Circuit Description
The body control module (BCM) monitors and supplies battery voltage to the appropriate relay on CKT 352:
| • | The headlamp power relay (all except Envoy) |
| • | The low beam relay (Envoy only) |
The BCM supplies battery voltage to the headlamp power relay in order to power up the headlamps when the following conditions exist:
| • | The ambient light sensor is in the DARK state. |
| • | The headlamp and panel dimmer switch is in the AUTO/OFF position. |
DTC B2577 sets when the following conditions exist:
| • | The ignition switch is in the RUN position. |
| • | The ambient light sensor is in the DARK state. |
| • | CKT 352 is shorted to ground. |
Conditions for Setting the DTC
| • | The ignition switch is in the RUN position. |
| • | The headlamps and panel dimmer switch is in the AUTO/OFF position. |
| • | The ambient light sensor is in the DARK state. |
| • | The park brake is released. |
| • | CKT 352 is shorted to ground. |
| • | The system voltage is within 9.0-16.0 volts. |
| • | The above conditions exist for 0.5 seconds. |
Action Taken When the DTC Sets
The automatic headlamps and the DRL will not operate properly when the headlamp and panel dimmer switch is in the AUTO/OFF position and the ambient light sensor is in the DARK state.
Conditions for Clearing the MIL/DTC
| • | DTC B2577 will clear immediately after the short to ground in CKT 352 is repaired. |
| • | The DTC will clear immediately after the next ignition cycle that does not contain a fault. |
| • | A history DTC will clear after 100 consecutive ignition cycles without a fault present. |
| • | History and current DTCs may be cleared using a scan tool. |
Diagnostic Aids
| • | If the headlamps are manually turned ON when CKT 352 is shorted to ground, the HDLP SW fuse will blow and the headlamp and panel dimmer switch may be damaged. |
| • | When the TBC fuse blows, the scan tool will lose communication with the BCM. |
| • | Always diagnose the first DTC that is listed on the scan tool. |
| • | Verify that the scan tool displays DTC B2577 as a current code before you perform diagnostics. |
| • | Inspect for loose or poor connections at all of the related components. |
| • | Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step determines if CKT 352 is shorted to ground or defective.
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This step determines if the BCM is causing the DTC to set or if the applicable relay (HDLP POWER or LOW BEAM) is causing the DTC to set.
-
This step determines if one of the following conditions exists:
| • | The HDLP PWR relay is shorted. |
| • | The LOW BEAM relay is shorted. |
| • | CKT 352 is shorted to ground. |
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the BCM Diagnostic System Check? | -- | Go to Step 2 | |
Does the test lamp light? | -- | Go to Step 4 | Go to Step 3 | |
Is DTC B2577 set as a current code? | -- | Go to Step 7 | Go to Step 5 | |
4 | Locate and repair the short to ground in CKT 352 (WHT) between the BCM and the headlamp and panel dimmer switch and the underhood fuse block. Is the repair complete? | -- | Go to Step 8 | -- |
Use a J 39200 digital multimeter (DMM) in order to measure the resistance between terminal 85 and terminal 86 (coil side) of the applicable relay ( HDLP PWR or LOW BEAM). Is the resistance less than the specified value? | 70 ohms | Go to Step 6 | Go to Diagnostic Aids | |
6 | Replace the applicable relay in the underhood fuse block. Is the repair complete? | -- | Go to Step 8 | -- |
7 |
Is the repair complete? | -- | Go to Step 8 | -- |
8 | Clear the DTCs. Is the repair complete? | -- | -- |
