Circuit Description
WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
The body control module (BCM) monitors and supplies battery voltage to the headlamp relay utilizing the headlamp relay coil supply voltage circuit. When the ambient light sensor is in the DARK state and the headlamp and panel dimmer switch is in the AUTO mode, the BCM supplies battery voltage to the headlamp relay to power up the headlamps. The BCM monitors the headlamp relay coil supply voltage circuit to determine when the daytime running lights (DRL) should be ON or OFF. When the BCM detects battery voltage on the headlamp relay coil supply voltage circuit it will turn OFF the DRL. When the BCM detects the headlamp relay coil supply voltage circuit is low it will turn ON the DRL when the DRL conditions are present.