For 1990-2009 cars

### Circuit Description

The body control module (BCM) monitors the ambient light sensor signal circuit in order to determine if the daytime running lights (DRL) or the auto headlights (ALC) should be turned ON in the AUTO mode. When the BCM senses the ambient light sensor signal voltage is between 1.75-4.9 volts, either the DRL will be commanded ON or the ALC will be commanded ON depending upon the amount of light received by the sensor.

### DTC Descriptors

This diagnostic procedure supports the following DTC:

• | DTC B2647 Ambient Light Sensor Circuit Low

• | DTC B2648 Ambient Light Sensor Circuit High

### Conditions for Running the DTC

The system voltage must be between 9-16 volts.

### Conditions for Setting the DTC

• | When the ambient light sensor signal voltage is less than 1.75 volts

• | When the ambient light sensor signal voltage is more than 4.9 volts

### Action Taken When the DTC Sets

The ALC will remain always ON or always OFF in any ambient light state.

### Conditions for Clearing the MIL/DTC

• | This DTC will clear immediately after the condition for setting the fault is corrected.

• | A history DTC will clear after 100 consecutive ignition cycles without a fault present.

• | Use a scan tool in order to clear history and current DTCs.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step verifies that the value displayed is within the specified range.

2. This step tests for the proper operation of the circuit in the high voltage range.

3. This step tests for the proper operation of the circuit in the low voltage range. If the fuse in the jumper opens when you perform this test, the signal circuit is shorted to voltage.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Headlights/Daytime Running Lights (DRL) Schematics Connector End View Reference: Lighting Systems Connector End Views or Data Communication Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. With a scan tool, observe the ambient light sensor parameter in the body control module (BCM) data list .

Does the scan tool indicate that the ambient light sensor parameter is within the specified range?

1.75-4.9 V

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3 |

1. Turn OFF the ignition.
2. Disconnect the ambient light sensor.
3. Turn ON the ignition, with the engine OFF.
4. With a scan tool, observe the ambient light sensor parameter.

Does the scan tool indicate that the ambient light sensor parameter is greater than the specified value?

4.9 V

Go to Step 4

Go to Step 5

4 |

1. Turn OFF the ignition.
2. Connect a 3-amp fused jumper wire between the ambient light sensor low reference circuit and the ambient light sensor signal circuit at the ambient light sensor harness connector.
3. Turn ON the ignition, with the engine OFF.
4. With a scan tool, observe the ambient light sensor parameter.

Does the scan tool indicate that the ambient light sensor parameter is less than the specified value?

1.75 V

Go to Step 6

Go to Step 7

5 | Test the ambient light sensor signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 10
6 | Test the ambient light sensor low reference circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 9
7 | Test the ambient light sensor signal circuit for a short to voltage, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 8
8 | Test the ambient light sensor low reference circuit for a short to voltage, a high resistance, or an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 10
9 | Inspect for poor connections at the harness connector of the ambient light sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 11
10 | Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12
11 | Replace the ambient light sensor. Refer to Ambient Light Sensor Replacement . Did you complete the replacement? | -- | Go to Step 13 | --
12 | Replace the BCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 13 | --
13 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK