Auto lamps or manual ?
The headlamps may be turned on 2 different ways. First, when the driver places the headlamp switch in the ON position, for manual operation. Second, with automatic lamp control ALC, and the headlamp switch placed in the OFF position.
The LH HDLP and RH HDLP fuse in the fuse block - underhood, supply battery positive voltage to both low and high beam lamps at all times. For manual operation , when the headlamp switch is in the ON position, the headlamp switch supplies ground directly to the headlamps and to the body control module (BCM) on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. The BCM uses this signal to determine if the conditions are present to illuminate the daytime running lamps or headlamps.
For ALC operation, the BCM receives a signal from the ambient light sensor indicating a low or high ambient (outside) light level. The ambient light sensor is a light sensitive transistor that varies its voltage signal to the BCM in response to changes to the outside (ambient) light level. Under the correct conditions the BCM grounds the headlamp relay control circuit energizing the headlamp control relay. The now closed relay supplies ground through the headlamp switch to the headlamps and the BCM on the headlamp low or high beam signal circuit, depending on the position of the headlamp dimmer switch. When the BCM grounds the headlamp relay control circuit, it also grounds the park lamp relay control circuit illuminating the park lamps. The headlamp relay is grounded at G203.
If the engine stalls with the headlamps in the ALC mode under low light conditions, the headlamps will be controlled OFF during cranking.
The conditions necessary for the BCM to activate the ACL mode are as follows:
The ignition switch is in the RUN position.
The headlamps are in the OFF position.
The park brake is released.
The BCM is receiving a low light condition from the ambient light sensor, if equipped with RPO T 82.