B2578

CIRCUIT DESCRIPTION
The Body Control Module (BCM) turns on the automatic lighting control (ALC) relay when the ambient light sensor senses a dark condition. Battery positive voltage is supplied to the ALC relay at all times through the LH HDLP fuse. The BCM grounds the ALC relay control circuit in order to activate the relay.

CONDITIONS FOR RUNNING THE DTC
^ The ignition switch is in the ON position.
^ The ambient light sensor senses a dark condition (ALC headlamps on).

CONDITIONS FOR SETTING THE DTC
The ALC relay control circuit is shorted to voltage for 1 second.

ACTION TAKEN WHEN THE DTC SETS
^ The BCM turns off the ALC relay control for the remainder of the ignition cycle.
^ The SERVICE indicator illuminates.
^ The check gauges chime sounds.

CONDITIONS FOR CLEARING THE DTC
^ The conditions for setting the DTC are no longer present.
^ A history DTC clears after 100 malfunction free ignition cycles.
^ The BCM receives the clear code command from the scan tool.

DIAGNOSTIC AIDS
If the DTC is a history DTC, the problem may be intermittent. Refer to Diagnostic Aids for Circuit Testing and Wiring Repair procedures.

TEST DESCRIPTION

Steps 1-6:

The numbers below refers to the step numbers on the diagnostic table.
2. Listen for an audible click when the ALC relay operates. Command both the ON and OFF states. Repeat the commands as necessary.