Cars Simplified: Everything Automotive Explained

Automatic Transmissions

An automatic transmission/transaxle is not just a manual transmission that shifts automatically! The fundamental structure of it is completely different, but aims to achieve the same mechanical goals as a manual transmission, while making the driving experience more simple.

Automatic transmissions/transaxles were once just an expensive option versus the "standard" manual transmission. Now, the typical transmission in North America is an automatic, with CVTs growing in popularity above manual transmissions as well.

There are several types of automatic transmissions: conventional transmissions, transaxles, dual clutch, continuously variable, a Honda/Saturn type of transmission, and hybrid transmissions. This page will focus on conventional automatic transmissions.

ATF Filter

The automatic transmission fluid filter keeps unwanted particles out of the fluid. It is typically found inside the transmission, under the transmission pan or as an external spin-on filter. Read the full article.

Automatic Transmission Gears

The Sun, Planetery, and Ring gears in an automatic transmission

As pictured at the right is a conventional automatic transmission gear style, a planetary gear set. These gear sets have three sections: the sun gear at the center, the planetary gears around the sun gear, and the ring gear around the rest.

These can be locked by either the outside casing or the carrier, and in some scenarios the sun gear can be locked. The different combonations of locks produce different gear ratios, including the reverse gear. In theory, most automatic transmissions could have multiple reverse speeds as well, if the right lock combinations were allowed.

Almost all automatic transmissions use helical-cut gears, as they are stronger and less noisy than straight-cut gears.

Ravigneaux Gears

A ravigneaux gear set is similar to a standard planetary gear set, but one of the sun gears is smaller to make room for a second set of planetary gears that are still held by the planetary carrier. The two sides share a common ring gear and outer set of planetary gears.

Torque Converter

Instead of using a clutch like a manual transmission, a torque converter is used.

Modern automatic transmissions can't be properly lubricated with engine oil; it uses automatic transmission fluid, which is specifically formulated for the stresses involved.


Servos use hydraulic pressure to activate or deactivate, converting that pressure into mechanical movement. A single servo is a piston within a cylinder that will have fluid push against one side of the piston or the other.

Gear Selection

The gear selection is done by holding some components of planetary gear sets still while other components are allowed to spin. The clamping components are operated hydraulically, and the hydraulics are controlled by shift solenoids.

Clamping Bands

Clamping bands grab on to the outer portion of the planetary gear assembly. They are typically metal with an inner friction material that is soaked in automatic transmission fluid.

Servo Activation

Servo motors push the bands together either directly or through levers. Direct application will use a larger servo, while the lever design allows for a smaller servo that travels further thanks to a leverage advantage. Which one is used depends on space and manufacturer preference.

Valve Body

The valve body is typically on the bottom of the transmission, just past the transmission pan, or to the side of the transmission behind a side cover. The valve body contains spool valves which direct the flow of ATF through the transmission components.

Multi-Disc Clutches

There are multi-disc clutches within a drum that spline to the drum. Those splines can be either on the steel plates or the friction material discs. When the clutch is applied, the two sets of plates lock together and drive the drum or get driven by the drum.

Clutch Springs

The clutch springs push back on the clutch discs as they are applied and while they are applied. Sometimes it is a single coil spring per clutch disc set, while on other applications use multiple small springs.

Front Pump

The front pump supplies hydraulic pressure to the rest of the transmission. It is driven by the input shaft of the transmission because the torque converter is the next driven component, and the torque converter doesn't function until it has fluid in it.

Hydraulic Pressure Regulator

Fluid from the front pump is regulated by the pressure regulator, which allows fluid higher than a selected pressure to drain out until the given pressure is achieved. This is the first component the fluid reaches before being sent to other components.

Other Components

These are parts that aren't directly involved in changing gears, but are still vital to the operation or life span of an automatic transmission.


An accumulator absorbs hydraulic shock to smooth out shifting by allowing the pressure of hydraulic fluid to push against it when it is activated. Without an accumulator, the hydraulic pressure would only have the push-back of the clamping band to act on. The accumulator is a second component for the pressure to act on, and can be designed for more or less smoothness by making it stiffer, softer, larger, or smaller. The piston type is a small cylinder with a seal around the piston and a spring behind the piston. A valve type is similar to a spool valve.

The smoothness can be adjusted in the accumulator, so performance shift kits will usually stiffen them up so the shift occurs more quickly. This typically isn't how the transmission leaves the factory because it causes a rough, unpleasant shift.

Transmission Pan

The transmission pan is a panel on the bottom of the transmission that allows access to the bottom internals, such as the valve body or internal filter. The pan is often used as a method of identifying transmissions, particularly in vehicle models that have multiple transmissions.

Automatic Transmissions Versus Manual Transmissions