For 1990-2009 cars
Lighting Systems Components
Headlights/Daytime Running Lights (DRL) Schematics

### Circuit Description

The ambient light sensor senses the light level to control the automatic headlamps and parklamps. The body control module (BCM) provides a 5 volt reference signal to the light sensor through CKT 1137. The BCM receives the modified voltage from the light sensor through CKT 1138. A low voltage (under 1.5V signals a dark condition to the BCM and vice-versa.

### Conditions for Running the DTC

The ignition switch is on.

### Conditions for Setting the DTC

Any of the following conditions will set this DTC:

• | CKT 1138 is shorted to battery for 5 seconds.

• | CKT 1137 is shorted to battery for 5 seconds.

### Action Taken When the DTC Sets

• | The ALC headlamp and parklamp relay control are disabled for the remainder of the ignition cycle.

• | The SERVICE VEHICLE SOON indicator illuminates.

• | The check gauges chime sounds.

### Conditions for Clearing the DTC

• | A current DTC B2648 will clear when the malfunction is no longer present and the ignition switch is cycled.

• | All BCM history codes clear after 100 ignition cycles (from OFF to ON) with no current codes active.

### Diagnostic Aids

If the DTC is a history DTC the problem may be intermittent. Perform the tests while wiggling the wiring and connectors. This may often cause the malfunction to appear.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. The normal operating voltage of the photocell is 0.5-4.5 volts.

2. If the value is above 0.5 volts, with the sensor disconnected, a short to voltage exists. If the value is below 0.5 volts, a short to ground exists.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Lighting Systems Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check - Lighting Systems
2 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. With a scan tool, observe the PHOTOCELL VOLTAGE data parameter in the LAMP STATUS data list.

Does the scan tool indicate that the PHOTOCELL VOLTAGE parameter is within the specified range?

0.5-4.5 volts

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

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 PHOTOCELL VOLTAGE data parameter.

Does the scan tool indicate that the PHOTOCELL VOLTAGE data parameter is greater than the specified value?

0.5 volts

Go to Step 5

Go to Step 4

4 | Test the signal circuit and the ground circuit of the ambient light sensor for a short to ground or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 6
5 | Test the signal circuit and the ground circuit of the ambient light sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 7
6 | 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 in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 8
7 | 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 10 | Go to Step 9
8 | Replace the ambient light sensor. Did you complete the replacement? | -- | Go to Step 10 | --
9 |

Important: Perform the set up procedure for the body control module.

1. Refer to Body Control Module (BCM) Programming/RPO Configuration .
2. Replace the body control module. Refer to Body Control Module Replacement in Body Control System.

Did you complete the replacement?

--

Go to Step 10

--

10 |

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