Circuit Description
The body control module (BCM) monitors CKT 278 in order to determine if either of the following lamps should be turned ON when the headlamp and panel dimmer switch is in the AUTO mode:
- The daytime running lamps (DRL)
- The headlamps
The DRL will be commanded ON when the following conditions exist:
| • | The BCM senses that the ambient light sensor voltage is between 2.2-4.7 volts. |
| • | The sensor resistance is low. |
The headlamps will be commanded ON when the following conditions exist:
| • | The BCM senses that the ambient light sensor voltage is between 0.2-1.3 volts. |
| • | The sensor resistance is high. |
If the BCM senses that the current on CKT 278 is less than 0.2 volts, the BCM will set DTC B2647.
Conditions for Setting the DTC
| • | The ambient light sensor input voltage is less than 0.2 volts. This low voltage occurs when CKT 278 is shorted to ground. |
| • | The system voltage is between 9.0-16.0 volts. |
| • | The above conditions exist for 0.5 seconds. |
Action Taken When the DTC Sets
The automatic headlight control will be turned ON.
Conditions for Clearing the MIL/DTC
| • | DTC B2647 will clear immediately when open or short to ground is no longer detected. |
| • | A history DTC will clear after 100 consecutive ignition cycles without a fault present. |
| • | History and current DTCs can be cleared using a scan tool. |
Diagnostic Aids
| • | Always perform the first DTC that is listed on the scan tool if more than one is displayed. |
| • | Verify that the scan tool displays DTC B2647 as a current code before you perform diagnostics. |
| • | If a self-powered test lamp is not available, connect a test lamp to battery voltage and probe the ground circuit in order to obtain the same results. |
| • | Inspect for loose or poor connections at all of the related components. |
| • | Refer to Wiring Systems for intermittent and poor connections. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step determines if the BCM is defective.
-
This step tests the ambient light sensor CKT 278 for an open.
-
This step tests CKT 278 for a short to ground.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the BCM Diagnostic System Check performed? | -- | Go to Step 2 | |
Is the voltage greater than or equal to the specified value? | 1 V | Go to Step 8 | Go to Step 3 | |
Is the resistance within the specified range? | 0-5 ohms | Go to Step 4 | Go to Step 5 | |
Connect a test lamp from CKT 278 cavity B to good power source. Does the test lamp light? | -- | Go to Step 6 | Go to Step 7 | |
5 | Locate and repair the open in CKT 278 (WHT) between the ambient light sensor and the BCM. Is the repair complete? | -- | Go to Step 9 | -- |
6 | Locate and repair the short to ground in CKT 278 between the ambient light sensor and the BCM. Is the repair complete? | -- | Go to Step 9 | -- |
7 | Replace the ambient light sensor. Is the repair complete? | -- | Go to Step 9 | -- |
8 |
Is the repair complete? | -- | Go to Step 9 | -- |
9 | Clear the DTCs. Is the repair complete? | -- | -- |
