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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 89 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 4.0L)DIAGNOSTIC TESTSDTC P1634: IMPROPER IGNITION 1 INPUTDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. 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, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Turn the ignition ON, with the engine OFF. Observe the ignition 1 signal with a scan tool. Is the ignition 1 signal near 12 V? If yes, go to step  4. If no, go to next step.
  3. Inspect the fuse to the ignition 1 voltage circuit. Is the fuse open? If yes, go to step  11. If no, go to next step.
  4. Turn OFF the ignition. Disconnect the PCM. Connect a test lamp between the ignition 1 voltage circuit and good ground. Turn the ignition switch to the ON and START positions. Does the test lamp illuminate with the ignition switch in the ON and START position? If yes, go to step  17. If no, go to next step.
  5. Did the test lamp illuminate for either ignition switch position? If yes, go to step  18. If no, go to next step.
  6. Turn the ignition OFF. Disconnect the ignition 1 relay from the underhood fuse block. Connect a test lamp between the battery positive voltage circuit of the ignition 1 relay and a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step  12.
  7. Connect a 5-amp fused jumper wire between the ignition 1 relay battery positive voltage circuit and the ignition 1 voltage circuit. Connect the test lamp between the ignition 1 voltage circuit at the PCM and a good ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step  10.
  8. Turn the ignition ON, with the engine OFF. Connect a test lamp between the control circuit of the ignition 1 relay and the ground circuit of the ignition 1 relay. Does the test lamp illuminate? If yes, go to step  16. If no, go to next step.
  9. Connect a test lamp between the control circuit of the ignition 1 relay and a good ground. Does the test lamp illuminate? If yes, go to step  14. If no, go to step  13.
  10. Test the ignition 1 voltage circuit for an open and for a high resistance. Repair circuit as necessary. After repairs, go to step  21.
  11. Test the ignition 1 voltage circuit and the voltage circuit of the start 1 relay for a short to ground. Did you find and correct the condition? If yes, go to step  21. If no, go to step  20.
  12. Test the ignition 1 relay battery positive voltage circuit for an open and for a high resistance. Repair circuit as necessary. After repairs, go to step  21.
  13. Test the ignition 1 relay control circuit for an open and for a high resistance. Did you find and correct the condition? If yes, go to step  21. If no, go to step  15.
  14. Test the ignition 1 relay ground circuit for an open and for a high resistance. Repair circuit as necessary. After repairs, go to step  21.
  15. Test for an intermittent and for a poor connection at the ignition switch. Did you find and correct the condition? If yes, go to step  21. If no, go to step  18.
  16. Test for an intermittent and for a poor connection at the ignition 1 relay. Did you find and correct the condition? If yes, go to step  21. If no, go to step  19.
  17. Test for an intermittent and for a poor connection at the PCM. Did you find and correct the condition? If yes, go to step  21. If no, go to step  20.
  18. Replace the ignition switch. After repairs, go to step  21.
  19. Replace the ignition 1 relay. 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. Use the scan tool in order to clear the DTCs. 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. With a scan tool, observe the stored information, Capture Info. 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.