B2578
CIRCUIT DESCRIPTIONThe 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 relay control circuit or the low beam headlamp relay 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 positive voltage in the low beam headlamp relay control circuit or the headlamp relay control circuit.
^ 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 Diagnostic Aids.
^ An intermittent short to voltage in the following locations may cause an intermittent malfunction to occur.
- The headlamp relay control circuit
- The headlamp relay
- The BCM
TEST
Steps 1-6:
Steps 7-13:
