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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 77 (ENGINE CONTROLS SELF-DIAGNOSTICS - 6.5L)DIAGNOSTIC TESTSDTC P0380: GLOW PLUG CIRCUIT PERFORMANCE (FED)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Install the scan tool. Turn ON the ignition, with the engine OFF. Use the scan tool in order to command the glow plugs ON. Observe the Glow Plug System on the scan tool display. Does the scan tool display the Glow Plug System as Enabled for 3 seconds? If yes, go to next step. If no, go to step  8 .
  3. Turn ON the ignition, with the engine OFF. Use the scan tool in order to command the glow plugs ON. Observe the glow plugs display on the scan tool. Does the scan tool display the glow plugs at battery voltage? If yes, go to next step. If no, go to step  6 .
  4. Does the scan tool display glow plug voltage present all the time? If yes, go to step  16 . If no, go to next step.
  5. The DTC is intermittent. If no additional DTCs are stored, see DIAGNOSTIC AIDS  . If any additional DTCs are stored, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  .
  6. Disconnect the glow plug relay connector. Turn ON the ignition, with the engine OFF. With a Unpowered Test Lamp (J 34142-B) connected to ground, probe the glow plug relay harness ignition feed circuit. Does the test lamp illuminate? If yes, go to next step. If no, go to step  10 .
  7. Turn ON the ignition, with the engine OFF. Connect a J 34142-B between the glow plug harness ignition feed circuit and the harness ground circuit. Does the test lamp illuminate? If yes, go to next step. If no, go to step  11 .
  8. Turn ON the ignition, with the engine OFF. Verify that the glow plug harness is still disconnected. With a Digital Multimeter (DMM) connected to ground, probe the glow plug relay control circuit at the glow plug harness connector. Use a scan tool in order to command the glow plugs ON. Does the voltage measure battery voltage? If yes, go to next step. If no, go to step  12 .
  9. Reconnect the glow plug relay. Turn ON the ignition, with the engine OFF. With a J 34142-B connected to ground, probe the glow plug side of the relay. Use a scan tool in order to command the glow plugs ON. Does the J 34142-B illuminate when the scan tool commands the glow plugs ON? If yes, go to step  14 . If no, go to step  18 .
  10. Repair an open or a short to ground in glow plug relay ignition feed circuit. After repairs, go to step  21 .
  11. Repair any opens or poor connections in the glow plug relay ground circuit. After repairs, go to step  21 .
  12. Check the glow plug relay control circuit for an open or short to ground. If the glow plug relay control circuit is open or shorted to ground, repair as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to next step.
  13. Check the glow plug relay control circuit for a poor connection at the PCM and replace the terminal if necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  20 .
  14. Check the glow plug relay signal control circuit for an open or short to ground. If the glow plug relay signal circuit is open or shorted to ground, repair as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to next step.
  15. Check the glow plug relay signal circuit for a poor connection at the PCM and replace the terminal if necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  20 .
  16. Disconnect the glow plug relay connector. Turn ON the ignition, with the engine OFF. Probe the glow plug relay control circuit with a J 34142-B connected to ground. Does the test lamp illuminate all the time? If yes, go to next step. If no, go to step  19 .
  17. Repair the short to voltage in glow plug relay control circuit. After repairs, go to step  21 .
  18. Check for poor connections at the PCM. Repair as necessary. Did you find and correct the condition? If yes, go to step  21 . If no, go to step  20 .
  19. Replace the glow plug relay. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. After repairs, go to step  21 .
  20. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  21. Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step  2 .
  22. Observe the stored information, Capture Info with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, system is okay.
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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.