The transmission control module (TCM) is the computer that runs an electronically controlled automatic transmission. It reads vehicle speed, engine speed, throttle position and fluid temperature, then commands the shift solenoids and line pressure to choose gears and shift smoothly. A failing TCM, or its wiring, causes erratic or harsh shifting, a transmission stuck in one gear, and warning lights.
Where the TCM is
Depending on the vehicle, transmission control is handled by a separate module, built into the engine computer (often called the PCM), or mounted inside or on the transmission itself. Your service information shows which layout your vehicle uses.
Bad TCM symptoms
- Erratic, late or harsh shifts
- Stuck in one gear or in limp mode
- Shift problems that come and go with temperature or bumps
- Check engine or transmission light, including loss-of-communication codes such as U0101
- Poor fuel economy if the converter clutch or overdrive is not engaging
TCM or solenoid?
Many of the same symptoms come from solenoids, low fluid or wiring, which are more common failures than the module itself. Check fluid level and condition, wiring and connectors, and the specific codes first; see transmission solenoid symptoms and how to check transmission fluid.
What damages a TCM
- Water getting into the module or its connectors
- Voltage spikes, for example from incorrect jump-starting or welding on the vehicle without disconnecting the battery
- Heat, especially for modules mounted inside the transmission when the fluid overheats; see transmission fluid temperature
- Corroded or loose connectors and damaged wiring
Low battery voltage can also make modules misbehave, so test the battery and charging system before condemning a TCM.
Do not overlook the range sensor
The transmission range sensor tells the TCM which gear the shifter is in. When it fails, the engine may not start in park, the gear display can be wrong, or the transmission may engage the wrong range, which can look like a module fault.
Replacing or reprogramming a TCM
A replacement TCM usually has to be programmed with the correct software for the vehicle and transmission, and the transmission often needs a relearn so the computer can adapt its shifts. The same applies after replacing a transmission on many modern vehicles, even when the TCM is reused.
When the transmission is the problem
If the codes point to internal failure, compare a rebuild with a replacement unit; see transmission rebuild vs replacement and find your unit by code on our transmissions page.
Related: how an automatic transmission works · transmission slipping.
More guides: transmission range sensor · car jerks when shifting.
Trouble codes: P0700 Transmission Control System · U0101 Lost Communication With TCM · P0715 Input Speed Sensor Circuit · P0720 Output Speed Sensor Circuit · P0750 Shift Solenoid A · P0882 TCM Power Input Low · P0705 Transmission Range Sensor Circuit · P0600 Serial Communication Link · P0716 Input/Turbine Speed Sensor Range · P0721 Output Speed Sensor Range · P0702 Transmission Control Electrical · P0613 TCM Processor · U0402 Invalid Data From TCM · P0884 TCM Power Input Intermittent · P0602 Control Module Programming Error · all OBD2 codes.
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