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Understanding How an Automatic Transmission "Thinks"

John is a fervent writer, gamer, and guitar lover. He is a former automatic-transmission repairer, welder, and hobbyist game developer.

Transmission control units are small computers that control how an automatic transmission operates.

Transmission control units are small computers that control how an automatic transmission operates.

There was a time when an automatic transmission was completely devoid of electronics. No switches, no sensors, no computers, just pure hydraulic engineering. Those days are long gone, however, and you’ll be hard-pressed to find an automatic car built in the last thirty years that doesn’t have some kind of electronic decision-making going on under the hood.

By making automatic transmissions “smarter”, manufacturers have been able to increase fuel efficiency and driving comfort considerably, but it’s not all upside. The transmission control units in modern cars, if they go faulty, can easily run up a bill of thousands to repair or replace. In contrast, all the necessary parts to completely overhaul one of those electronic-less transmissions of yore would barely break a few hundred.

But what are these control units? And what do they do exactly?

What Is It?

Before we get into how it works, let's look at what an automatic transmission's control module actually is.

The control unit of a modern automatic transmission can come under a few different names. The most obvious one is TCU, which stands for Transmission Control Unit. Another variant of this is TCM, Transmission Control Module. Alternatively, these modules are sometimes referred to as a “mechatronic”, which is technically specific to VAG vehicles (Volkswagen Audi Group), but is often used for others. While there is no official reason that I’m aware of, the difference between a TCU and a mechatronic seems to be in the fact that a mechatronic lives inside the transmission itself, whereas a TCU is typically found elsewhere on the vehicle. A mechatronic will also have a range of sensors built directly into it, whereas a TCU will be connected to its sensors remotely.

The control module is also sometimes referred to as a “Transmission ECU.” ECU stands for Engine Control Unit, which is essentially the brains of the engine. Transmission ECU is an inaccurate label (Transmission Engine Control Unit?), but "ECU" has become almost interchangeable with "control module" in automotive trade (in my experience, at least).

This control module from a Powershift transmission is an example of a mechatronic that is housed inside the transmission itself. Note the black sensors protruding from the back.

This control module from a Powershift transmission is an example of a mechatronic that is housed inside the transmission itself. Note the black sensors protruding from the back.

As far as “what it is”, the control module is a computer of sorts. Its job is to take in a range of information from the vehicle and decide how the transmission should behave based on that information. The more sophisticated the transmission, the more information it takes into account. The control module is usually responsible for driving “modes” as well, since the only difference between, say, “Comfort” and “Sport” modes is the way the transmission shifts gears, and that is all controlled by the control module.

How It Works

Now you know what a control module is, let's look at how it controls your transmission.

There are four main things that happen when an automatic transmission takes action. Starting from the transmission itself and working back up the chain of command they are:

  • Action
  • Mechanical Trigger
  • Electronic Trigger
  • Command

An automatic transmission has a number of major components inside it that are used to achieve gear ratios. Depending on the transmission, these components can be clutch packs, shift forks, brake bands, and more. The last step along the route of action is the engagement of whichever component is being used. A clutch pack is applied, a shift fork moved, etc.

Next up the chain, we have the mechanical trigger. This takes the form of valves which sit inside a component called a “valve body", and exist to direct the hydraulic flow to where it needs to be. Hydraulic fluid is channelled through the valve body—it’s course determined by the opening and closing of valves—until reaches it destination where it will cause a major component (mentioned above) to operate.

A valve is just a piece of metal (usually) however, and can’t move on its own. That’s where the electronic trigger comes in. This most often takes the form of solenoids, which receive electrical signals and, depending on the solenoid, can let the oil flow, prevent the oil flow, or let a variable amount of the oil flow. This oil then flows (or doesn’t flow) to the valves, which alters the course of the oil through the valve body, causing action to be taken in the major components.

But before any of that can happen, an electrical signal needs to be sent to the solenoids to activate them, and that is handled by the control module. Taking in information from around the vehicle, the control module decides what the transmission should be doing, and sets that in motion by controlling the appropriate solenoids, which control the appropriate valves, which engages or disengages the appropriate components.

Still with me?

The mechatronic unit from a ZF6HP26 automatic transmission, together with the valve body which it controls.

The mechatronic unit from a ZF6HP26 automatic transmission, together with the valve body which it controls.


We know what the brain is, and we know how it thinks, but where does it get it's information from?

At it’s heart, the transmission could theoretically operate with little more than engine speed and vehicle speed. Indeed, older automatic transmission without control modules achieved gear shifting using a modulator which used a combination of the output shaft rotation speed and the vacuum pressure created by the engine speed to initiate gear changes without any "thinking" involved. Of course, if that had been the best driving experience that could be created, we’d still be using that type of setup in today’s transmissions.

The core information that the transmission needs remains the input speed (ie, the engine RPM) and the output speed of the transmission, however. In addition to deciding when to change gear, this also gives the control module a way of checking for problems. The control module will know what the gear ratio should be at any given time, and so by comparing the input and output speed, it can tell if the transmission is not behaving as it should. For this reason, many transmissions will also have an “intermediate” speed sensor which monitors the internal speed of part of the transmission. This simple check of expected speed against actual speed actually allows the control module to be fairly detailed with its fault reporting. It may not have a sensor on each clutch, but if the actual speed varies significantly from the expected speed every time a particular clutch engaged, the control module can deduce that there is an issue with that clutch.

To supplement this information, however, the control module also takes a number of other variables into account. Some examples of this data include the throttle position, engine torque, vehicle incline (how steep the road is), and even engine airflow.

All of these variables allow modern, sophisticated automatic transmissions to provide a smooth, intuitive driving experience that older transmissions could never have hoped to produce. Of course, the downside is that transmissions can misbehave for a whole range of new reasons that have nothing to do with the transmission itself. I personally have seen a number of shifting faults cured by replacing the MAF (Mass Air Flow) sensor on the engine, and at least one instance of a transmission going into fault mode because the vehicle had been fitted with a replacement tyre that was the wrong size!

Fail State

What happens when things go wrong? And how does the control module know?

The control module has a number of ways of detecting a problem within the transmission. This includes the aforementioned speed sensor comparisons, but can also include monitoring the electrical resistance of electrical components, the temperature of the transmission fluid, the pressure of the hydraulic circuit, and even current gear selector position (if you’re doing 70mph and the selector says you’re in “Park”, that’s a problem).

Typically when the control module detects such a problem, it will put the transmission into a fail state, often referred to as “limp mode”, “default”, or “fault mode”. In this mode, the transmission will be locked into a single gear—usually third—in order to minimise the potential of exacerbating the problem. Automatic transmissions typically have a default state (hence the name of the fail state) whereby the unpowered state of all the electronic components leaves one hydraulic circuit open, and that circuit will engage whatever gear that particular transmission defaults to.

So that’s how an automatic transmission “thinks”. Hopefully you have a better understanding of what’s going on under the hood of your automatic vehicle, but if you have any questions, feel free to leave a comment below.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2017 John Bullock