For 1990-2009 cars
Body Control Module Components
Body Control System Schematics

### Circuit Description

This is a trouble code (both current and history) that indicates that the Automatic Lighting Control (ALC) headlamp relay output is shorted to the battery.

### Conditions for Setting the DTC

Meet all of the following conditions:

• | The ambient light sensor indicates dark conditions

• | The ALC headlamp output is inactive

• | The ALC headlamp feedback is present for 1 second

### Action Taken When the DTC Sets

• | Turns off the ALC headlamp relay.

• | Stores the DTC B2578 in the body control module (BCM) memory.

• | The SERVICE VEHICLE SOON indicator illuminates.

• | The CHECK GAUGES chime sounds.

### Conditions for Clearing the DTC

• | The fault state is exited upon program return to the ALC headlamp normal state.

• | All the body control module (BCM) history codes will be cleared after 100 ignition cycles (from OFF to ON) with no current codes active during the 100 ignition cycles.

• | Use the Scan Tool .

### Diagnostic Aids

If the DTC is a history DTC, the problem may be intermittent. Perform 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.

1. This step verifies proper operation of the ALC headlamp relay.

2. This step determines if a short to battery exists in CKT 1970 within the underhood wiring harness junction block.

3. This step determines if a short to battery exists in CKT 1970 between the underhood wiring harness junction block and the BCM.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Important: A short to ground will cause an open fuse(s). Before performing this diagnostic procedure, inspect the fuse(s) for an open.
1 | Was the Body Control Module (BCM) System Check performed? | -- | Go to Step 2 | Go to Body Control Module System Check
2 |

1. Replace the Automatic Lighting Control (ALC) headlamp relay with a known good relay on the underhood wiring harness junction block.
2. Use the Scan Tool in order to enable the ALC headlamp relay.

Did the ALC headlamp relay activate?

--

Go to Step 8

Go to Step 3

3 |

1. Remove the ALC headlamp relay on the underhood wiring harness junction block.
2. Disconnect the underhood wiring harness junction block connector C3.
3. Use the J 39200 DMM in order to measure the voltage in CKT 1970 between the underhood wiring harness junction block connector C3 terminal A9 and ground.

Is the measured voltage at the specified value?

0V

Go to Step 5

Go to Step 4

4 | Replace the underhood wiring harness junction block. Refer to Underhood Fuse Block Replacement in Engine Electrical. Is the repair complete? | -- | Go to Step 8 | --
5 |

1. Disconnect the BCM connector C1.
2. Use the J 39200 DMM in order to measure the voltage in CKT 1970 between the underhood wiring harness junction block harness connector C3 terminal A9 and ground.

Is the measured voltage at the specified value?

0V

Go to Step 6

Go to Step 7

6 | Replace the BCM. Refer to the following procedures:

• | Body Control Module Replacement

• | Body Control Module (BCM) Programming/RPO Configuration

Is the repair complete?

--

Go to Step 8

--

7 | Repair the short to B+ in CKT 1970 between the Body Control Module (BCM) connector C1 terminal A1 and the underhood wiring harness junction block harness connector C3 terminal A9. Is the repair complete? | -- | Go to Step 8 | --
8 | Clear the DTCs from the memory. Are the DTCs cleared from the memory? | -- | Go to Body Control Module System Check | --