GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Malibu 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC B2578 Headlamp Relay Output Short to Battery

Object Number: 501984 Size: MF
Body Control Module Components
Cell 51: Ignition Switch and Left Front Door Ajar Switch

Circuit Description

This is a trouble code (both current and history) that indicates that the 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 one 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

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.

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

    2.

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

    3.

  3. This step determines if a short to battery exists in CKT 1970 between the underhood wiring harness junction block and the body control module (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 BCM System Check performed?

--

Go to Step 2

Go to the Body Control Module System Check

2

  1. Replace the 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. Backprobe the underhood wiring harness junction block connector C3 terminal C11.
  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 C11 and ground.

Is the measured voltage at the specified value?

0 V

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 C11 and ground.

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 wiring harness 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?

--

Go to Body Control Module System Check

--