Circuit/System Description

WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.

The stop lamp switch signal circuit is a direct input to the throttle actuator control (TAC) module from the body control module (BCM). The TAC module monitors the stop lamp switch signal circuit in order to detect when the brake pedal has been applied. When the brake pedal is pressed, the BCM sends a serial data message to the powertrain control module (PCM) indicating that the brake pedal has been applied. The BCM also sends a high voltage signal on the stop lamp switch signal circuit to the TAC module and the PCM. The TAC module sends a serial data message to the PCM indicating that a brake pedal application has occurred. The PCM compares the serial data messages from the BCM and TAC modules in order to confirm that both serial data messages match.

This diagnostic test functions on the assumption that a sudden decrease in vehicle speed is caused by a brake pedal application. When the PCM detects that there is a 4.2 km/h (2.6 mph) or greater decrease in vehicle speed within 0.25 seconds and a transition of the torque converter clutch (TCC) brake/cruise release switch without a transition of the stop lamp switch, the PCM sets DTC P1574.