Circuit/System Description
WARNING: This page is about a different car, the 2022 Cadillac CT5. However, it is still accessible from the selected car via links, so may be relevant.
The S78 Turn Signal Switch is used to control the fog lights when equipped (T79). The K9 Body Control Module (BCM) provides 12 V signal circuit 187 to the turn signal switch for fog light controls. When the rear fog lamp button is pressed, the signal circuit becomes grounded, the BCM interprets the signal and responds by sending a serial data message to the K219 Lighting Control Module. The lighting control module responds by applying pulse width modulated (PWM) voltage to the appropriate fog light(s) through control circuit 122.