The body control module (BCM) turns on the headlamp relay when the ambient light sensor senses a dark condition. Battery positive voltage is supplied to the headlamp relay at all times through the BACKUP/TURN fuse. The BCM grounds the headlamp relay control circuit in order to activate the relay.
This diagnostic procedure supports the following DTC:
DTC B2578 Headlamp Relay Output Circuit High
• | The ignition switch is in the ON position. |
• | The ambient light sensor senses a dark condition (automatic lamp control (ALC) headlamps on). |
The headlamp relay control circuit is shorted to voltage for 1 second.
• | The BCM turns off the headlamp relay control for the remainder of the ignition cycle. |
• | The SERVICE indicator illuminates. |
• | The conditions for setting the DTC are no longer present. |
• | A history DTC clears after 100 malfunction-free ignition cycles. |
• | The BCM receives the clear code command from the scan tool. |
If the DTC is a history DTC, the problem may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections .
The number below refers to the step number on the diagnostic table.
Step | Action | Yes | No |
---|---|---|---|
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics | |||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |
Does the headlamp relay turn ON and OFF with each command? | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 3 | |
3 | Test for a short to voltage in the control circuit of the headlamp relay. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 6 | Go to Step 4 |
4 | 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 . Did you find and correct the condition? | Go to Step 6 | Go to Step 5 |
5 | Replace the BCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 6 | -- |
6 |
Does the DTC reset? | Go to Step 2 | System OK |