Circuit Description
The body control module (BCM) controls the headlamp relay by grounding the control circuit of the headlamp relay coil.
Conditions for Running the DTC
| • | Voltage supplied to the module is in the normal operating voltage range (approximately 10-16 volts). |
| • | The headlamps must be ON. |
Conditions for Setting the DTC
| • | The BCM detects a short to voltage in the headlamp relay control circuit or the low beam headlamp relay control circuit when the BCM energizes the headlamp relay. |
| • | The condition must be present for more than 3 ms. |
Action Taken When the DTC Sets
| • | The BCM stores DTC B2578 in memory. |
| • | The SERVICE VEHICLE SOON telltale illuminates. |
| • | The BCM activates the daytime running lamps. |
Conditions for Clearing the DTC
| • | A current DTC B2578 clears if the BCM no longer detects a short to battery positive voltage in the low beam headlamp relay control circuit or the headlamp relay control circuit. |
| • | A history DTC B2578 clears after 100 consecutive ignition cycles. |
| • | The BCM receives the clear code commend from the scan tool. |
Diagnostic Aids
| • | If the DTC B2578 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
| • | An intermittent short to voltage in the following locations may cause an intermittent malfunction to occur: |
| - | The headlamp relay control circuit |
| - | The headlamp relay |
| - | The BCM |
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics Connector End View Reference: Master Electrical Component List | |||
1 | Did you perform the Diagnostic System Check? | Go to Step 2 | |
2 |
Does the HDLP low beam relay turn ON and OFF with each command? | Go to Diagnostic Aids | Go to Step 3 |
3 |
Does the test lamp illuminate? | Go to Step 4 | Go to Step 10 |
4 |
Does the test lamp turn ON and OFF with each command? | Go to Step 8 | Go to Step 5 |
5 | Does the test lamp remain illuminated with each command? | Go to Step 7 | Go to Step 6 |
6 | Test the control circuit of the HDLP low beam relay and the headlamp relay control circuit for a short to battery positive 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 |
7 | Test the control circuit of the HDLP low beam relay and the headlamp relay control circuit 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 |
8 | Inspect for a poor connection at the HDLP 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 a high resistance 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 | Replace the underhood fuses block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems. Did you complete the repair? | Go to Step 13 | -- |
11 | Replace the HDLP low beam relay. Refer to Relay Replacement in Wiring Systems. Did you complete the replacement? | Go to Step 13 | -- |
12 |
Important: Perform the set-up procedure for the Body Control Module. Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control Systems. Replace the body control module. Refer to Body Control Module Replacement in Body Control System. Did you complete the replacement? | Go to Step 13 | -- |
13 |
Does the DTC reset? | Go to Step 2 | System OK |
