Circuit Description
When the ignition is in the On position, the Body Control Module (BCM) supplies exterior lamp power to the high side of the low beam relay coil. When the BCM receives an input from the headlamp switch to turn the low beam lamps ON, then the BCM grounds the low beam relay coil, energizing the relay. When the relay is energized, battery power is supplied to the low beam lamps.
Conditions for Setting the DTC
The BCM monitors the output to the low beam relay every second at the low beam relay control circuit. The criteria for the fault code to set is as follows:
| • | Exterior lamp power is active. |
| • | A DTC B1970 is not set. |
| • | Headlamp switch is inactive. |
| • | The commanded state doesn't match the feedback for 3 seconds, open circuit, short to ground or B+. |
Action Taken When the DTC Sets
| • | The BCM continues to command the requested states until it is achieved or until it changes. |
| • | Stores a DTC B2472 in the BCM memory. |
Conditions for Clearing the DTC
| • | The low beam relay is pulled low when commanded ON. |
| • | The conditions for setting the DTC are no longer present. |
| • | A history DTC will clear after 50 consecutive malfunction free ignition cycles. |
| • | The BCM receives the clear code command from the scan tool. |
Diagnostic Aids
Refer to Power Distribution Component Views in Wiring Systems for an end view and for terminal assignments of the relay box when the diagnostics direct you to take electrical measurements at the relay box.
Check for the following conditions if a history or intermittent DTC is indicated:
| • | A intermittent open, or short to B+ in the low beam relay control circuit or the headlamp switch redundant low beam relay signal circuit. |
| • | A intermittent open in the exterior lamps power circuit. |
| • | A short to ground in the low beam relay control circuit or the headlamp switch redundant low beam relay signal circuit may cause the low beams to illuminate at all times with the ignition On and the headlamp switch in the Off position. |
| • | A damaged wiring harness. Inspect the wiring harness for damage. If the wiring harness appears to be in good condition, observe the display on the DMM while moving the connectors and wiring harnesses related to the circuits. A change in the DMM display during this test will indicate the location of the fault. |
| • | For additional information, refer to Inducing Intermittent Fault Conditions in Wiring Systems. |
Test Description
The following numbers refer to the step numbers on the diagnostic table:
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This step uses a scan tool to test the low beam operation and to verify if the fault is present.
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This step tests for voltage at the coil side feed of the low beam relay.
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This step verifies that the BCM is providing a ground signal to the low beam relay.
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This step tests if ground is being constantly applied to the low beam relay.
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After verifying that no circuit problems exist, the BCM is most likely at fault. The BCM needs to be programmed after installation.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics | |||
1 | Did you perform the Lighting System Diagnostic System Check? | Go to Step 2 | |
Does the low beam relay turn ON and OFF with each command? | Go to Diagnostic Aids | Go to Step 3 | |
Does the test lamp illuminate? | Go to Step 4 | Go to Step 11 | |
Does the test lamp turn ON and OFF with each command? | Go to Step 8 | Go to Step 5 | |
Does the test lamp remain illuminated with each command? | Go to Step 10 | Go to Step 6 | |
6 | Test the control circuit of the low beam relay for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 7 |
7 | Test for a short to B+ in the control circuit of the low beam relay or the headlamp switch redundant low beam relay signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 9 |
8 | Inspect for poor connections at the low beam relay. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 13 |
9 | Inspect for poor connections at the Body Control Module. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 14 |
10 | Test for a short to ground in the control circuit of the low beam relay or headlamp switch redundant low beam relay signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 12 |
11 | Locate and repair an open circuit in the exterior lamp power circuit of the low beam relay. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Is the repair complete? | Go to Step 15 | -- |
12 | Replace the headlamp switch. Refer to Headlamp Switch Replacement . Is the replacement complete? | Go to Step 15 | -- |
13 | Replace the low beam relay. Is the replacement complete? | Go to Step 15 | -- |
|
Important: Perform the programming procedure for the BCM. Replace the BCM. Refer to Body Control Module Replacement in Body Control System. Is the replacement complete? | Go to Step 15 | -- | |
15 |
Does the DTC reset? | Go to Step 2 | System OK |
