Transmission problems often cause driver frustration, but before you schedule a repair, make sure it’s the transmission and not just a bad transmission control module.
This guide will explain what a transmission control module is, the signs of a bad control module, and how much it is going to cost you to replace ( or might repair ) the TCM.
What is a Transmission Control Module?
You already know the job of your transmission is to direct the power your engine produces to the wheels. In simple terms, it makes your car move; without it, your car is just a brick or paperweight.
The transmission needs some backup to consistently do its job properly and efficiently, hence, each transmission is equipped with a transmission control module. TCM is the brain of a transmission.
The Transmission Control Module is an electronic component that receives data from the transmission, interprets signals from other components (throttle position sensor, turbine speed sensor, fluid temperature sensor, etc.), and manages gear shifts and speed changes.
Essentially, the TCM controls every aspect of how your vehicle shifts and changes speeds at the right times. Additionally, the TCM may signal OBD-II codes to tell you that something is not working properly with the transmission.
The check engine light is a common error that may display.
Manual vs. Automatic
As you can imagine, this part operates a little differently in manual transmissions than in automatic transmissions in as it serves the function of timing gear shifts. In fact, this part is redundant in manual transmissions.
In a manual, the driver is supposed to manually shift the gears of their vehicle as required, thus completely negating the need for the detecting capabilities in the control module. So, if you’re a fortunate driver of a manual, this is most likely a part you won’t have to deal with.
Where is the TCM Located?
If you don’t know the location of the transmission control module, you’re not alone. The location will be different from vehicle to vehicle.
You’ll usually find the TCM under the cover on the backside of the case, just below the engine control module location. Sometimes it will be located under the center console, under the dash by the firewall, under the hood by the battery, inner fender panel, or in the glovebox.
An example of where a TCM is located would be in a Chevy 4L60E transmission; the TCM would be located in the rear of the transmission case (just below the cover). In contrast, the same would not apply to an Allison transmission like the T87, which has its TCM located at the battery side of the fan shroud in the engine compartment.
Don’t start taking things apart by guessing where it is. The best resource for determining the location of the TCM is a vehicle-specific service manual, but Googling the vehicle’s year, make, and model will also point you in the right direction.
Signs You Have a Bad Transmission Control Module
If you ever have an issue with your transmission control module, you need to get it looked at right away. A bad TCM will cause problems for your car and may compromise your ability to drive it.
The good news is that there are a few basic warning signs you will likely experience before it gets to that point.
#1. Check Engine or Transmission Light.
If the Check Engine warning light (on the dashboard) lights up, it could signify several things. But if you are experiencing any shifting issues, along with the Check Engine light being on, then it is likely a bad transmission control module or another part of the transmission system.
In any case, make sure to get your vehicle checked out by a mechanic as soon as possible, or use your scan tool to check for any Diagnostic Trouble Codes (DTC). If P0700 shows up, you will probably have a bad transmission control module!
#2. Unpredictable Shifting.
Because an automatic transmission system relies on the transmission control module to do the shifting, any malfunction of the transmission control module will cause incorrect or unexpected shifts. This can be anything from an annoyance to causing a serious accident.
A lot of drivers have reported that the car felt like it was pulling back at times when they’re driving. Or it can feel like the car jerks every time they accelerate.
For example, if your car goes to neutral or any other gear out of nowhere that it wouldn’t normally shift into, then you likely have a bad control module that should be replaced. Otherwise, it could be very unsafe and dangerous to continue driving.
If your vehicle does not have a manual override option (such as paddle shifters), you will have to have your vehicle towed to a shop and have a new module installed.
Note that more issues, such as a broken shift solenoid, can cause the same symptom.
#3. Trouble Shifting Into Higher Gears.
Your transmission should shift into the higher gears as you gradually increase speed, and you should notice that your engine’s RPM returns to a normal increase and drop during an upshift, but if your TCM is broken, you will notice simply an increase in RPM.
If your TCM won’t shift to a higher gear, you might never reach your desired speed, because each gear has a max speed it can go to.
#4. Trouble Downshifting.
This condition is similar to the above but reversed. If you need to pass a slower vehicle and your transmission fails to shift into a lower gear, you give the gas pedal a big stomping to help you accelerate.
Or simply cruising on the freeway, an off-ramp to the street should have you gradually shifting down as your speed decreases, and a complete stop should have your automatic transmission shifting down to 1st gear so it is ready to accelerate when you are ready to start moving again.
If your TCM is bad, you might be in a higher gear when coming to a stop. When you’re ready to go, you are stuck in too high a gear to accelerate, which can mean you won’t be able to move at all.
#5. Stuck in the same gear.
This normally shows itself as being stuck in either neutral or first gear. The transmission will not shift, and you will either not go anywhere (stuck in neutral) or will only go as fast as you can in first gear.
#6. Delayed Shifting.
Your speed relies on shifting to different gears at the right time. If you have a bad TCM, it will delay the shifting into the next gear.
This will impact your acceleration and, in turn, cause you to lose speed instead of picking up speed. But it will get worse if you approach a hill.
The worst situation occurs when you are approaching a passing situation on a two-lane road, and your TCM does not allow the vehicle to downshift immediately so you can have more torque and make a quick pass.
#7. Poor Fuel Economy.
Any problems in your transmission system will mean poor fuel economy. Your automatic transmission is designed to give you the optimal gear selection and timing for the best fuel economy.
If you have a TCM that is bad and is affecting this timing, it means your engine is working harder than it needs to make up for the TCM. If your engine is working harder, it is causing it to spend more fuel, which means more money spent at the gas station.
#8. Limp Mode.
One very obvious and frustrating symptom of a bad TCM is that your vehicle is stuck in limp mode. This self-preservation mode of most vehicles is initiated when the TCM senses a major/transient fault in the transmission, it returns incorrect data, or stops communicating with the other control modules in the vehicle.
Limp mode keeps your car from shifting into higher gears and limits your power and speed.
How to Diagnose TCM Problems
If you start to experience any of the symptoms listed above, the first step is to use an OBD-II scanner to obtain diagnostic trouble codes, or DTCs. This will help you see what possible TCM problems you are experiencing.
Next, it is commonly wise to check wiring and connections, as it is common that the problems are not related to the TCM. Look for wires or connectors that are loose, damaged, or corroded, and repair anything you find that is questionable.
More often than not, diagnosing a TCM just comes down to eliminating other possible issues. After you check the TCM codes and the wiring connections, observing the overall performance of the vehicle can help determine if you have a TCM concern.
When you have multiple concerns from the list above, especially if there are no related DTCs, this can help narrow it down as a possible TCM issue.
If you would like a more thorough diagnosis, take your vehicle to a professional mechanic. They have the equipment, such as testing tools and experience, to carry out advanced testing, including live data and TCM testing.
If the TCM is determined to be bad, it will need to be replaced. The replacement is relatively straightforward. Locate the TCM and remove it, replace the TCM, and then program the new TCM to the vehicle.
Causes of TCM Failures
It’s important to understand what can contribute to TCM problems so you can perform preventive maintenance if possible. The most likely causes of failure include:
- Electrical problems. Electrical issues, such as corrosion, may impair the communication between the TCM and other electronic systems, causing the TCM to inadvertently malfunction.
- Overheating. Too much heat can cause the TCM circuitry to fail or become damaged. A poor cooling system can cause the TCM to eventually fail due to the excessive heat in the engine compartment of your vehicle.
- Water damage. Water inside the TCM housing usually results in short circuits and corrosion. Water usually gets inside the housing as a leak or from flooding.
- Normal wear and tear. As your vehicle ages, the TCM and its components may fail simply because of natural wear and tear.
TCM Replacement Cost
The replacement cost of a transmission control module will be anywhere from $500 to $900 total. Of that cost, the parts cost is probably around $450 to $700, and the labor part of the cost will be a range from $50 to $200.
Sure, you can order a new TCM on the internet and then ask a mechanic about their hourly labor rates, but isn’t that not the point? Most of the cost for this type of work is incurred in the cost of the parts.
The parts and labor charge can vary greatly with most car manufacturers and their locations, taxes, fees, and the geography and locality are considerations.
TCM Repair Cost
The other option is to simply repair your TCM. In just about every case, the repair cost will be far less expensive than the replacement of the new module. In general, expect to pay around $300 to repair your transmission control module.
Most TCM repair shops will offer a warranty of some kind; however, if the repair is not correctly completed, there is a small amount of risk.
Also, understand that you might need to ship your broken TCM if it is to a non-local repair shop, so if you cannot afford not to have your vehicle for a few days, then obviously, replacement would be the quickest alternative.