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Diagnostic Procedures

WARNING: This page is about a different car, the 1999 GMC Sierra and 1999 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. If BCM diagnostic system check has been performed, go to next step. If BCM diagnostic system check has not been performed, perform BCM diagnostic system check. See BCM DIAGNOSTIC SYSTEM CHECK  under SYSTEM TESTS.
  2. Turn ignition on. Place a light close to ambient light sensor in order to increase voltage on circuit No. 278. Using DVOM connected to ground, backprobe Purple BCM connector C3 terminal A6 (White wire). See Figure . If reading is 1.1-4.7 volts, go to step  10 . If reading is not 1.1-4.7 volts, go to next step.
  3. Turn ignition off. Disconnect ambient light sensor. Using DVOM, measure resistance between Purple BCM connector C3 terminal A6 and ambient light sensor connector terminal "B" (White wire). See WIRING DIAGRAMS  . If resistance is 0-5 ohms, go to next step. If resistance is not 0-5 ohms, go to step  7 .
  4. Using test light connected to battery voltage, backprobe ambient light sensor connector terminal "B" (White wire). If test light illuminates, go to step  8 . If test light does not illuminate, go to next step.
  5. Using DVOM, measure resistance between ambient light sensor connector terminal "A" and Purple BCM connector C3 terminal A4. See WIRING DIAGRAMS  . If resistance is 0-5 ohms, go to step  9 . If resistance is not 0-5 ohms, go to next step.
  6. Repair open in Gray wire between ambient light sensor connector terminal "A" and Purple BCM connector C3 terminal A4. After repairs, go to step  11 .
  7. Repair open in White wire between ambient light sensor connector terminal "B" and Purple BCM connector C3 terminal A6. See WIRING DIAGRAMS  . After repairs, go to step  11 .
  8. Repair short to ground in White wire between ambient light sensor connector terminal "B" and Purple BCM connector C3 terminal A6. See WIRING DIAGRAMS  . After repairs, go to step  11 .
  9. Replace ambient light sensor. After replacing ambient light sensor, go to step  11 .
  10. Replace BCM and program. See BODY CONTROL MODULE (BCM)  under REMOVAL & INSTALLATION.
  11. Reconnect all components. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES (DTCS)  under SELF-DIAGNOSTIC SYSTEM.
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