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4 Most Common Transmission Control Module Symptoms

Recent automatic transmissions are one of the most complicated and technologically-advanced components on your vehicle. Some of them are so well-built and versatile, they can be found on a variety of different vehicles. The ZF 8HP eight-speed transmission, for example, is used on the Ram 1500, the Aston Martin DBS Superleggera, the Toyota Supra, the BMW M5, the Grand Cherokee Trackhawk, and even the legendary Rolls-Royce Phantom. Wondering how can the same gearbox be suitable for such different vehicles at the same time? It’s all thanks to their transmission control module and their model-specific programming.

The TCM is the component dictating how a transmission behaves and how great it will perform under certain conditions. But what happens when a component as vital as the transmission control module goes bad? In this article, we will aim to explain how TCM work, the most probable cause of TCM failures as well as to demonstrate the most common symptoms of a faulty transmission control module.

What Is A Transmission Control Module And What Does It Do?

The TCM is the unit controlling everything related with the transmission on your vehicle. The transmission control module is made of a processor, a memory and input and output circuits. 

The processor is the component used to process the information received by the different sensors positioned on the transmission such as the input shaft, output shaft, temperature and oil pressure sensors. The memory, on the other hand, is where the mapping data informing the processor of how to react and which valve should be opened or closed on a given situation is stored. The input circuits are used to convert the signals received by the transmission sensors into readable data for the processor while the output circuits send back actionable signals to various actuators. 

To put it simply, input sensors take in the data and send it to the processor. The memory tells the processor what to do with it and the processor then sends it back to the transmission using the output circuits.  

Most Common Symptoms Of A Faulty Transmission Control Module

Since the transmission completely relies on the commands of the TCM to operate properly, any faulty component inside the transmission control module will inevitably have an impact on how the transmission behaves. Here’s a list of some of the most common symptoms that the TCM on your car may be faulty and some of the probable causes of the failure.

Transmission Not Changing Gears

One of the most common problems with today’s transmissions is gears not changing at all. The transmission will either shift a couple of gears and won’t downshift the next time you stop or will simply stay stuck in the same gear no matter what you do.  

While this can be related to multiple other malfunctions like a low transmission fluid level or a faulty speed sensor, if the input or output circuits are burned, the signal coming from the sensors will stop reaching the processor. When this happens, the processor will have no idea what speed the car is going at and will consequently stop shifting gears altogether.  

Early And Late Gear Changes

This symptom can be described as when your vehicle shifts gears at a lower or higher rpm than normal. If, when gradually accelerating from a stop, the gears change below 2000 rpm or higher than 4000 rpm, your transmission is definitely switching gears too soon or too late. 

A transmission not changing gear at the right moment can be related to more than one faulty component inside the transmission control module. For example, if the TCM’s memory gets corrupted, the processor won’t be able to rely on the mapping included in the memory and will have no idea when to shift gears. Luckily, such a condition doesn’t always mean that you’ll have to have your TCM replaced. Sometimes, a simple re-flash might do just fine.  

Another probable cause is if the input circuits have an internal short. The signal coming from the output shaft sensor might be delayed, causing the processor to react later than expected. If the processor takes a couple of seconds more to send the signal to shift back to the transmission, it will obviously delay the time required to switch gears and will create late gear changes happening at a higher rpm than they should. 

It’s worth mentioning that not all early or late gear change problems are automatically related to a malfunctioning TCM, though. Faulty sensors and an incorrect transmission fluid level could also create similar symptoms.

Transmission Stuck In Park

Automotive technology is constantly evolving and so do car problems and their associated symptoms. What was once a simple thing to fix may now be related to a completely different system and require a much more complicated troubleshooting process? 

A good example of that is when you start your car in the morning and, even though you are pressing the brake pedal all the way down, the shifter won’t budge from the Park position. In the past, such a condition was usually related to a faulty shift lock solenoid, a blown relay or a bad brake switch. The system was much simpler then and the shift lock solenoid circuit was only controlled by the brake switch. 

On recent cars, however, it’s not rare that the solenoid is activated through the TCM. If for some reason, the transmission control module stops doing its job and doesn’t send a signal to the shift lock solenoid to retract when the brake switch input is detected, you won’t be able to move the shifter from the Park position, no matter how hard you press on the brake pedal. 

Luckily, if this is your case, most cars have a specifically designed shift lock override button hidden somewhere on the center console. If you aren’t sure where it’s located, look at the transmission section inside your car’s owner manual for more details.

Transmission Stuck In Neutral

A car stuck in Neutral is probably the worst problem on this list. When it happens, the transmission will not engage in any gear no matter how the shifter is positioned. Consequently, the car won’t move at all.  

The explanation for this condition is quite simple. When you move the shifter from Park to Drive, the transmission position switch, also called the Neutral switch, sends a signal to the processor through the input circuits to let it know what gear the driver wants the transmission to be in. The processor then sends a signal back to the transmission to activate the valves required to engage the 1st gear.

The most frequent cause of transmission not engaging and staying stuck in Neutral is a faulty transmission position switch. If the Neutral switch doesn’t send a signal to the processor, it obviously can’t be aware that the driver has moved the shifter to engage the transmission. With that being said, the same thing will happen if something is broken inside the TCM and the communication between the Neutral switch and the transmission can’t be established.

Keep in mind that a faulty Neutral switch is a lot more frequent than a malfunctioning transmission control module. Always make sure to correctly test the switch before jumping to conclusions and ordering a new TCM right away.  

To Conclude…

The transmission control module on your vehicle is the main unit controlling how your transmission behaves and, therefore, is a vital part of your vehicle’s proper operation. Luckily, TCM installed on recent vehicles are quite reliable and rarely come into the problem. Computers have evolved considerably in the last decade and control units don’t come into fault as often as they used to. Moreover, car manufacturers now design processors and memories that can be updated and reprogrammed if the OEM mapping turns out not as efficient as originally planned. In most cases, when a transmission starts to misbehave, a visit to your car dealer to have your TCM re-flashed should do the trick. If you notice any of the faulty transmission control module symptoms mentioned in this list, a quick call to your dealer might save you a ton of money and the trouble of having your TCM replaced.

Read More: Top 10 Transmission Problems and How to Fix Them

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Auto Diagnostic Tools and Repair Guides". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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