Circuit Description
The body control module (BCM) turns on the automatic lighting control (ALC) headlamp relay when the light sensor senses a dark condition. Battery voltage is supplied to the relay at all times through the LH HDLP fuse. The BCM grounds CKT 1970 in order to activate the relay.
Conditions for Running the DTC
| • | The ignition switch is on. |
| • | The light sensor senses a light condition (ALC headlamps off). |
Conditions for Setting the DTC
Either of the following conditions will set this DTC:
| • | CKT 1970 is open for 5 seconds. |
| • | CKT 1970 is shorted to ground for 5 seconds. |
Action Taken When the DTC Sets
| • | The SERVICE VEHICLE SOON indicator will illuminate. |
| • | The check gauges chime sounds. |
Conditions for Clearing the DTC
| • | A current DTC B2577 will clear when the malfunction is no longer present and the ignition switch is cycled. |
| • | All the BCM history codes will be cleared after 100 ignition cycles (from OFF to ON) with no current codes active during the 100 ignition cycles. |
Diagnostic Aids
| • | If CKT 1970 is open, the ALC headlamps will be inoperative. |
| • | In CKT 1970 is shorted to ground, the ALC headlamps will be always on. |
| • | If the DTC is a history DTC, the problem may be intermittent. Try performing the tests while wiggling the wiring and connectors. This can often cause the malfunction to appear. |
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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Listen for an audible click when the ALC headlamp relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
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Tests for voltage at the coil side of the ALC headlamp relay. The LH HDLP fuse supplies power to the coil side of the ALC headlamp relay.
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Verifies that the body control module is providing ground to the ALC headlamp relay.
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Tests if ground is constantly being applied to the ALC headlamp relay.
Step | Action | Yes | No |
|---|---|---|---|
1 | Did you perform the Lighting System Diagnostic System Check? | Go to Step 2 | |
Does the ALC 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 | |
Does the test lamp illuminate? | Go to Step 4 | Go to Step 10 | |
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 7 | Go to Step 6 | |
6 | Test the control circuit of the ALC headlamp relay for an open. 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 ALC headlamp relay 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 poor connections at the ALC headlamp 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 poor connections at the harness connector of the body control module. 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 | Repair the coil side feed circuit of the ALC headlamp relay. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 13 | -- |
11 | Replace the ALC headlamp relay. Did you complete the replacement? | Go to Step 13 | -- |
12 |
Important: Perform the set up procedure for the body control module. Did you complete the replacement? | Go to Step 13 | -- |
13 |
Does the DTC reset? | Go to Step 2 | System OK |
