GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2007 🢒 CTS 🢒 Body Systems 🢒 Lighting 🢒 DTC B2646

Circuit Description

The HVAC control module determines the ambient light conditions either DAYTIME 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 out a class 2 message to the dash integration module (DIM) indicating the outside light condition based on the signal from the ambient light sensor.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B2646 Ambient Light Sensor Circuit Range/Performance

Conditions for Running the DTC

The HVAC control module is powered.

Conditions for Setting the DTC

The twilight 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 twilight status is set to NIGHT.

Conditions for Clearing the DTC

The HVAC control module does not detect a voltage that is less than or greater than the predetermined value during the diagnostic self test.

Test Description

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

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

    2.

  2. 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: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

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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 Photocell parameter in the Climate Control Panel (CCP) Lighting Data list.

Does the scan tool indicate that the Photocell 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 Photocell parameter.

Does the scan tool indicate that the Photocell data 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 of the ambient light sensor and the low reference circuit of the ambient light sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, observe the Photocell data parameter.

Does the scan tool indicate that the Photocell 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 of the ambient light sensor 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 of the ambient light sensor 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 12

Go to Step 7

7

Test the ambient light sensor low reference circuit for a high resistance or an open. 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 assembly. 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 assembly. Refer to Sun Load Sensor Replacement .

Did you complete the replacement?

--

Go to Step 12

--

11

Important: Perform the setup procedure for the HVAC control module.

Replace the HVAC control module. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 12

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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