Circuit/System Description
WARNING: This page is about a different car, the 2007 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The ambient light sensor is a light sensitive transistor that varies the voltage signal to the body control module (BCM) based on outside lighting conditions. When the ambient light sensor indicates daytime conditions with engine running, the body control module (BCM), will then apply battery voltage through the front turn signal lamp supply voltage circuits illuminating the left and right front turn lamps. Any function or condition that turns the headlamps ON will cancel DRL operation.