only engage (start rotating) when the impeller reaches a certain (higher) speed.

Both the impeller and the turbine are similar to propellers with many long blades. As the impeller rotates, it pushes the fluid through the stator, propelling it further to the turbine.

The stator can only rotate in the same direction as the impeller and turbine, thanks to a one-way clutch.

Furthermore, the stator is designed so that when the transmission fluid is propelled less “violently,” the stator, which does not move, will direct the fluid to the upper ends of the turbine’s “blades.”

This, due to fancy physics, increases the torque output by increasing the pressure. Lo and behold, the torque has been converted and increased!

The amplified torque is needed, especially when the car starts to move. In that case, the stator is static.

When the turbine reaches a higher speed, the turbulent flow will get another “angle of attack,” pressing down the blades of the stator and making it spin.

Due to the activation of the stator, the fluid in the torque converter will be less turbulent, hitting the turbine more to the center and therefore mitigating the torque amplification.

If it sounds a bit complicated, don’t worry, it is.

To get a better understanding, check out this YouTube video, which illustrates the torque converter mechanism well.

Lock-Up Clutch

The fourth substantial part of the torque converter is the lock-up clutch.

As the automatic transmission fluid plays a key role in the torque converter, it also poses a problem when greater speeds are evident, as there is a loss of energy due to fluid mechanics.

This loss of energy makes the impeller and turbine spin at slightly different speeds, which is not optimal when the car is moving relatively fast (around 35mph).

To combat this issue, the lock-up clutch engages and locks the turbine with the impeller, mitigating any loss of energy that would otherwise be present.

Planetary Gears

gears technology mechanic transmission planetary system 3D illustration

Both manual and automatic transmissions use gears to dictate how much torque should be sent to the wheels.

In a manual transmission, the gears are mechanically moved by the operator with the help of a gearshift, which is connected to a shift fork and, lastly, to the gears.

In an automatic transmission, things are much more complicated. Imagine the gears on a bike, but with different types of gears surrounding the standard ones, which engage “independently.”

This setup enables the transmission to automatically change gears with the help of brake bands and clutches (mentioned further down in the article).

To simplify this complicated part of an automatic transmission, we will keep it short and sweet.

Three types of gears constitute the automatic transmission:

  1. Sun gear – located in the center of the whole gear system.
  2. Planet gear – located between the sun gear and the ring gear, multiple planet gears are present in the system.
  3. Ring gear – the outermost gear in the system.

Refer to the image below to see the three types of gears:

Planetary Gears

These three types of gears enable different amounts of torque to be finally delivered to the wheels, just like the gears in a bicycle.

All these different gears provide different rotational speeds depending on which gear is engaged, at what speed, and even the direction.

The gears are engaged with the help of bands and clutches.

Brake Bands & Clutches

The bands and clutches are important parts of an automatic transmission. They work in different ways but are both needed to engage or disengage a gear properly.

So, what do the bands in an automatic transmission do? To put it simply, the bands halt the clutches that are engaged, which is necessary for the gear selection and output.

The bands are powered by hydraulic fluid pumped by the torque converted. When needed, a sensor will engage the hydraulic pressure, and the band will then connect and prevent a planetary gear from spinning.

The clutches are also powered by hydraulic fluid. They consist of several clutch plates that come together when engaged and may then, for example, make a planetary gear move.

The best way to visualize this is through a diagram.

The video below shows how the planetary gear set works together with the clutch plates. A word of caution is that it is extremely complex.

Output Shaft

All the things previously mentioned, from the impeller in the torque converter to the planetary gears helped by brake bands and clutches, all have one goal – to get the energy to the output shaft.

The output shaft is what makes your wheels rotate and, thus, makes the car move.

Is An Automatic Transmission Complicated?

Yes, an automatic transmission is complicated. Nevertheless, it is a great feat of engineering that makes driving much easier for many people.

There are many other parts that we have not touched, such as bearings, sensors, oil reservoirs, and many others transmission parts.

Either way, we hope you have a greater understanding of how an automatic transmission works.

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Bajro Muric

Bajro is an ASE-certified master technician and car enthusiast with a love for writing and teaching. He writes about anything regarding cars, from common problems and fixes to racing.