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.
What is a Transmission Control Module?
You already know your transmission is responsible for funneling the power your engine creates to the wheels. In laymen’s terms, your transmission is what makes your car move, and without it your vehicle would be a paperweight.
But even the transmission needs support to make sure it is doing its job properly and efficiently. That’s where the transmission control module comes into the picture! If the transmission had a brain, it would be the TCM.
The TCM is an electronic mechanism that collects data from the transmission, interprets signals sent from other parts of the vehicle (i.e. throttle position sensor, turbine speed sensor, fluid temperature sensor, etc.), and helps to regulate the gears shifting.
It helps regulate how and when your vehicle shifts gears and changes speeds. In addition to supporting the transmission, the TCM may send OBD-II codes that alert you to any issues going on with the transmission.
One common error to display is the check engine light.
Manual vs. Automatic
Needless to say, this part functions just a bit differently in manual transmissions than with automatic transmissions as it’s responsible for timing gear changes. In fact, this part is entirely obsolete in manual transmissions.
In a manual, drivers are expected to switch their vehicle’s gears as needed by hand, making the detecting powers of the control module completely unnecessary. Therefore, if you’re a proud driver of a manual, this is probably a part you don’t need to worry about.
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.
#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.
How to Diagnose TCM Problems
When you encounter any of the symptoms mentioned above, the first step is to use an OBD-II scanner to retrieve diagnostic trouble codes, or DTCs. These codes can provide valuable insights into the specific TCM-related issues your vehicle is having.
From there, it’s often a good idea to check wiring and connections because many problems aren’t related to the TCM itself. Look for loose, damaged, or corroded wires or connectors, and fix anything that looks less than ideal.
Often, diagnosing a TCM comes down to eliminating other potential problems. After checking the codes and inspecting connections, pay close attention to the vehicle’s performance. If you have multiple symptoms listed above, especially with no related DTCs, it can help narrow it down to a TCM issue.
For a more comprehensive diagnosis, consult a professional mechanic n. They have the tools and expertise to perform advanced testing, including live data analysis and TCM-specific diagnostics.
If your TCM is indeed found to be faulty, it may need to be replaced. The replacement process typically involves locating the TCM, removing and replacing it, and then programming the new TCM to work with your vehicle.
Causes of TCM Failures
Understanding what can lead to TCM problems is essential for preventive maintenance. The most likely reasons for failure include:
- Electrical issues. Electrical problems, such as damaged wiring, loose connections, or short circuits, can disrupt the communication between the TCM and sensors, leading to malfunctions.
- Overheating. Excessive heat, often caused by a poorly functioning cooling system, can overheat the TCM and other components, causing damage over time.
- Water damage. Water that gets into the TCM housing, often due to leaks or flooding, can lead to short circuits and corrosion.
- Age or wear. As your vehicle ages, the TCM and its components may wear out naturally.
TCM Replacement Cost
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.