Circuit Description
The body control module (BCM) controls the headlamp relay by grounding the control circuit of the headlamp relay coil.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC B2578 Headlamp Control Circuit High
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 . |
| • | 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 - Vehicle? | 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 . 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 . 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 . 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 . 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 . Did you complete the repair? | Go to Step 13 | -- |
11 | Replace the HDLP low beam relay. Refer to Relay Replacement . Did you complete the replacement? | Go to Step 13 | -- |
12 | Replace the BCM. Refer to Control Module References 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 |
