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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 533 (LIGHTING SYSTEMS)DIAGNOSTIC INFORMATION AND PROCEDURESDTC B2645: AMBIENT LIGHT SENSOR CIRCUITTEST DESCRIPTION
1999 GMC Forward Control 5.7 R, Automatic

Test Description

1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Description
WARNING: This page is about a different car, the 2004 Pontiac Aztek and 2004 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

The number below refers to the step number on the diagnostic table.

  1. 2:  This step tests the ambient light sensor reference voltage circuit for a short to battery positive voltage.
DTC B2645

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? - Go to Step 2  Go to Diagnostic System Check - Lighting Systems 
2 
  1. Disconnect the ambient light sensor.
  2. Turn OFF the ignition.
  3. Measure the voltage from the ambient light sensor reference voltage circuit.
Does the voltage measure near the specified value?		 12 V Go to Step 4  Go to Step 3 
3 Inspect the ambient light sensor return circuit for a short to battery positive voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 7  Go to Step 6 
4 Inspect the ambient light sensor reference voltage circuit for a short to battery positive voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 7  Go to Step 5 
5
IMPORTANT: Perform the set up procedure for the body control module (BCM). Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control Systems.
Replace the body control module. Refer to Body Control Module Replacement in Body Control System.Did you complete the replacement?		 - Go to Step 7  -
6 Replace the ambient light sensor. Refer to Daytime Running Lamps (DRL) Ambient Light Sensor Replacement .
Did you complete the replacement?		 - Go to Step 7  -
7
  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
RENDER: 1.0x

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.