Symptoms of a bad Transmission Control Module

Vehicles with automatic transmission usually have a control module that tells other parts of your vehicle what to do – most importantly the gear.

If this control module develops a fault or starts showing some negative symptoms, you must pay attention to it and fix it without delay.

There are some symptoms of a bad transmission control module that are quite obvious, while others require paying close attention to the vehicle. Still, ignoring these symptoms can be dangerous and can result in catastrophic damages if you continue using the vehicle.

Before we start discussing what happens when a transmission control module goes bad and how to fix it, you must understand this part of the vehicle you’re dealing with.

What is a transmission control module?

Symptoms of a bad transmission control module

This is a vital part of a vehicle that works in harmony with other transmission system components. It’s a computer that communicates with other components of the vehicle and helps regulate gear shifting.

Several parts of a vehicle perform vital functions in making your journey as pleasant as possible, and this control module is an important one.

Now you ask, “what does a transmission control module do?” It’s simple. This is a component of your vehicle that sends out signals and helps your vehicle choose the gear that is best for it. This will make your drive enjoyable.

Usually, what causes a transmission control module to fail is voltage overload. When this happens, other transmission components of the vehicle start malfunctioning. This is when you need to pay close attention to the signs.

Symptoms of a bad transmission control module

Engine warning signal

When this warning light shows up on your dashboard, there could be several reasons for this. However, if you start noticing any unusual changes or malfunction with the shifting, it’s usually because of a bad transmission control module.

If you’re good with mechanical tools, you can simply diagnose the vehicle for trouble codes with a scan tool, otherwise, you should ask for the help of a mechanic without delay.

Irregular gear shifting

An automatic transmission system depends on the control module to shift the gear. So, if a vehicle control module is bad, you’d notice an irregular or hard shifting. It could also shift when you don’t intend to.

Let’s say your vehicle shifts to another gear without any prior notice, it’s most likely a problem with the TCM. You have to call the attention of a mechanic to it, as it’s unsafe to ignore and keep using the vehicle.

Gear upshift problem

Have you tried accelerating and your vehicle fails to speed up even as you keep mashing the gas pedal?  It’s most likely a problem with the control module not transmitting the signal to the gear.

This can be dangerous, especially when you’re trying to overtake a vehicle. When your vehicle gear downshifts instead of accelerating when you feed it with gas, you should call the attention of a mechanic to it.

Gear downshift problem

This is quite related to the previous symptom. What you see here is that the gear becomes unusually unresponsive. It happens when the control module is bad and it won’t transmit signals to decrease gear.

For instance, when trying to slow down or park the vehicle, a fault like that could result in a gear downshifting problem. When you start noticing signs like this, you should check your vehicle or call a mechanic to do it for you.

Stuck gear position

Unlike the irregular shifting, this time your vehicle gear gets stuck in a mode – usually the first or neutral mode. When this happens, it is usually a problem with the control module not transmitting the signal to the gear.

This means your vehicle won’t move in the neutral mode or you may struggle with the high speed of the first gear. This can be dangerous, especially when driving on a highway. It’s important to call a TCM expert to help when these signs start showing.

Delay in gear shifting

How fast your vehicle will move depends on the timing accuracy of the shifting. This means that a bad control module won’t transmit well, which will in turn slow down your vehicle.

This bad transmission relay symptom will also affect the acceleration, making your vehicle slow down instead of speeding up. Things could get worse when you’re trying to get past a slow vehicle and the gear won’t shift so that you can overtake.

Quick fuel exhaustion

A bad TCM in a vehicle can result in quick fuel exhaustion. The job of a TCM is to regulate the timing and change in your vehicle gear to function on an ideal level.

However, when there’s a problem with this part of your vehicle, your vehicle will start consuming more fuel resulting in quick fuel exhaustion.

what happens when a transmission control module goes bad

How to test transmission control module

Diagnosing a TCM for problems requires the use of a scan tool and understanding the trouble codes. However, there’s a way you can troubleshoot a TCM without going through the tedious pages of a TCM manual.

Here, you’ll need a screwdriver – Phillip-head preferably, a socket set, and a spray cleaner for electrical connectors. To begin troubleshooting, kill the engine completely and detach the battery.

Apply the vehicle emergency brake and shift the gear to neutral mode. Then, open the bonnet and find the TCM of your vehicle – it’s usually in the distributor cap or between the engine and transmission below the hood. You can also find it using the vehicle manual.

Next, you’ll have to remove the TCM. Unscrew the distributor cap using the Philip-head screwdriver. Carefully do this to avoid creating electrical jolts. Then unscrew the bolts or nuts holding the TCM before removing it.

Remove the fluid in the TCM and clean the connectors and the module using the electronic spray cleaner. Carefully install the TCM and screw the nuts and bolts back in place.

If troubleshooting doesn’t fix the problem, you may decide to run a test with a scan tool if you can understand the error code results. However, you’ll eventually have to call an expert to help after detecting the issue. It’s better to call a mechanic from the start to save you some trouble.

How do you fix the transmission control module?

Most often than not, this component of a vehicle can’t be fixed but replaced. This is why you need to call the attention of a TCM expert to the problem when you start noticing the malfunctioning.


Q: Can you drive with a bad transmission control module?

Since this part is responsible for transmitting signals that control the vehicle’s movement and speed, it’s dangerous to use a faulty vehicle like this.

You will experience difficulty in shifting the gear automatically or manually, and this could result in an accident. Also, this can cause severe damage to the transmission wiring.

Q: Can a bad ECM cause transmission problems?

A bad ECM can cause transmission problems to your vehicle. This means that a vehicle control module can start showing signs of fault as a result of a bad ECM.

Q: How long does it take to replace a transmission control module?

There’s no specific amount of time it’ll take to change a vehicle’s TCM. However, if your vehicle is a rear-wheel type, you may have to be without it for a day or two.

If your vehicle drive is front-wheel, replacing this component of your vehicle can take 3 or 4 days. Still, you should leave the vehicle with the mechanic till it’s properly fixed and ready to function.

Q: How Much Does it cost to replace the transmission control module

Changing a transmission control module can cost you about $500 to $750. Replacement parts only can cost about $400 to $550, while you may have to pay about $100 to $200 for labor.

A TCM replacement cost can also vary, depending on the mechanic you hire and how you get the new TCM. Sometimes, mechanics and vehicle owners prefer getting their vehicle parts online since it’s cheaper. However, delivery fees and tax are also added to the total cost.

Final Words:

The control module is a vital part of a vehicle that should be given attention, especially when the vehicle starts showing signs of unusual gear shifting and acceleration.

When these symptoms start to surface, you must call a transmission expert to examine the TCM, and replace it if necessary. This will save you all the stress of buying a manual and diagnosing the problem yourself.

Eventually, the vehicle’s TCM will have to be taken to the mechanic for proper examination and repair. It’s also dangerous to use a vehicle already showing symptoms of a bad transmission control module – similar to the ones mentioned in this article. It could result in an accident or severe damages.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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