GM Service Manual Online
For 1990-2009 cars only

The automatic transmission shift lock control system prevents the automatic transmission from being shifted out of the PARK position unless the brake pedal is depressed and the ignition switch is in the RUN position. When the brake pedal is depressed, the automatic transmission shift lock control solenoid releases a lock paw. The released lock paw enables the movement of the shift linkage and the operation of the shift lever.

When service operations are required, overriding the automatic transmission shift lock control is possible. In order to override the automatic transmission shift lock control, turn the ignition switch to the OFF (not LOCK) position, and remove the battery voltage from the solenoid.