Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Circuit/System Description
The stop lamp switch signal circuit is a direct hardwire input to the transmission control module (TCM) from the body control module (BCM). The TCM module monitors the stop lamp signal circuit in order to detect when the brake pedal has been applied. When the brake pedal is pressed, the BCM sends a High Speed GM Lan message to the engine control module (ECM) indicating that the brake pedal has been applied. The BCM also sends a high voltage signal on the stop lamp signal circuit to the TCM module. The TCM module sends a universal asynchronous receiver transmitter low speed GM Lan message to the ECM indicating that a brake pedal application has occurred. The ECM will compare the GM LAN Low speed and GM LAN High Speed data message in order to confirm that both serial data messages match.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.