Daytime Running Lamp: Description and Operation

DAYTIME RUNNING LAMPS (DRL)
The ambient light sensor is a light sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes to the outside (ambient) light level. When the BCM receives this signal it will either turn on the daytime running lamps (DRL) or the headlight relay for auto headlamp operation. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. The daytime running lamps (DRL) illuminate the high beam headlights at a reduced intensity. With the headlight switch in the OFF position, the headlights will either be turned ON or OFF, after an approximate 30 second delay depending on whether daylight or low light conditions are sensed. Voltage is applied at all times to both the switched and coil side of the DRL relay. When the ambient light sensor detects a low light condition, the BCM applies a ground to the DRL relay control circuit. This energizes the relay and applies voltage to the DRL fuse. Current flows from the DRL fuse to the right high beam lamp to the left hand high beam lamp. The high beam headlamps are now in series, and split the voltage. The headlamps illuminate at half intensity. The ground is provided from G101. The DRL operate 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.