For 1990-2009 cars

### Circuit Description

The HVAC control module determines the ambient light conditions, DAY or NIGHT, based on the voltage from the ambient light sensor. The HVAC control module provides the ambient light sensor with a 5-volt reference voltage and a signal return circuit. The ambient light sensor signal circuit is a direct input to the HVAC control module. The HVAC control module sends a message via GMLAN serial data to the instrument panel module (IPM) indicating the outside light conditions based on the signal from the ambient light sensor. For more detailed information concerning interior lighting operation, refer to Interior Lighting Systems Description and Operation .

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2645 Ambient Light Sensor Circuit

The vehicle has DTCs including the DTC symptoms. Refer to DTC Symptom Description for more DTC symptoms.

DTC Symptom | DTC Symptom Descriptor
--- | ---
00 | Voltage Out of Range

### Conditions for Running the DTC

• | The HVAC control module must be powered.

• | DTC B1327 Battery Voltage Low, must not be current.

### Conditions for Setting the DTC

Ambient light sensor voltage is less than 0.09 volt or greater than 4.75 volts on 3 consecutive tests.

### Action Taken When the DTC Sets

• | The ambient light sensor will default to NIGHT status.

• | The HVAC control module commands the Service A/C System indicator ON via GMLAN serial data.

### Conditions for Clearing the DTC

• | The condition responsible for setting the DTC no longer exists.

• | You issue a scan tool CLEAR DTCs command.

• | A history DTC will clear once 40 consecutive malfunction free ignition cycles have occurred.

### Diagnostic Aids

Thoroughly inspect the wiring and connectors. An incomplete inspection of the wiring and connectors may result in misdiagnosis causing part replacement with the reappearance of the malfunction. If an intermittent malfunction exists, refer to Testing for Intermittent Conditions and Poor Connections .

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Headlights Schematics Connector End View Reference: Master Electrical Component List
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 HVAC control module open/short data list.

Does the scan tool indicate the Ambient Light Sensor parameter is within the specified range?

0.09-4.75 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 assembly.
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?

4.75 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 signal circuit and the ambient light sensor low reference circuit.
3. Turn ON the ignition, with the engine OFF.
4. With a scan tool, observe the Ambient Light Sensor data parameter.

Does the scan tool indicate the Ambient Light Sensor data parameter is less than the specified value?

0.09 V

Go to Step 8

Go to Step 6

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

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