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Home >> GMC >> 2003 >> Safari Van Cargo, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 105 (Engine Control System Self-Diagnostics - 3.8L) >> Diagnostic Tests >> DTC P1639: 5-Volt Reference Circuit "B" Or No. 2 (Grand PRIX, Impala, LESABRE, Monte CARLO & Park Avenue) >> Diagnostic Procedures

Diagnostic Procedures

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  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS . After performing Diagnostic System Check - Engine Controls, go to next step.
  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 as specified in the supporting text 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  12. If no, go to next step.
  4. Turn off the ignition. Disconnect the A/C pressure 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 volts? If yes, go to step  10. If no, go to next step.
  5. Does the voltage measure more than 5 volts? If yes, go to next step. If no, go to step  7.
  6. Turn off the ignition. Disconnect the PCM and check for a short to voltage on the 5-volt reference circuit and repair as necessary. Did you find and repair a problem? If yes, go to step  12. If no, go to step  11.
    7. NOTE: The ignition must be turned OFF for this test.
  7. Turn off the ignition. Disconnect the PCM. Using a DMM check for continuity between the 5-volt reference circuit and the low reference circuit at the A/C pressure sensor connector. Is the resistance infinite? If yes, go to next step. If no, go to step  9.
  8. 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  11. If no, go to next step.
  9. Repair the circuit for a short to chassis ground or a short to A/C pressure sensor low reference. After repairs, go to step  12.
  10. Replace A/C pressure sensor. After repairs, go to step  12.
  11. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  12. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  13. Using scan tool, observe stored information. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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