B2577

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 light condition (ALC headlamps off).

CONDITIONS FOR SETTING THE DTC
Either of the following conditions will set this DTC:
^ The ALC relay control circuit is open for 5 seconds.
^ The ALC relay control circuit is shorted to ground for 5 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ The SERVICE indicator will illuminate.
^ 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 ALC relay control circuit is open, the ALC headlamps will be inoperative.
^ In the ALC relay control circuit is shorted to ground, the ALC headlamps will be always on.
^ 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 1-13:

The numbers below refer 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.
3. Tests for voltage at the coil side of the ALC relay. The LH HDLP fuse supplies battery positive voltage to the coil side of the ALC relay.
4. Verifies that the body control module is providing ground to the ALC relay.
5. Tests if ground is constantly being applied to the ALC relay.