Sierra 1500 4WD V8-5.3L VIN Z Flex Fuel (2005)
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, for automatic transmissions (A/T), the gear selector is
not in the PARK position, the headlamp switch is in the AUTO position, and for manual transmissions (M/T), the park brake is released. When these
conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate. 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 headlights
will cancel the daytime running lamps operation. The DRL can be manually turned OFF by depressing the AHL/DRL 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 and headlamps
switch to manual control. 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 headlight 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-amp 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 , the DRL receive
voltage on the DRL supply voltage circuit. Ground for the DRL is at G100.
