Thinking about how automatic transmissions work usually results in the same type of headache you get when eating ice cream too quickly, brain freeze. Automatic transmissions are one of the most complicated components in a car. And their use of epicyclic (aka planetary) gear sets, one way clutches, brake bands, hydraulic fluid, torque converters, solenoids and a rat’s maze of fluid passages has confounded more than one person trying to come to grips with how they work.

Maybe it’s time to put fear aside and try to tackle the subject in light of the latest developments in transmission technology such as ZF’s new 9HP nine-speed transverse mounted automatic transmission.

First how do you define what an automatic transmission is? Some will say anything that lacks a clutch pedal and can be put in ‘D’, and left there, qualifies as an automatic transmission. That would lump the new double clutch manuals, SMG, and Powerglide into the same bowl of soup (or is that a bowl of ATF).

No, that really isn’t fair to either the double clutch and SMG transmissions or to the true automatics. Yes all true automatics have no clutch pedal and can be put in ‘D’, but they also have some other important features that distinguish them from fancy manuals.

A true automatic transmission can shift gears without having to interrupt torque transfer. Ever wonder why people railed about the shift quality of SMGs? It’s because in order to shift smoothly you need to lift the throttle a bit before the shift (interrupt the transfer of torque).

An automatic transmission can also remain in gear without stalling the engine when at idle. And it can change engine speed when engine torque is inadequate to maintain road speed. And that requires more than a little bit of monkey motion to accomplish.

But before we get into it, be aware that this is not a comprehensive look at automatic transmissions, it is merely a glimpse at best. It does not include any math, without which the ‘automagic’ properties will have to be believed, rather than understood. But, more importantly, there will be NO quiz at the end. However, there will be links to resources that you can explore if you want to dive deeper.

We’ll begin at the torque converter and work our way back towards the driveshaft (as if the example is a rear wheel drive layout).

The Swamp Vortex

At the nose of the transmission is a ‘torque converter’. It consists of an impeller, a turbine, and a stator. The impeller is attached to the engine, the turbine is attached to the transmission. The stator slips in between the impeller and turbine. Most torque converters also have a lock-up clutch that ‘locks’ the impeller and turbine together. In other words it directly couples the input from the engine to the transmission.

When bundled together the stator (which has a smaller diameter than the impeller and turbine) fits inside the impeller and turbine which, when assembled, looks like a metal donut. But all three pieces can move separately (the stator’s motion is usually controlled by a one way clutch).

This whole assembly is swimming in a sea of automatic transmission fluid (hydraulic fluid when it comes right down to it). The impeller’s fins move the fluid from the center and flings it outwards towards the edges. The impeller is acting as a fluid pump. At idle the pump action of the impeller is too weak to effectively move the turbine, so the car doesn’t stall at idle. As the speed of the impeller is increased more pumping action results in the turbine spinning, effectively transferring power to the transmission. The stator, when held stationary by a one way clutch, actually increases the pumping action by increasing the force and speed of the fluid flow from the impeller to the turbine. The stator works most effectively at low speeds where its torque multiplication is most useful.

That torque is then presented to the transmission’s gear sets by the output shaft of the turbine.

In Orbit

The gear sets that the transmission uses determine the number of forward gears the transmission has and the torque capacity of the transmission. Most automatic transmissions use planetary gear sets. They have decent torque handling capacity and are relatively compact.

Planetary gear sets have been in use forever (in dog’s years at least). In their simplest form, a planetary (epicyclic) gear set consists of a sun gear, planet gears, planet carrier, and a ring gear. The sun gear sits in the middle with its teeth on the outside of the gear. The planet gears, also known as pinions, rotate around the sun gear and are attached to each other via a planet carrier. The ring gear encloses the sun and planet gears and its teeth are on the inside.

So, if you turn the sun gear clockwise, and hold the planet gears in place by stopping the planet carrier from moving, the planet gears will rotate counterclockwise. The ring gear will then rotate counterclockwise also. (And that’s how you get reverse!) The Model T used a planetary gear set to provide two forward speeds and reverse.

But why do simple when you can make them even more complicated. Subsequently by mixing and matching combinations of multiple sun, planet, and ring gears you can create specialized gear sets that provide more than two forward speeds. These specialized gear sets go by the names of Wilson, Simpson, Ravigneax, and Lepelletier. And one or more of those gear sets (or variations thereof) can be found in every modern automatic transmission.

Catch and Release

However, to change gears you have to be able to start and stop, connect and disconnect, various pieces of the gear sets to get the engine’s input converted to the wheel speed output needed to propel the vehicle. You can manipulate the motion of the planet gear carrier with clutches, usually grouped in clusters or clutch packs. Clutches and/or band brakes are used to manipulate the ring gears. Sun gears can also be held stationary or allowed to rotate.

Shifting gear sets to change output ratios is accomplished through manipulating clutch packs. It is done by increasing or diminishing hydraulic pressure to the clutch pack required to provide the output ratio requested.

There’s a schematic language to describe the power flow through an automatic transmission. At first glance the language appears to be pretty opaque, but run through a few simple examples and it will become transparent. A link to a transmissions fundamentals document is provided in the Resources section of this article.

The Maze

The trick to all this is how to get the car in the right gear at the right time. Early automatic transmissions did not have electronics to determine what gear the transmission needed to be in. They relied on fluid pressure to move piston (plungers) that change the flow of hydraulic fluid within the maze of the valve body which then engages or releases specific clutches and or brakes bands. This valve body provides the control over how and when the transmission shifts.

The gearshift lever was directly connected to the valve control body and moving the lever manipulated the plungers which controlled where hydraulic pressure was routed. Later, as electric solenoids came into greater use, the shift lever was used to send an electrical signal to a solenoid which operated the plungers.

Now, electronics are used. The software code used by the transmission’s computer is the input providing the electrical signals that operate the solenoids that operate the plungers that control the flow of hydraulic fluid that manipulate the clutch packs that change the output ratio of the transmission. Sounds like that old kid’s song, “There’s a Hole In the Bottom of the Sea”.


Quite frankly it’s pretty amazing that automatic transmissions work and even more amazing that they worked well when there were no electronic controls. They work well as long as you keep clean fluid and a clean filter in them. The hydraulic fluid is critical to their successful operation and heat is the enemy of hydraulic fluid.

Over the years we’ve gone from two speed automatics, like GM’s Powerglide (still a very useful transmission for drag racing – referred to by the cognoscenti as a ‘glide’) to marvels like the new ZF 8HP and the coming ZF 9HP.

Automatic transmissions have allowed many more people to drive than would be if we were relegated to manual transmissions only. And sometimes I’m not sure that’s a good thing. Regardless, automatic transmissions are in the vast majority of cars in North America and as they get even more compact, and efficient they’ll take over more and more of the share in other markets.


PDF Files

Transmission Fundamentals

Transmission Fundamentals 2


Planetary Gearset

Ravigneaux Gear Train

Simpson Gear Train

TQ Lockup



Assembling An Automatic Transmission