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Home › GMC › 1999 › Envoy › Repair and Diagnosis › Body & Frame › Body, Cab Control Systems › Body Control Modules › Diagnostic Tests › DTC B2577: Headlight Control Circuit Low › Diagnostic Procedures

Diagnostic Procedures

  1. Perform BCM diagnostic system check. See BCM DIAGNOSTIC SYSTEM CHECK  under SYSTEM TESTS. After performing system check, go to next step.
  2. Ensure headlight switch is in AUTO/OFF position. Turn ignition off. Remove headlight power relay (all except Envoy) or low beam relay (Envoy) from Underhood Bussed Electrical Center (UBEC). See COMPONENT LOCATIONS  . Disconnect Light Blue 24-pin BCM connector. Remove connector comb from Light Blue BCM connector. Using test light connected to battery voltage, backprobe Light Blue BCM connector terminal A11 (White wire). See Figure . If test light illuminates, go to step 4). If test light does not illuminate, go to next step.
  3. Install known-good relay into position for headlight power relay (all except Envoy) or low beam relay (Envoy) in UBEC. Reconnect all components. Connect scan tool to Data Link Connector (DLC). Turn ignition on. Check for DTCs. If DTC B2577 is set as a current code, go to step 7). If DTC B2577 is not set as a current code, go to step 5).
  4. Repair short to ground in White wire between Light Blue BCM connector terminal No. A11, headlight switch connector terminal "C" and appropriate relay connector terminal in UBEC. See WIRING DIAGRAMS  . After repairs, go to step 8).
  5. Using DVOM, measure resistance of headlight power relay (all except Envoy) or low beam relay (Envoy) between relay terminals No. 85 and 86. If less than 70 ohms exists, go to next step. If 70 ohms or more exists, see DIAGNOSTIC AIDS.
  6. Replace headlight power relay (all except Envoy) or low beam relay (Envoy). After replacing appropriate relay, go to step 8).
  7. Replace BCM. See BODY CONTROL MODULE (BCM)  under REMOVAL & INSTALLATION. Program BCM and perform relearn procedure. See PROGRAMMING  . After programming module, go to next step.
  8. Clear DTCs. After clearing DTCs, perform BCM diagnostic system check. See BCM DIAGNOSTIC SYSTEM CHECK  under SYSTEM TESTS.
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