Automatic Transmission Shift Lock Control System with Allison Transmission

WARNING: This page is about a different car, the 2006 GMC Sierra and 2006 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The automatic transmission shift lock control is a safety device that prevents an inadvertent shift out of PARK when the ignition is ON. 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 automatic transmission shift lock control switch.
The body control module (BCM).
The engine control module (ECM), if equipped with a 6.6L engine.
The powertrain control module (PCM), if equipped with a 8.1L engine.

With the ignition in the ON position and the shift lever is in PARK, battery positive voltage is supplied from the BCM to the automatic transmission shift lock control solenoid through the normally-closed automatic transmission shift lock control switch. When the transmission control module (TCM) indicates the shift lever is in the PARK position, the ECM/PCM sends a class 2 serial data message to the BCM indicating the transmission is in the PARK position. This allows current to flow through the switch to the automatic transmission shift lock control solenoid. Since the automatic transmission shift lock control solenoid is permanently grounded, the shift lock control solenoid energizes, locking the shift linkage in the PARK position. When the driver presses the brake pedal the contacts in the automatic transmission shift lock control switch open, causing the automatic transmission shift lock control solenoid to de-energize, unlocking the shift lever from the PARK position.