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How Do You Know If Your Transmission Control Module Is Bad? – Here’s What You Need To Know

How Do You Know If Your Transmission Control Module Is Bad

As we know, any part in your car that has a “module” is sure to be a key component of your vehicle. The electronic control module is basically the computer of the vehicle, while the transmission control module is in control of the shifting, the changing of gears, and the proper working of the engine. If the engine is not working well, this can be a way to know if your transmission control module is bad. 

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What is the function of my transmission control module?


In a nutshell, the transmission control module is a collection of various electrical circuit boards and processors that are key to the electrical functioning of the car. It is in charge of activating the transmission solenoids, which then tell the hydraulic fluid within the system the final destination. This hydraulic fluid is also called transmission fluid, and is directed by the transmission control module to the proper clutch or the gearset.


When locating the transmission control module in your vehicle if you just want to know a little bit more about your car if you need to find it for a repair or a fix, it is generally found outside the transmission, near the wiring harness. If it is not located here, it can be inside the transmission right adjacent to the valve body. In order to figure out what gear the car has to use next, the TCM, or transmission control module, must be able to analyze the data it is getting from various sensors in the car. 

Signs of a Failing Transmission Control Module


Now that you know the function of the transmission control module, you need to know the signs of a damaged TCM to determine how to know if your transmission control module is bad. 

Check Engine Light 


Although your check engine light might appear on your dashboard for many reasons in your car, some serious and some minor, if the check engine light comes on it can direct you directly towards your transmission control module as the culprit. If you have any issues with shifting along with this light on your dash, the two together can indicate how you know if your transmission control module is bad. 

Poor Shifting 


If you have a light on your dashboard along with erratic shifting that is unpredictable and occurring at the wrong time, then this is a clear sign of an issue with the TCM. Since the automatic transmission relies on the TCM for smooth shifting to ensure optimal performance, a bad TCM causes dangerous shifting. 

Problems Upshifting or Downshifting


Although these sound like opposites, problems with the TCM directly indicate that you have problems either shifting up into high gears or downshifting smoothly. If you notice your transmission is not able to move into high gears while you are driving uphill or accelerating, this is how you know if your transmission control module is bad. In addition, if you need to slow down to make a turn or pass a slow moving car and you are not able to, this is a sign that your transmission control module is bad. 

Transmission control module sensors

Throttle position sensor


One of the sensors that sends signals to the transmission control module is the throttle position sensor. This sensor provides an important data source, coming directly from the throttle position in the vehicle. If the throttle sensor cannot send the proper location, then this is a way to figure out how you know if your transmission control module is bad. 


The signal from the throttle position sensor lets the module know the amount of load being placed on the engine at one time, or how much stress the engine is under. This information can help the transmission control module figure out if it needs to direct the car to upshift or downshift based on the position. The TCM can also analyze this data, and then compare this information to the vehicle sensor speed data, to figure out which shift is most appropriate. If your vehicle is having trouble shifting, this is a way how you know if your transmission control module is bad

Vehicle speed sensor 


The transmission control module utilizes the vehicle speed sensor to analyze how fast the car is traveling so it knows the proper time to change gears based on the performance. If this sensor is not working properly or is functioning incorrectly, the transmission control module can select the wrong gear at the wrong time, causing the transmission to slip. This is how you know if your transmission control module is bad. 

Wheel speed sensor


The wheel speed sensor data can help the transmission control module better know what the car is currently doing by the actions it is performing, so the TCM can properly adjust the shifting and gear changing accordingly. The wheel speed sensor also notifies the TPM of when to control and alter the function on the torque converter.

Turbine speed sensor


The turbine speed sensor, also known as the input speed sensor, is in charge of telling the transmission control module to determine the correct amount of slippage to put forth on the transmission clutches, bands, and torque converter clutch. Without this ability, the transmission control module will not apply the proper tension, showing how you know your transmission control module is bad

Transmission fluid temperature sensor


The transmission fluid temperature sensor is usually used by older transmission control units that have to monitor and analyze the current health and longevity of the transmission system as a whole. If the transmission overheats during use, then the transmission control unit might turn on a special mode in your car to protect you and the drivers – this special mode is called limp mode to protect the transmission from further damage. If this does not turn on, then this is how you know if your transmission control module is bad. 


Newer transmission control modules in more modern cars can use the sensor data to change the functioning and control of the torque converter lockup clutch or the line pressure accordingly. 

Brake light switch


Modern transmission control modules in newer cars are generally programmed with an engine braking code that will be able to sense when a downshift is needed to properly stop the car and brake at the right time. This engine brake algorithm needs data directly from the sensor, so if this is not working, this is how you know if your transmission control module is bad. 


The TCM might also use this specific information to function correctly in tandem with the shift lock solenoid, preventing the driver from choosing the right gear unless the brake pedal is currently being pressed by the driver. 

Cruise control module 


The transmission control module needs to receive an input of data from the cruise control system, which is the system that keeps your car driving at a steady speed, so it can properly modify the shifts to happen at the right time and reach the desired speed. If this cannot occur, then your car will not be able to maintain a consistent speed, meaning your transmission control module is bad. 

Traction control system 


Have you ever been driving in the rain and feel your car slipping? This is a scary event – and a sign your traction control system might not be working correctly. The traction control system is in charge of letting the transmission control module that it needs to modify the shifting behavior in certain treacherous conditions, like preventing lockup, and having 1st gear starts.


Once the correct gear ratio has been chosen by the transmission control module, the module then sends signals to the related components within the transmission. 


Shift solenoids – the shift solenoid in your transmission is a device that is used to direct the flow of pressurized hydraulic fluid. If the transmission control module sends an electric charge through the wire, then it will move the fluid, or block the fluid, to the correct passageway in the system. If the shift solenoids are not functioning, then the traction control system is not working, showing how you know if your transmission control module is bad. 


Pressure control solenoids – as we know now, an automatic transmission system in your car requires the right pressurized hydraulic fluid to work correctly. The transmission control module is the mechanism that is in charge of controlling that pressure. The pressure solenoid is the little part in this system that allows the TCM To regulate the pressure necessary to work and operate the clutches and bands.


This means that if one of the pressure control solenoid fails in your car, the transmission can slip or begin rough shifting. If this occurs, this is how you know if your transmission control module is bad.


Torque converter clutch solenoids – when the torque converter clutch is locked, the engine and the transmission are working at the same level and at the same speed. Once the torque converter clutch slips, this means that only half of the lockup is working. The transmission control module is able to control the converter and prevent this from happening by sending the right signal to the torque converter solenoid, so if the torque converter slips, this is a sign of how you know if your transmission control module is bad. 



The engine control unit is the brain of your vehicle. This component and the transmission control module go hand in hand, working together to make the vehicle operate properly and safely down the road. Some of the functions that the ECM is in charge of include turning on the check engine light during failure, storing error codes that can help diagnostic processes, and temporarily changing the RPM of the engine to spur the proper gear change. 


Since the transmission control module is electrical with circuit boards and other wiring, it is in charge of holding various shifting programs. These codes let the transmission control module analyze the data from the sensors against the preset parameters for your specific vehicle, then adjust the shift times based on the solenoids to fit the driving situation. If the ECU and the TCM are not working, this is how you know if your transmission control module is bad. 

Troubleshooting the TCM


In most cases with modern cars, problems with the transmission control module are directly caused by faulty circuits or a problem with one of the electrical resistors. When this occurs, if you bring your car to a professional mechanic or a worker at an auto body shop, they should be able to easily diagnose the issue and quickly repair the faulty part. In more advanced cases of damage, it might be even necessary to replace the entire control module if you can tell the transmission control module is bad.


When troubleshooting the TCM, it is recommended for car owners to visit an experienced mechanic or technician. Since the parts are very expensive and any slip-ups can cause further issues, improper handling of this repair or replacement can lead to personal injury or an expensive cost for you. 


However, if you feel confident, some problems with the transmission control module can actually be resolved by cleaning and draining the entire module instead of replacing it. You will need your specific car’s manual and be able to locate the TCM. This will require some advanced knowledge, so if you have not worked on cars before, this should not be your first attempt.


Step 1 – turn off your car’s ignition. If you feel comfortable, completely disconnect the battery before you begin working on your vehicle.

Step 2 – shift the Emergency Brake into position to prevent your car from moving at all during the process.

Step 3 – use the auto manual for your car to figure out the precise spot where your transmission control module is located. In most modern cars, it is located in the distributor cap.

Step 4 – open the hood of the car and prop the door open.

Step 5 – after locating the distributor cap, remove the cap by loosening the screws with a screwdriver. 

Step 6 – remove the transmission control module once you have determined that you know if your transmission control module is bad. You can do this by separating the two connectors in the TCM and then removing the supporting nuts and bolts. 

Step 7 – drain the transmission control unit’s fluid and use a cleaner to ensure there is no debris in the connector that can cause loose connections. 

Step 8 – replace the bolts, the nuts, and the wire connectors to reinstall the connector back to the original spot in the car. 


If these steps do not solve the transmission control module issues in your car, then you should bring your car to a mechanic to determine how you know if your transmission control module is bad. 

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