Circuit Description
This is a trouble code (both current and history) that indicates that the automatic light control (ALC) headlamp relay control output is shorted to the battery.
Conditions for Setting the DTC
Meet all of the following conditions:
| • | The ambient light sensor indicates bright conditions. |
| • | The ALC option is enabled. |
| • | The ACL headlamp output is active. |
| • | The ALC headlamp feedback is present for 1 second. |
Action Taken When the DTC Sets
| • | Turns off the ALC headlamp relay. |
| • | Enters the fault state. |
| • | Stores the DTC B2578 in the BCM memory. |
| • | The SERVICE VEHICLE SOON indicator will illuminate. |
Conditions for Clearing the DTC
| • | The fault state is exited upon program return to the ALC headlamp OFF state. |
| • | All the BCM history codes will be cleared after 100 ignition cycles (from OFF to RUN) with no current codes active during the 100 ignition cycles. |
| • | Use the Scan Tool . |
Diagnostic Aids
| • | When the diagnostics direct you to take electrical measurements at the junction blocks, refer to Power Distribution Schematics in Wiring Systems for terminal assignments of the junction blocks. |
| • | If the DTC is a history DTC, the problem may be intermittent. Try performing the above tests while wiggling the wiring and connectors. This can often cause the malfunction to appear. |
Test Description
The numbers below refer to the numbers on the diagnostic table.
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This step verifies proper operation of the ALC headlamp relay.
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This step determines if a short to battery exists in CKT 1970 within the underhood wiring harness junction block.
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This step determines if a short to battery exists in CKT 1970 between the underhood junction block and the body control module (BCM).
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the BCM diagnostic system check performed? | -- | Go to Step 2 | Go to the Body Control Module System Check |
Did the ALC headlamp relay activate? | -- | Go to Step 8 | Go to Step 3 | |
Is the measured voltage at the specified value? | 0 V | Go to Step 5 | Go to Step 4 | |
4 | Replace the underhood junction block. Refer to Underhood Fuse Block Replacement in Engine Electrical. Is the repair complete? | -- | Go to Step 8 | -- |
Is the measured voltage at the specified value? | 0 V | Go to Step 6 | Go to Step 7 | |
6 | Replace the BCM. Refer to the following procedures: Is the repair complete? | -- | Go to Step 8 | -- |
7 | Repair the short to B+ in CKT 1970 between the BCM connector C1 terminal A1 and the underhood junction block harness connector C3 terminal C11. Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | Go to Step 8 | -- |
8 | Clear the DTCs from the memory. Are the DTCs cleared from the memory? | -- | -- |
