GM Service Manual Online
For 1990-2009 cars only

Automatic Transmission Shift Lock Control System

The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:

    • The automatic transmission shift lock control solenoid
    • The body control module (BCM)
    • The powertrain control module (PCM)

The BCM controls the voltage to the shift lock control solenoid through the shift lock control solenoid controlled voltage circuit. The following conditions must be met before the BCM will supply voltage to the shift lock control solenoid:

    • The ignition is in the ON position.
    • The PCM sends a class 2 message to the BCM indicating the transmission is in the PARK position.
    • The BCM determines the brake pedal is not applied according to the stop lamp switch.

When the ignition is in the ON position, the ignition 1 fuse supplies voltage to the automatic transmission shift lock control solenoid as the BCM supplies a ground to the control circuit, mechanically locking the shift lever in the PARK position as the solenoid energizes. When the brake pedal is applied, the BCM turns the ground of the control circuit OFF, de-energizing the shift lock control solenoid. The de-energized solenoid releases the mechanical lock allowing the driver to move the shift lever out of the PARK position. When the transmission is out of the PARK position, the shift lock control solenoid remains de-energized.