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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 260 (ENGINE CONTROLS - SELF-DIAGNOSTICS)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 is about a different car, the 2003 Cadillac Seville and 2003 Cadillac DeVille. 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 for this DTC. Turn off the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  3. Turn ignition off. Disconnect the Air Conditioning (A/C) pressure sensor. Turn ignition on, with engine off. Measure the voltage between the A/C pressure sensor 5-volt reference circuit and a good ground. Is the voltage 4.8-5.2 volts? If yes, go to next step. If no, go to step  5.
  4. Connect the A/C pressure sensor. Disconnect the Fuel Tank Pressure (FTP) sensor. Measure the voltage between the FTP sensor 5-volt reference circuit and a good ground. Is the voltage 4.8-5.2 volts? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to step  12.
  5. Is the voltage measured in the previous step more than 5.2 volts? If yes, go to step  8. If no, go to next step.
  6. Monitor the DVOM while disconnecting the FTP sensor. Does the voltage return to 4.8-5.2 volts when the FTP sensor is disconnected? If yes, go to step  11. If no, go to next step.
  7. Turn ignition off. Disconnect the PCM. Test the 5-volt reference circuit for a short to ground or to any sensor low reference circuit. Did you find and correct the condition? If yes, go to step  14. If no, go to step  13.
  8. Monitor the DVOM while disconnecting the FTP sensor. Does the voltage return to 4.8-5.2 volts when the FTP sensor is disconnected? If yes, go to step  10. If no, go to next step.
  9. Test all 5-volt reference circuits for a short to voltage. Did you find and correct the condition? If yes, go to step  14. If no, go to step  13.
  10. Test the FTP sensor signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  14. If no, go to step  13.
  11. Replace the FTP sensor. See FUEL TANK PRESSURE SENSOR under SENSORS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to step  14.
  12. Replace the A/C pressure sensor. See A/C REFRIGERANT PRESSURE SENSOR under REMOVAL & INSTALLATION in appropriate AUTOMATIC A/C-HEATER SYSTEMS article in HVAC. Did you complete the replacement? If yes, go to step  14.
  13. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to next step.
  14. Clear the DTCs with a scan tool. Turn off the ignition for 30 seconds. Start the engine and operate vehicle within the Conditions For Running DTC. You may also operate vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition cycle? If yes, go to step  2. If no, go to next step.
  15. Observe the Capture Info with a scan tool. Are there any DTCs that have not been 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.