B2578

Diagnostic Chart:

Schematics:

CIRCUIT DESCRIPTION
The body control module (BCM) turns on the automatic lighting control (ALC) headlamp relay when the light sensor senses a dark condition. Battery voltage is supplied to the relay at all times through the LH HDLP fuse. The BCM grounds CKT 1970 in order to activate the relay.

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

CONDITIONS FOR SETTING THE DTC
CKT 1970 is shorted to battery positive voltage for 1 second.

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

CONDITIONS FOR CLEARING THE DTC
^ A current DTC 2578 will clear when the malfunction is no longer present and the ignition switch is cycled.
^ All BCM history codes will clear after 100 ignition cycles (from OFF to ON) with no current codes active during the 100 ignition cycles.

DIAGNOSTIC AIDS
If the DTC is a history DTC, the problem may be intermittent. Perform the tests while wiggling the wiring and connectors. This can often cause the malfunction to appear.

TEST DESCRIPTION
The number(s) below refer to the step number(s) on the diagnostic table.
2. Listen for an audible click when the ALC headlamp 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 headlamp relay. The LH HDLP fuse supplies power to the coil side of the ALC headlamp relay.
4. Verifies that the body control module is providing ground to the ALC headlamp relay.
5. Tests if ground is constantly being applied to the ALC headlamp relay.