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.
Module, Transmission Control (TCM): Operation
The TCM regulates the amount of hydraulic pressure used to engage the clutches and the Torque Converter Clutch (TCC), in addition to directing hydraulic pressure to engage or release any given clutch for any given required gear.
The TCM will actuate the valves via solenoids based on the position of the shifter, transmission fluid temperature, engine operating conditions, traction conditions and driver demands.
During a shift, the TCM will actuate the solenoids to match the gear ranges to the optimal torque range of the engine based on the position of the accelerator pedal, shifter and vehicle speed as determined by the PCM based on input from the Vehicle Speed Sensor (VSS) and ABS module.
Due to the complexity of the 948TE transmission control system, always refer to the transmission electrical/electronic diagnostics in this service information when attempting to diagnose transmission problems.
Some examples of direct inputs to the TCM are:
- Battery (B+) voltage
- Ignition Status
- Transmission Range Sensor (TRS)
- Cruise Control Switch
- Throttle Position
- Body Control Module (BCM)
- Shifter Position
- Pressure Switches
- Transmission Temperature Sensor
- Input Shaft Speed Sensor
- Output Shaft Speed Sensor
- Line Pressure Sensor
Some examples of indirect inputs to the TCM are:
- Torque Reduction Confirmation
- Engine Coolant Temperature
- Scan Tool Communication
Based on the information received from these various inputs, the TCM determines the appropriate shift schedule and shift points, depending on the present operating conditions and driver demand. This is possible through the control of various direct and indirect outputs.
Some examples of TCM direct outputs are:
- Solenoids
- Torque Reduction Request
Some examples of TCM indirect outputs are:
- Transmission Temperature
- PRNDL Position
In addition to monitoring inputs and controlling outputs, the TCM has other important responsibilities and functions:
- Storing and Selecting Appropriate Shift Schedules
- System Self-diagnostics
- Diagnostic Capabilities (with scan tool)
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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.