Circuit Description
The body control module (BCM) turns ON the headlamp relay when the ambient light sensor senses a dark condition. Voltage is supplied to the headlamp relay coil side at all times. The BCM grounds the headlamp relay control circuit in order to activate the relay. With the HEADLAMP relay activated ground is supplied to the low or high beam signal circuit depending on the position of the headlamp dimmer switch illuminating the headlamps, and signaling the BCM.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC B2578 Headlamp Relay Control Circuit High.
Conditions for Running the DTC
| • | The ignition switch is in the RUN position. |
| • | The ambient light sensor senses a dark condition, automatic lamp control (ALC) headlamps ON. |
Conditions for Setting the DTC
The headlamp relay control circuit is shorted to voltage for 1 second.
Action Taken When the DTC Sets
The BCM turns OFF the headlamp relay control for the remainder of the ignition cycle.
Conditions for Clearing the DTC
| • | The BCM receives the clear code command from the scan tool. |
| • | A current DTC will clear when the conditions for setting the DTC are no longer present. |
| • | A history DTC clears after 100 malfunction free ignition cycles. |
Diagnostic Aids
| • | If the DTC is a history DTC, the problem may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. |
| • | Headlamps will remain ON with key OFF until battery runs down. |
Test Description
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 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
Does the headlamp relay turn ON and OFF with each command? | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | 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 in Wiring Systems. 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 in Wiring Systems. Did you find and correct the condition? | Go to Step 6 | Go to Step 5 |
5 | 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 6 | -- |
6 |
Does the DTC reset? | Go to Step 2 | System OK |
