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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 181 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 2.2L)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 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
  2. Observe the Freeze Frame/Failure Records for this DTC. Turn ignition off for 90 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See 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? If yes, go to next step. If no, problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 2.2L & 3.0L VUE article.
  3. Visually and physically inspect the ECM and engine grounds. Ensure that the grounds are clean and secure. If a condition is found, repair as necessary. Did you find and correct a condition? If yes, go to step  12. If no, go to next step.
  4. Turn ignition off. Disconnect the Manifold Absolute Pressure (MAP) electrical connector. Turn ignition on, with engine off. Measure the voltage between the 5-volt reference circuit of the MAP sensor and a good ground with a DVOM. Is the voltage 4.8-5.2 volts? If yes, go to step  6. If no, go to next step.
  5. Is the voltage greater than 5.2 volts? If yes, go to step  9. If no, go to step  7.
  6. Connect MAP sensor electrical connector. Disconnect throttle position sensor. Measure the voltage between the 5-volt reference circuit of the throttle position sensor No. 2 and a good ground with a DVOM. Is the voltage 4.8-5.2 volts? If yes, problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 2.2L & 3.0L VUE article. If no, go to step  10.
  7. Monitor the DVOM while disconnecting all other devices connected to 5-volt reference circuit 2, one at a time. If voltage changes when one of the components are disconnected, replace the affected component. Was a component replaced? If yes, go to step  12. If no, go to next step.
  8. Turn ignition off. Disconnect the ECM. Test the 5-volt reference circuit 2 for a short to ground or a short to any sensor low reference circuit. Did you find and correct a condition? If yes, go to step  12. If no, go to step  11.
  9. Turn ignition off. Disconnect the ECM. Turn ignition on, with engine off. Inspect the following circuits for a short to voltage: The MAP sensor signal circuit, Accelerator Pedal Position (APP) 1 sensor signal circuit, TP sensor No. 2 sensor signal circuit and the 5-volt reference circuit for all sensors listed. If a condition is found, repair as necessary. Did you find and correct a condition? If yes, go to step  12. If no, go to step  11.
  10. Replace MAP sensor. After replacing sensor, go to step  12.
  11. Turn ignition off. Replace ECM. Perform ECM relearn procedure. See ELECTRONIC CONTROL MODULE REPROGRAMMING  under PROGRAMMING. After replacing ECM, go to next step.
  12. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Operate vehicle within the conditions for running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate vehicle within the conditions observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  13. 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.