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Home >> GMC >> 1996 >> Yukon 4D Utility, 4WD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures - Gasoline >> On-Board Diagnostic (OBD) System Check >> A-1, Malfunction Indicator Light (Mil) >> Notes

A-1, Malfunction Indicator Light (Mil): Notes

When vehicle operator initially turns ignition ON, MIL momentarily flashes ON then OFF and then stays ON. When the engine is running and no DTCs are stored, the VCM turns OFF the MIL. Ignition switch supplies battery voltage directly to MIL. VCM controls MIL by providing a ground path through the MIL control circuit to turn OFF MIL.

  1. Perform Powertrain OBD System Check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK  . Turn engine OFF for 15 seconds. Turn ignition ON, engine OFF. If MIL is ON, go to step 2). If MIL is not ON, attempt to start engine. If engine starts, go to step 5). If engine does not start, go to step 6).
  2. Install scan tool. Command MIL ON and OFF. If MIL does not function as specified, go to next step. If MIL turns ON and OFF when commanded, allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  3. Turn OFF the ignition. Disconnect black VCM connector. Turn ON ignition. If MIL does not turn ON, go to step 8). If MIL turns ON, go to step 17).
  4. Turn OFF ignition. Disconnect black VCM connector. Turn ON ignition. Probe MIL control circuit with a fused jumper connected to ground. If MIL is not ON, go to step 6). If MIL is ON, go to step 11).
  5. Inspect fuses 9 (ECM-BATT) and 10 (ECM-I) for an open. If fuses are okay, go to step 7). If fuses are not okay, go to step 8). 6)If fuse in the fused jumper blew, go to step 11). If fuse is okay, go to step 12).
  6. Turn OFF ignition. Disconnect black VCM connector. Turn ON ignition. Probe VCM ignition feed circuit (terminal BK18) with a test light to ground. If test light illuminates, go to step 16). If test light does not illuminate, go to step 18).
  7. Check for a short to ground in the circuit of the fuse that was open and repair if necessary. Replace faulty fuse. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  8. Check for faulty VCM connections. If problems were found, Repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay. If no problems were found, replace VCM.
    NOTE: If VCM is faulty, reprogram VCM.
  9. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  10. Repair the open in the VCM ignition feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  11. 11) Repair short to voltage in MIL control circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  12. Check for an open in MIL control circuit. If control circuit is okay, go to next step. If control circuit is not okay, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  13. Check for an open ignition feed circuit or fuse (gauge 4) to MIL. If no problem was found, go to next step. If circuit was open, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  14. Check for an open MIL bulb. If MIL bulb is okay, go to next step. If bulb is not okay, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  15. Replace the instrument panel. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  16. Turn ignition OFF. Disconnect Gray VCM connector. Probe battery feed circuit (terminal GR21) with a test light connected to ground. If test light illuminates, go to step 19). If test light does not illuminate, go to step 20).
  17. Repair short to ground in MIL control circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  18. Repair open in VCM ignition feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  19. Check for faulty VCM ground connection at engine block. If ground is okay, go to step 9). If ground is faulty, repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  20. Repair open battery feed circuit. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
  21. Repair as necessary. Allow engine to idle until normal operating temperature is reached. If DTCs are set, refer to appropriate TESTS W/CODES article. If no DTCs are set, system is okay.
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