Where is the transmission control module located in a vehicle?

This blog will answer the question where is the transmission control module located in a vehicle? Which types of transmission use a transmission control module? What are the common problems in a transmission control module?

Where is the transmission control module located in a vehicle?

The transmission control module can be located in a vehicle in the following places:

• Front passenger footwell
• Under the cowl top
• Inside the transmission

The transmission control module or TCM is responsible for the automatic gear shifting in a vehicle. If the transmission is fully automatic then the TCM can have additional functions like remote start and automatic engine cutoff.

The details of each installation location are given below:

Front passenger footwell

This location is very common for cars with automatic transmission. The transmission control unit is located either under the front passenger seat or in the footwell. In order to remove the transmission control unit, the floor carpet or the seat must be removed.

This location is the easiest place to access the TCM; the control unit is also safe from harsh elements like dirt and moisture. The transmission control unit is connected to the CAN data bus so that it can communicate with the engine control module.

The power supply for the transmission control unit is taken from the vehicle interior or the cockpit fuse box and the ground signal is given by connecting the ground electrical line to the vehicle ground point. The output couplers contain more electrical lines that are used to actuate the shift solenoids.

Under the cowl top

The cowl top is located between the engine hood and the windshield of the vehicle. There is a control unit box on the left or the right side under the cowl top. The control module box houses the Transmission control unit and the engine control unit along with the engine compartment fuse box.

All the electrical lines to the shift solenoids go under the cowl top; this location is more common in vehicles manufactured before 2005. The control unit box has a sealing gasket so that moisture and dirt can stay out of the box.

Despite the sealing gasket on the top cover, water can enter the control unit box if the cowl top drain is choked due to foliage. If water goes into the control unit and fuse box then the electrical contact points get corroded and the control unit becomes inoperative.

Therefore, car manufacturers are not using this location in the latest cars despite the ease of access to the removal and installation of the transmission control unit.

Inside the transmission control unit

This location is common in the latest automatic transmission; the control unit is directly located on the electrohydraulic valve body inside the transmission. There is an electrical coupler protruding out of the aluminum housing of the automatic transmission.

The electrical coupler has O rings so that automatic transmission fluid is not leaking between the housing and the coupler. The transmission control unit is fully submerged in the automatic transmission fluid, which offers better cooling for the control unit.

There is an electrical plate connecting the transmission control module to the shifting solenoid valves and the turbine speed sensors. The electrical coupler only contains 3 to 4 electrical lines, the power supply is given directly from the vehicle pre-fuse box and the CAN data bus lines are connected to the potential distributor.

The procedure to remove and install the transmission control unit located in the automatic transmission is not easy. All the automatic transmission fluid is drained and the oil pan is removed. The electrohydraulic valve body is also removed from the transmission housing. 

The transmission control unit located in the transmission cannot be replaced separately. It comes with an electrohydraulic valve body. Hence, the repair costs are very high compared to the transmission control modules located in the other two locations.

Which types of transmission use a transmission control module?

There are three types of transmission currently used in passenger cars, they are Manual transmission, automatic transmission, and semi-automatic transmission.

The transmission control module is not needed in the manual transmission because the gear shifting is done by the driver. There is no need to monitor the input data like engine speed, throttle position, and engine load. The driver is responsible for selecting the correct gear according to the vehicle’s speed.

Automatic and semi-automatic transmissions are not like manual transmissions. The gear shift is done automatically after analyzing the input data from the engine control unit. There are only two pedals; the clutch is engaged and dis-engaged automatically by the transmission control unit.

The timing required for filling the shift elements is stored in the transmission control unit. It is constantly adjusting the filling time in the range of -20 to 20 in order to improve the shift quality. Semi-automatic transmission is less complicated compared to a fully automatic transmission.

There are intention sensors in the gearshift lever and the transmission control unit activates the clutch solenoid when the driver is about to change the gear. Fully automatic transmissions are the most expensive transmission as there are planetary gears that are locked with the help of multi-plate clutches and brakes.

The semi-automatic transmission provides the convenience of automatic transmission with the construction of a manual transmission. Therefore, it is found in entry-level passenger cars.

What are the common problems in the transmission control module?

The common problems in the transmission control module are explained below:

• No communication with the engine control unit

The transmission control unit is connected with the engine control unit via the CAN data bus. The engine control unit acts as a gateway between the transmission control unit and the other control units installed in the vehicle.

When the transmission control unit fails to communicate with the engine control unit, the vehicle does not start. Engine warning light and a warning message ‘without changing gear consult workshop’ is displayed in the instrument cluster.

If the CAN signal and power supply to the transmission control module are within a specified range then the transmission control unit has an internal fault and it must be replaced.

• Abnormal jerk in the gear changes or poor shift quality

The transmission control module controls the opening and closing of the solenoid valves. The solenoid valves are located on the electrohydraulic valve body; the gear changes when these valves are actuated by the TCM.

TCM records the filling time and the end time of each shift element but if the adaptation of the gearbox is not completed successfully then the shift quality becomes poor. Drivers can experience jerks during the upshift and downshift of the gears.

Conclusion

This blog answered the question regarding the location of the transmission control module in the vehicle. All the latest transmissions have the control module located in the transmission. The diagnostic scanners are also capable of reading EEPROM data and rectifying any customer concerns without any disassembly of the automatic transmission.