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Home >> Oldsmobile >> 1998 >> Regency >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control Systems Tests W/Codes >> Diagnostic Trouble Codes >> DTC P1639 - 5-Volt Reference "B" Circuit >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK . Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. After performing OBD system check, go to next step.
  2. Ensure PCM and engine grounds are secure and clean. If a problem is found, go to step 11). If no problem is found, go to next step.
  3. Ensure A/C is off. Observe A/C high side pressure on scan tool. If pressure displays a voltage of greater than 4.8 volts or less than 0.1 volt, go to DTC P0530 for diagnosis of pressure sensor circuit. If voltage displayed is not greater than 4.8 volts or less than 0.1 volt, go to next step.
  4. Unplug A/C pressure sensor connector. Using DVOM, check voltage between sensor ground circuit and sensor 5-volt reference circuit. If voltage is greater than 5.5 volts, go to next step. If voltage is not greater than 5.5 volts, go to step 7).
  5. Turn ignition off. Disconnect PCM harness connectors. Turn ignition on, engine off. Using DVOM, check voltage between PCM harness connector ground and 5-volt reference "B" circuits. If voltage is greater than 5.5 volts, go to next step. If voltage measurement is not greater than 5.5 volts, go to step 10).
  6. Locate and repair short to voltage in 5-volt reference "B" circuit. After repairs, go to step 13).
  7. If voltage is less than 4.5 volts, go to next step. If voltage is not less than 4.5 volts, go to step 9).
  8. Check for short to voltage in A/C pressure sensor signal circuit. Repair as necessary. After repairs, go to step 13). If no repairs are required, go to step 10).
  9. Ignition off. Disconnect PCM connectors. Turn ignition on. Check 5-volt reference "B" circuit for short to ground. Repair as necessary. After repairs, go to step 13). If circuit is okay, go to step 11).
  10. Turn ignition on. Ensure A/C pressure sensor is disconnected. Wait 10 seconds. Using scan tool, check if DTC P1635 is set. If DTC P1635 is set, go to next step. If DTC P1635 is not set, go to step 12).
  11. Replace PCM. Program replacement PCM using required equipment. After replacing PCM, go to step 13).
  12. Replace A/C pressure sensor. After replacing sensor, go to next step.
  13. Using scan tool, read and record FAIL RECORDS DATA, and clear DTCs. Operate vehicle within conditions noted in FAIL RECORDS data. Using scan tool, select SPECIFIC DTC INFO for DTC P1639. If scan tool displays DTC P1639 FAILED THIS IGN, return to step 2). If scan tool does not display DTC P1639 FAILED THIS IGN, repair is complete.
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