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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 59 (ENGINE CONTROLS SELF-DIAGNOSTICS - 4.3L)DIAGNOSTIC TESTSDTC P1639: 5-VOLT REFERENCE 2 CIRCUITDIAGNOSTIC 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 Sonoma, 2002 GMC Safari, 2002 GMC Jimmy, 2002 Chevrolet S10 Pickup, and 2002 Chevrolet Blazer. 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. Observe the Freeze Frame/Failure Records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC or as close to the Freeze Frame/Failure Records data that you observed. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  3. Visually and physically inspect the PCM and engine grounds. Ensure that the grounds are clean and secure. If a problem is found, repair as necessary. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  4. Turn the ignition OFF. Disconnect the Fuel Tank Pressure (FTP) sensor connector. Connect a Digital Multimeter (DMM) between the FTP sensor signal circuit and a good ground. Turn the ignition ON, with the engine OFF. Does the voltage measure near 0 V? If yes, go to next step. If no, go to step  7.
  5. Turn the ignition OFF. Disconnect the Fuel Tank Pressure (FTP) sensor connector. Connect a DMM between the 5-volt reference and a good ground. Turn the ignition ON, with the engine OFF. Does the voltage measure near 5 V? If yes, go to step  11. If no, go to next step.
  6. Does the voltage measure more than 5 V? If yes, go to next step. If no, go to step  8.
  7. Turn OFF the ignition. Disconnect the PCM and check for a short to voltage on the following circuits:
    • The 5-volt reference circuit.
    • The FTP sensor signal circuit.

    If you find a problem repair as necessary. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.

    8. NOTE: The ignition must be turned OFF for this test.
  8. Turn OFF the ignition. Using a DMM check for continuity between the 5-volt reference circuit and the low reference circuit at the FTP sensor connector. Is the resistance infinite? If yes, go to next step. If no, go to step  10.
  9. Leave the ignition OFF. Using a DMM check for continuity between the 5-volt reference circuit and a good chassis ground. Is the resistance infinite? If yes, go to step  12. If no, go to next step.
  10. Repair the circuit for a short to chassis ground or a short to FTP sensor low reference. After repairs, go to step  13.
  11. Replace the FTP Sensor. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. After repairs, go to step  13.
  12. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  13. Select the Diagnostic Trouble Code (DTC) option and the Clear DTC Information option using the scan tool. Idle the engine at the normal operating temperature. Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool. Operate the vehicle within the Conditions for Running DTC, if applicable. Does the scan tool indicate that this test ran and passed? If yes, go to next step. If no, go to step  2.
  14. 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.