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Home >> GMC >> 1998 >> Forward Control 7.4 B >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - 7.4L >> Self-Diagnostic System >> Mil Inoperative
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Mil Inoperative

  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK , then go to next step.
  2. Turn engine off for 15 seconds. Turn ignition on with engine off. If Malfunction Indicator Light (MIL) illuminates, go to step 4). If MIL does not illuminate, go to next step.
  3. Start engine. If engine starts, go to step 6). If engine does not start, go to step 7).
  4. Install scan tool. Turn ignition on. Using scan tool, command MIL to turn on and off. If MIL turns on and off as commanded, go to step 25). If MIL does not turn on and off as commanded, go to next step.
  5. Turn ignition off. Disconnect VCM harness connectors. Turn ignition on. If Malfunction Indicator Light (MIL) illuminates, go to step 19). If MIL does not illuminate, go to step 11).
  6. Turn ignition off. Disconnect VCM harness connectors. Turn ignition on. Using a fused jumper connected to ground, probe VCM harness connector MIL control circuit. See WIRING DIAGRAMS - 7.4L article. If MIL illuminates, go to step 11). If MIL does not illuminate, go to step 8).
  7. Check for open in ignition and battery fuses. If fuses are okay, go to step 9). If fuse(s) is open, go to step 10).
  8. Check for open in fused jumper. If fused jumper is okay, go to step 13). If fused jumper is open, go to step 14).
  9. Turn ignition off. Disconnect VCM harness connectors. Turn ignition on. Using a test light connected to ground, probe VCM harness connector ignition feed circuit. See WIRING DIAGRAMS - 7.4L article. If test light illuminates, go to step 18). If test light does not illuminate, go to step 20).
  10. Check for short to ground in fuse circuit that was open. Repair as necessary and replace fuse. After repairs, go to step 25).
  11. Check for faulty VCM connections. If connections faulty okay, go to step 23). If connections are okay, go to step 24).
  12. Repair open in ignition feed circuit. After repairs, go to step 25).
  13. Repair short to voltage in MIL control circuit. After repairs, go to step 25).
  14. Check for open in MIL control circuit. See WIRING DIAGRAMS - 7.4L article. Repair as necessary. After repairs, go to step 23). If circuit is okay, go to next step.
  15. Check for open in ignition feed circuit or fuse to MIL. See WIRING DIAGRAMS - 7.4L article. Repair as necessary. After repairs, go to step 23). If circuit or fuse is okay, go to next step.
  16. Check for an open MIL bulb. If light bulb is faulty, go to step 23). If light bulb is okay, go to next step.
  17. Replace instrument panel assembly. After replacing instrument panel, go to step 25).
  18. Turn ignition off. Disconnect VCM harness connectors. Turn ignition on. Using test light connected to ground, probe VCM harness connector battery feed circuit. See WIRING DIAGRAMS - 7.4L article. If test light illuminates, go to step 21). If test light does not illuminate, go to step 22).
  19. Repair short to ground in MIL control circuit. After repairs, go to step 25).
  20. Repair open in ignition feed circuit. After repairs, go to step 25).
  21. Check for faulty VCM ground connection at engine block. Repair as necessary. After repairs, go to step 23). If connections were okay, go to step 11).
  22. Repair open in battery feed circuit. After repairs, go to step 25).
  23. Repair as necessary. After repairs, go to step 25).
  24. Replace VCM. Perform VCM relearn procedures. After repairs, go to next step.
  25. Warm engine to normal operating temperature. Check if any DTC(s) are set. Diagnose any DTC present. If DTC(s) are not present, system is okay.
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