Daytime Running Lamp: Description and Operation

The ambient light sensor is a light-sensitive transistor that varies its voltage signal to the Body Control Module (BCM) in response to changes in the outside (ambient) light level. When the BCM receives this signal, the BCM turns ON either the DRL or the headlamp relay for auto headlamp operation. Any function or condition that turns ON the headlamps will cancel the DRL operation.

Thirty seconds after you start the ignition with the headlamp switch set in the OFF position, the DRL will perform one of two actions:
^ Illuminate the high beam headlights at a reduced intensity in daylight conditions
^ Illuminate the low beam headlights in low light conditions
The DRL/EXT lamps fuse in the top of the engine fuse block supplies battery positive voltage to both the DRL switch contacts and to the DRL relay coil.

When the BCM energizes the DRL relay coil, current flows through the right high beam lamp, to the left hand high beam lamp, to the energized DRL relay switch contacts, to ground G101. The high beam headlamps are now in series, and split the voltage so that the headlamps illuminate at half intensity. The DRL operates when the ignition switch is in the RUN position, and the parking brake is not set or the transmission is not in park. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.