Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Transmission Control Module (TCM)
The transmission control module (TCM) is mounted in the engine compartment and connects directly to the wiring harness. Two 64-way connectors are used to make connections between the vehicle wiring, the transmission, and the TCM. The TCM is an electronic control module that receives input and provides output to control the operation of the VT25-E automatic transmission.
The TCM receives the following inputs from the engine control module (ECM):
- Engine speed and torque values
- Engine intake air temperature (IAT), and throttle position (TP) information
- Engine coolant temperature (ECT)
- Air-conditioning (A/C) status
- Cruise control status
The engine control module (ECM) provides this data to the TCM through the GMLAN high-speed controller area network (HSCAN). A two-wire circuit is used to communicate data between the ECM and TCM, CAN H and CAN L. Other TCM inputs are the following:
- Battery and ignition voltage
- Brake switch status
- Transmission fluid temperature (TFT)
- Transmission input shaft speed (ISS) sensor
- Transmission output shaft speed (OSS) sensor
The TCM provides the following outputs in order to control the automatic transmission:
- Ratio control motor to control the speed ratio of the transmission
- Line pressure control solenoid (PCS) valve regulates the transmission line pressure
- The TCC enable solenoid valve is used to hydraulically select whether the TCC pressure control solenoid valve will control the fluid pressure used to apply the forward and reverse clutches or the fluid pressure used to apply the torque converter clutch
- TCC pressure control solenoid valve is used to control the fluid pressure used to apply and release the forward and reverse clutches, and the torque converter clutch
Other TCM outputs provided to the ECM are the following:
- MIL illumination request
- Vehicle speed
- Transmission input speed
- Transmission fluid temperature
- TCC status
- Torque reduction requests
- Park/Neutral Position Assembly status
- Transmission service status
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.