Circuit Description
The body control module (BCM) monitors and supplies battery voltage to the headlamp relay utilizing the headlamp relay coil supply voltage circuit. 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 headlamp relay to power up the headlamps. The BCM monitors the headlamp relay coil supply voltage circuit to determine when the daytime running lights (DRL) should be ON or OFF. When the BCM detects battery voltage on the headlamp relay coil supply voltage circuit it will turn OFF the DRL. When the BCM detects the headlamp relay coil supply voltage circuit is low it will turn ON the DRL when the DRL conditions are present.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC B2577 Headlamp Control Circuit Low
Conditions for Running the DTC
| • | The battery voltage must be between 9-16 volts. |
| • | Ignition switch in the RUN position. |
| • | The headlamp and panel dimmer switch must be turned to the ON position or the ambient light sensor must be in the DARK state with the headlamp and panel dimmer switch in the AUTO position. |
| • | The park brake must be released. |
Conditions for Setting the DTC
| • | The headlamp and panel dimmer switch in the AUTO mode. |
| • | The ambient light sensor state is in the DARK state. |
| • | The headlamp relay coil supply voltage circuit shorted to ground. |
| • | The above conditions must be met for 0.5 seconds. |
Action Taken When the DTC Sets
The headlamps will not operate with the headlamp and panel dimmer switch in the AUTO position and the ambient light state is DARK.
Conditions for Clearing the MIL/DTC
| • | The DTC will clear immediately after the short to ground in the headlamp relay coil supply voltage circuit 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 DTCs can be cleared using a scan tool. |
Diagnostic Aids
| • | If the headlamps are manually turned ON when the headlamp relay coil supply voltage circuit is shorted to ground the TBC fuse will open and the headlamp and panel dimmer switch can be damaged. When the TBC fuse opens, the scan tool will lose communication with the BCM. |
| • | Perform a visual check for loose or poor connections at all related components. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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Listen for an audible click when the headlamp power or low beam relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
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This step verifies that the BCM is providing voltage to the headlamp power or low beam relay.
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This step tests for an open in the ground circuit to the headlamp power or low beam relay.
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This step tests if voltage is constantly being applied to the headlamp power or low beam relay.
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This step tests for a short to ground on the control circuit of the headlamp power or low beam relay.
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After replacement of the BCM you must calibrate the new module for proper operation.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics Connector End View Reference: Lighting Systems Connector End Views or Computer/Integrating Systems Connector End Views in Computer/Integrating Systems | |||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
Does the headlamp power or low beam relay turn ON and OFF with each command? | Go to Diagnostic Aids | Go to Step 3 | |
Does the test lamp turn ON and OFF with each command? | Go to Step 4 | Go to Step 5 | |
Does the test lamp turn ON and OFF with each command? | Go to Step 8 | Go to Step 10 | |
Does the test lamp remain illuminated with each command? | Go to Step 7 | Go to Step 6 | |
Test the control circuit of the headlamp power or low beam relay for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 13 | Go to Step 9 | |
7 | Test the control circuit of the headlamp power or low beam relay for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 13 | Go to Step 9 |
8 | Inspect for poor connections at the headlamp power or low beam relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 13 | Go to Step 11 |
9 | Inspect for poor connections at the harness connector of the body control module (BCM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 13 | Go to Step 12 |
10 | Repair the ground circuit of the headlamp power or low beam relay. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 13 | -- |
11 | Replace the headlamp power or low beam relay. Refer to Relay Replacement in Wiring Systems. Did you complete the replacement? | Go to Step 13 | -- |
Replace the BCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | Go to Step 13 | -- | |
13 |
Does the DTC reset? | Go to Step 2 | System OK |
