Brake Light Switch: Description and Operation

Typical Brake Pedal Position Switch:

PURPOSE
The Brake ON/OFF (BOO) switch signals the Powertrain Control Module (PCM) with Battery Positive Voltage (B+) when the brakes are APPLIED. The PCM adjusts engine idle speed with the brakes APPLIED and A/C turned ON, or will turn OFF the A/C for a calibrated time.

On vehicles with locking torque converters, the BOO signal directs the PCM to disengage the torque converter.

A high voltage is present at the PCM due to a pull-up resistor if a stop lamp bulb is OPEN (burned out). This provides fail safe operation in the event the circuit to the BOO switch has failed.