B2578

CIRCUIT DESCRIPTION
The Body Control Module (BCM) controls the headlamp relay by grounding the control circuit of the headlamp relay coil.

CONDITIONS FOR RUNNING THE DTC
^ Voltage supplied to the module is in the normal operating voltage range (approximately 10-16 volts).
^ The headlamps must be ON.

CONDITIONS FOR SETTING THE DTC
^ The BCM detects a short to voltage in the headlamp coil control circuit when the BCM energizes the headlamp relay.
^ The condition must be present for more than 3 ms.

ACTION TAKEN WHEN THE DTC SETS
^ The BCM stores DTC B2578 in memory.
^ The SERVICE VEHICLE SOON telltale illuminates.
^ The BCM activates the daytime running lamps.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC B2578 clears if the BCM no longer detects a short to battery voltage in the control circuit of the headlamp relay coil.
^ A history DTC B2578 clears after 100 consecutive ignition cycles.
^ The BCM receives the Clear Code commend from the scan tool.

DIAGNOSTIC AIDS
^ If the DTC B2578 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent and Poor Connections in Diagrams.
^ An intermittent short to voltage in the following locations may cause an intermittent malfunction to occur:
- The control circuit of the headlamp relay coil
- The headlamp relay
- The BCM

TEST DESCRIPTION

Steps 1-13:

The number(s) below refer to the step number(s) on the diagnostic table.
2. Listen for an audible click when the HDLP low beam relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
3. This step tests for voltage at the coil side of the HDLP low beam relay. The left IP fuse supplies power to the coil side of the HDLP low beam relay.
4. This step verifies that the body control module is providing ground to the HDLP low-beam relay.
5. This step tests whether ground is constantly being applied to the HDLP low beam relay.