For a full page schematic, refer to
Cell 51: Head Lights
in Body Control Module Schematics.
Circuit Description
The body control module (BCM) monitors and supplies battery voltage to the automatic headlamp (AHL) relay on CKT 352. When the ambient light sensor is in the DARK state and the headlamp and panel dimmer switch is in the AUTO mode, the BCM supplies battery voltage to the AHL relay to power up the headlamps. The BCM monitors CKT 352 to up the headlamps. The BCM monitors CKT 352 to determine when the DRL should be ON or OFF. When the BCM detects battery voltage on CKT 352 it will turn OFF the DRL. When the BCM detects CKT 352 is low it will turn ON the DRL.
Conditions for Setting the DTC
To set B2577, the following conditions must be present:
| • | The ignition switch in the RUN position. |
| • | The headlamps and panel dimmer switch in the AUTO mode. |
| • | The ambient light sensor state is in the DARK state. |
| • | The park brake must be released. |
| • | CKT 352 shorted to ground. |
| • | The system voltage must be between 9.0 volts and 16.0 volts. |
| • | The above conditions must be met for 0.5 seconds. |
Action Taken When the DTC Sets
The headlamps and the DRL will not operate with the headlamp and panel dimmer switch in the AUTO position and the ambient light state in DARK state.
Conditions for Clearing the MIL/DTC
| • | The DTC will clear immediately after the short to ground on CKT 352 is repaired or the next fault free ignition cycle. |
| • | A history DTC will clear after 100 consecutive ignition cycles without a fault present. |
| • | History and current DTC(s) can be cleared using a scan tool. |
Diagnostic Aids
| • | If the headlamps are manually turned ON when CKT 352 is shorted ground the TBC fuse will blow and the headlamp and panel dimmer switch can be damaged. When the BCM fuse blows the scan tool will loose communication with the BCM. |
| • | Always diagnose the first DTC listed on the scan tool. |
| • | The scan tool must display DTC B2577 as a current code before performing diagnostics. |
| • | Perform a visual check for loose or poor connections at all related components. |
| • | Refer to Intermittents and Poor Connections in Wiring Systems. |
Test Description
Step number(s) below refer to the same step number(s) in the Diagnostic Table
-
This step determines if the BCM is defective.
-
This step determines if the AHL relay is shorted or CKT 352 is shorted to ground.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the BCM Diagnostic System Check performed? | -- | ||
2 |
Does the test lamp light? | -- | ||
3 |
Does the test lamp light? | -- | ||
4 | Locate and repair the short to ground in the WHT (352) wire between the BCM and the headlamp and panel dimmer switch. Is the repair complete? | -- | -- | |
5 | Replace the auto headlamp relay (AHL). Is the repair complete? | -- | -- | |
6 |
Is the repair complete? | -- | System OK | -- |
7 | Clear the DTCs. Is the action complete? | -- | System OK | -- |
