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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 (BONNEVILLE & Regal) >> 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 FREEZE FRAME/FAILURE RECORDS data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC or as close to FREEZE FRAME/FAILURE RECORDS data as possible. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  3. 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  17. If no, go to next step.
  4. Turn ignition off. Disconnect A/C pressure sensor connector. Connect 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 next step. If no, go to step  6.
  5. Turn ignition off. Reconnect A/C pressure sensor. Disconnect FTP sensor. Connect a DMM between the 5-volt reference at the FTP sensor connector and a good ground. Does the voltage measure near 5 volts? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to step  13.
  6. Does the voltage measure more than 5 volts? If yes, go to next step. If no, go to step  8.
  7. Turn ignition off. Leave DMM connected as in step  4. Disconnect PCM. Turn ignition on, engine off. Does the voltage measure zero volts? If yes, go to step  15. If no, go to step  12.
  8. Turn ignition off. Leave DMM connected from step  4. Disconnect FTP sensor. Turn ignition on, engine off. Does the voltage measure near 5 volts? If yes, go to step  14. If no, go to next step.
    9. NOTE: The ignition must be turned off for this test.
  9. Turn ignition off. Using DMM, check continuity between the 5-volt reference circuit and the low reference circuit of the A/C pressure sensor or the FTP sensor. Is the resistance infinite? If yes, go to step  11. If no, go to next step.
  10. Leave ignition off. Using DMM, check continuity between the 5-volt reference circuit and a good chassis ground. Is the resistance infinite? If yes, go to step  15. If no, go to next step.
  11. Repair the circuit for a short to chassis ground or a short to the FTP or A/C pressure sensor low reference circuits. After repairs, go to step  17.
  12. Locate and repair the short to voltage in the 5-volt reference circuit or the FTP signal circuit. After repairs, go to step  17.
  13. Replace A/C pressure sensor. After repairs, go to step  17.
  14. Replace FTP sensor. After repairs, go to step  17.
  15. Check the 5-volt reference circuit for an open, short to chassis ground or poor connection at the PCM. Did you find a problem? If yes, go to step  17. If no, go to next step.
  16. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  17. 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.
  18. 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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