GM Service Manual Online
For 1990-2009 cars only

Transmission Control Module (TCM)


Object Number: 2018891  Size: SH
  1. Rotating magnet
  2. Stator magnet
  3. IC circuit, Hall sensor
  4. Coil
  5. Magnetic field

The TCM is mounted directly on top of the gearbox. The selector lever is integrated in the control module. The TCM is mounted so that the selector lever shaft goes through the control module. The selector lever position is calibrated with a scan tool, there is no mechanical adjustment.

An exterior 16-pin connector connects to the car's electrical system. Underneath the control module is another connector (22-pin) that connects directly to the gearbox. TCM makes contact with all the gearbox solenoids and sensors here.

Note: Magnetic fields from e.g. magnets and from high-current cables, such as starter cables and cables to auxiliary equipment, can interfere with the gear position sensor. A rule of thumb is max 1A per mm in distance from the control module. A starter cable carrying a current of 200A must therefore be kept at least 20 cm away from the control module.

The TCM has a microprocessor with clock, RAM memory and a programmable ROM. An internal bus connects the processor and memory with the I/O unit. The I/O unit reads values from the A/D converter for analogue inputs, digital inputs and bus, as well as activates the transistors' output stages.

Adaptive values are saved in a non-volatile memory (ROM). When changing gearbox, these values must be zeroed using scan tool. After changing TCM they will be zeroed automatically after adding the control module using the scan tool.

Solenoid Valve S1

The normally open solenoid valve is mounted in the gearbox valve housing and is used by TCM when activating C2. The pressure to SLC2 is then raised from system pressure to D-pressure. The valve is also used to apply pressure to brake B2 for engine braking in 1st gear.


Object Number: 2018887  Size: LF
  1. On
  2. Off

The control module activates the valve by applying B+ to it. The valve is powered from control module pin 5 in the connector in direct contact with the gearbox on the bottom of the control module. The valve is grounded in the valve housing.

Solenoid Valve S2

The normally closed solenoid valve is mounted in the gearbox valve housing and is used together with S1 to apply pressure to brake B2 for engine braking in 1st gear.


Object Number: 2018887  Size: LF
  1. On
  2. Off

The control module activates the valve by applying B+ to it. The valve is powered from control module pin 2 in the connector in direct contact with the gearbox on the bottom of the control module. The valve is grounded in the valve housing.

Solenoid Valve SLB1

The PWM controlled solenoid valve is mounted in the gearbox valve housing and controls the pressure in brake B1. The valve is active in gear positions P, R, N, D (gears 1, 3, 4 and 5).


Object Number: 2018882  Size: LF
  1. In
  2. Out
  3. Ex

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 21 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 16. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases

Solenoid Valve SLC1

The PWM controlled solenoid valve is mounted in the gearbox valve housing and controls the pressure in clutch C1. The valve is active in gear positions P, R, N, D (gears 5 and 6).


Object Number: 2018882  Size: LF
  1. In
  2. Out
  3. Ex

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 11 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 10. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLC2

The PWM controlled solenoid valve is mounted in the gearbox valve housing and controls the pressure in clutch C2. The valve is active in gear positions P, R, N, D (gears 1, 2 and 3).


Object Number: 2018882  Size: LF
  1. In
  2. Out
  3. Ex

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 17 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 18. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLC3

The PWM controlled solenoid valve is mounted in the gearbox valve housing and controls the pressure in clutch C3. The valve is active in gear positions P, N, D (gears 1, 2, 4 and 6).


Object Number: 2018882  Size: LF
  1. In
  2. Out
  3. Ex

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 14 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 22. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLT

The PWM controlled solenoid valve is mounted in the gearbox valve housing and controls the gearbox system pressure. The value is controlled so that it is not higher than what is currently needed for any friction element. Brake B2 does not have its own pressure control valve and is therefore supplied by SLT.


Object Number: 2018884  Size: LF
  1. In
  2. Out
  3. Ex

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 3 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 1. The current varies between 0-1000mA. The solenoid is normally open and the hydraulic pressure drops as the current increases.

Solenoid Valve SLU

The solenoid valve is mounted in the gearbox valve housing.

The valve controls the pressure used for the torque converter clutch. The pressure is modulated so that full, partial or no engagement is achieved.


Object Number: 2018884  Size: LF

The control module activates the valve by pulsing it with B+ (300 Hz PWM). The valve is powered from control module pin 9 in the connector that makes contact directly with the gearbox on the bottom of the control module. The valve is grounded from pin 4. The current varies between 0-1000mA. The solenoid is normally closed and the hydraulic pressure rises as the current increases.

Input Shaft Speed Sensor

The sensor is mounted inside the valve housing in the front part of the gearbox and measures the speed of the gearbox input shaft. This value is used by TCM to calculate the actual gearchange time and to regulate lock up. The Hall sensor measures a toothed wheel with 36 teeth.


Object Number: 2018890  Size: LF
  1. Output shaft gear with output speed sensor
  2. C2 clutch drum with input speed sensor

X = High voltage level

Y = Low voltage level

The sensor is powered from TCM pin 12 and is grounded through pin 13.

Output Shaft Speed Sensor

The sensor is mounted inside the valve housing in the rear part of the gearbox and measures the gearbox output shaft speed. This value is used by TCM mainly to determine the gearchange points. The hall sensor measures a toothed wheel with 56 teeth


Object Number: 2018890  Size: LF
  1. Output shaft gear with output speed sensor
  2. C2 clutch drum with input speed sensor

X = High voltage level

Y = Low voltage level

The sensor is powered from TCM pin 19 and is grounded through pin 20.

Gearbox Oil Temperature Sensor

The temperature sensor is mounted in the gearbox valve housing and informs the control module about the current temperature. The value is used to correct the PWM ratio to the solenoids so that the gear changes are not affected by the viscosity of the oil. It is also used to activate the special gearchange program in case of overheating.


Object Number: 2018899  Size: SH

The sensor comprises an NTC resistor built into a plastic casing. The NTC resistor characteristic means that the resistance drops as the temperature rises. The NTC resistor is supplied with 5V through a pull up from control module pin 8 and is grounded through pin 7.