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 lights (DRL) or the headlights. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. 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. The LT HEADLAMP fuse in the underhood junction block supplies ignition positive voltage to the daytime running lamp (DRL) relay coil circuit. The DRL relay switch contacts are connected to the low beam circuit. When the headlamp switch is placed in the OFF position, the BCM will either turn on or off the daytime running lamps, depending on whether daylight or low light conditions are sensed. The BCM energizes the DRL relay, closing the switch contacts and grounding the low beam circuit. Ground is provided at G103. The DRL illuminate the low beam headlights at a reduced intensity. The DRL will operate when the ignition switch is in the ON position, the gear selector is not in the PARK position and the parking brake is released. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.

DRL operation in Manual Transmission equipped vehicles will occur when the ignition switch is in the ON position, and the parking brake is released.

The DRL telltale is turned ON by the cluster when it receives a command from the BCM. The telltale will be ON whenever DRL is in operation.

NOTE: Low resistance high beam bulbs or high charging system voltage can cause a flickering DRL telltale in Service Precautions. The DRL telltale will turn OFF if the High or Low Beams headlamps are activated or if a High Beam Headlamp fails. However, if both high beams fail, the DRL will turn ON again. This is because the DRL circuitry is unable to identify this condition.

Daytime running lamps (DRL) are controlled by the BCM based on input from the high beam headlamp circuit 1200, the low beam headlamp circuit 1201, the parking brake circuit 1134 and ignition switch state.

When DRL operation is desired, the BCM switches circuit 592 to ground. This action energizes the DRL relay. When energized, the DRL relay operates high beam headlamps or low beam headlamps (later models) as a series circuit, one-half of battery voltage across each high beam headlamp.

When DRL operation is not desired, the BCM does not provide ground for the DRL relay. When not energized, as when high beam headlamps or low beam headlamps (later models) are desired, the DRL relay operates the high beam headlamps or low beam headlamps (later models) as a parallel circuit, full battery voltage across each headlamp.




For DRL operation, the parking brake must be fully released, the ignition switch must be in the RUN position and headlamp switch must be in the Off position. Under normal operating conditions, the feedback voltage to the BCM measured at circuit 1200 as shown in the table.