Daytime Running Lamp: Description and Operation

DAYTIME RUNNING LAMPS (DRL)

The daytime running lamps (DRL) will operate when the ignition switch is in the RUN position, the gear selector is not in the PARK position, and the headlamp switch is in the AUTO position. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate. Some export vehicles are not equipped with automatic lamp control (ALC), disabling the ALC, DRL and wiper activated headlamp (WAH) functions in the module. With ALC disabled, the headlamps will operate in the manual mode. In these vehicle diagnostics, the term AUTO shall be equivalent to OFF when referencing headlamp switch positions.

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 DRL or the headlamps for auto headlamp operation. Any function or condition that turns on the headlamps will cancel the DRL operation. The DRL can be manually turned OFF by depressing the AHL/DRL defeat switch in the headlamp switch. The AHL/DRL defeat switch sends a ground signal through the DRL switch signal circuit to the BCM. The BCM then turns OFF the DRL. When the AHL/DRL are manually turned OFF at the headlamp switch, the AHL/DRL will default to ON with the next ignition cycle. The DRL are separate lamps independent of the headlamps. With the headlamp switch in the AUTO position, the DRL will either be turned ON or OFF, after an approximate 8 second delay depending on whether daylight or low light conditions are sensed. The DRL 10 A fuse in the underhood fuse block supplies battery positive voltage to the DRL relay switch contacts and relay coil. When the BCM energizes the DRL relay on the DRL relay control circuit in daylight conditions, the DRL receives voltage on the DRL supply voltage circuit. Ground for the DRL is at G100.