8 Symptoms of a Bad Transmission Control Module (& Replacement Cost)

Transmission issues are among the most frustrating. But before you start getting quotes for a new transmission, make sure the culprit isn’t actually a faulty transmission control module.

Keep reading to learn what this component does, common warning signs of failure, and how much it’s going to cost you to replace (or may repair) the module.

Table of Contents

What is a Transmission Control Module?

The transmission control module (TCM) is a vital component of any vehicle with an automatic transmission system. Rather than relying on hydraulic or mechanical control over a transmission system, newer vehicles use a transmission control module instead.

Think of the TCM as the brain of your transmission; it communicates with various sensors and components to ensure that your vehicle shifts gears smoothly and efficiently.

The TCM interprets and processes data received from components such as the throttle position sensor, vehicle speed sensor, and engine load. This information is used to calculate the optimal timing and gear shifting required for your car.

In short, the TCM is continuously working behind the scenes to deliver a smooth driving experience with optimal fuel efficiency.

A car or truck’s TCM will likely never need replacing, but if you have a bad transmission control module, immediate replacement (or repair) is necessary.

Where is the TCM Located?

If you don’t know where to find the transmission control module, don’t worry, you’re not alone. Its exact location will vary from one vehicle to the next.

The TCM can often be found below the cover at the back of the transmission case. It should be just below the engine control module’s position. Sometimes it’s found under the center console in the interior, under the dash by the firewall, under the hood near the battery, inner fender panel, or even the glovebox.

For example, the TCM of Chevy’s 4L60E transmission is normally found near the back of the transmission case (right under the cover). But an Allison T87 transmission would have its TCM located by the driver’s side battery on the fan shroud.

Don’t start taking things apart trying to guess where it might be. Getting your hands on a vehicle-specific service manual is the best way to confirm where it’s mounted but searching online for your vehicle’s year, make, and model will also get you there.

Signs of a Bad Transmission Control Module

If you ever have a problem with your transmission control module, you’ll need to have it inspected right away. A bad TCM will spell trouble for your vehicle and your ability to drive it. Fortunately, there are a few basic symptoms that you’ll most likely notice before this happens.

#1 – Check Engine or Transmission Light

If the Check Engine warning light (or transmission warning light) illuminates on your dashboard, it could mean several things. But if you notice any problems with your shifting in addition to the Check Engine light being on, then it is most likely a problem with your transmission control module or some other component of your transmission system.

In any event, get your vehicle checked out immediately by a mechanic or use your own scan tool to check for any Diagnostic Trouble Codes (DTC). If code P0700 shows up, you may have a problem with your transmission control module.

See Also: Code U0100

#2 – Unpredictable Shifting

Since an automatic transmission system relies on the transmission control module to do the shifting, a bad transmission module will cause incorrect or unintended shifting. This can prove to be anything from an annoyance to causing a serious accident.

Many drivers have reported that the car feels like it’s pulling back when driving. Or it can feel like the car jerks when accelerating.

For instance, if your car shifts to neutral or any other gear that it normally wouldn’t shift into without warning, then your control module will likely need to be replaced. Otherwise, it will be very dangerous to continue driving.

Unless you have a manual override option in your vehicle (example: paddle shifters), you will need to get your car towed to a repair shop and have the module replaced.

Keep in mind that other issues, like a faulty shift solenoid, can cause a similar symptom.

#3 – Problems Shifting Into Higher Gears

As you gradually increase speed, your transmission should correctly shift into higher gears at the correct time. When this doesn’t happen, you’ll notice that your engine’s RPM simply keeps increasing instead of slightly dropping an upshift occurs.

Because each gear has a maximum speed it can go to, you may never reach your intended speed if the faulty TCM refuses to shift to a higher gear.

#4 – Problems Downshifting

This is similar as above but in reverse order. Maybe you need to pass a slow moving vehicle and your transmission fails to shift into a lower gear when you mash the gas pedal to help you accelerate.

Or you’re cruising down the freeway and take an off-ramp, your transmission should gradually be going into lower gears as your speed decreases. At a stop, an automatic transmission should be in first gear so it’s ready to go when you need to start moving again.

If your transmission control module is bad, you may still be in a high gear as you’re coming to a stop. When it’s time to start moving again, you’re in a too high a gear for proper acceleration which may even prevent you from moving at all.

#5 – Stuck in the Same Gear

This usually presents itself as being either stuck in neutral or first gear. The transmission simply will not shift and you either won’t be going anywhere (stuck in neutral) or you’ll be limited by the maximum speed of first gear.

#6 – Delayed Shifting

The speed of your vehicle relies on cycling to various gears at the appropriate timing. If you have a bad transmission control module, then it will cycle to next gear too slowly.

This will impact your acceleration, causing you to lose speed rather than picking up speed. When you approach a hill, the shifting performance will be even worse.

Worst case is when you’re attempting to pass a slower vehicle on a 2-lane road and your vehicle refuses to downshift immediately so you have more torque available to make a quick pass.

#7 – Poor Fuel Economy

Problems with your transmission system usually result in bad fuel economy. An automatic transmission is designed for optimal gear selection and timing to provide you with the best possible fuel economy.

If you have a transmission control module that’s bad and affects this timing, then your engine will end up working harder than usual. This means it will consume more fuel than usual, causing you to spend more money at the gas pump.

#8 – Limp Mode

One very obvious and frustrating symptom of a bad TCM is your car getting stuck in “Limp Mode.” This self-preservation feature in most vehicles is initiated when the TCM detects a severe issue with the transmission, give false data, or simply stops communicating with other control modules within the vehicle.

Limp mode in essence prevents the car from shifting to higher gears, limiting your vehicle’s power and speed.

TCM Replacement Cost

Best places to order parts? See: 19 Best Online Auto Parts Stores

The replacement cost of a transmission control module will be anywhere from $500 to $900. You can expect the parts costs to be around $450 to $700 while the labor costs will be around $50 to $200.

Of course, you can order a new TCM online and ask a mechanic what their hourly labor rates are. However, most of the cost for this replacement job goes toward the parts cost itself and it can vary greatly by car manufacturer. You can expect taxes and fees to be added to the total cost as well.

TCM Repair Cost

Another option is to repair your TCM. The transmission control module repair cost will in almost all cases be lower than replacing with a new module. On average, expect to pay around $300 to repair your transmission control module.

While most TCM repair businesses will offer a warranty of some sort, it does pose a bit more risk if the repair is not done right.

Also, you may need to ship your bad TCM to a non-local location so if you can’t afford not having your vehicle for a few days, replacement would be the fastest method.

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Mark Stevens

Owner at Car Treatments

Mark is a veteran auto technician who has honed his skills with over 19 years of service working at various independent shops. While Fords and Mazdas are his specialty, he enjoys working on practically any vehicle (except a Nissan 300ZX).

He’s known for his attention to detail and problem-solving skills and enjoys coming up with out-of-the box solutions when the situation requires it.

When he's not troubleshooting customers' cars, you'll most likely find him working on his current project ('96 Bronco) or cooking Italian food for his family.

8 Symptoms of a Bad Transmission Control Module (and Replacement Cost)