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Home >> Oldsmobile >> 1998 >> Regency >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control Systems Tests W/Codes >> Diagnostic Trouble Codes >> DTC P1635 - 5 Volt-Reference "A" 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 14). If no problem is found, go to next step.
  3. Using scan tool, select DTC INFO, LAST TST FAIL. If DTC P0107, P0108, P0122, P0123, P1106, P1107, P1121, P1122 and/or P1406 also set, diagnose affected DTC before proceeding. If none of these DTCs are set, go to next step.
  4. Turn ignition off. Disconnect PCM connectors. Turn ignition on. Using a DVOM, check voltage between PCM harness connector ground and 5-volt reference "A" circuits. If voltage is greater than 5.5 volts, go to next step. If voltage is not greater than 5.5 volts, go to step 9).
  5. Using DVOM, check voltage between PCM harness connector ground and 5-volt reference "A" circuits. Disconnect EGR valve. If voltage is greater than 5.5 volts, go to next step. If voltage is not greater than 5.5 volts, go to step 8).
  6. Using DVOM, check voltage between PCM harness connector ground and 5-volt reference "A" circuits while sequentially disconnecting fuel tank pressure (VIN K), MAP and TP sensors. If voltage changes whenever any of these sensors is disconnected, locate and repair short to voltage in affected sensor signal circuit and go to step 14). If voltage does not change, go to next step.
  7. Turn ignition off. Disconnect PCM harness connectors. Turn ignition on. Check 5-volt reference "A" circuit for short to voltage. Repair as necessary. After repairs, go to step 14). If circuit is okay, go to step 13).
  8. Replace EGR valve. After replacing EGR valve, go to step 14).
  9. Turn ignition off. Reconnect PCM. Disconnect TP sensor, MAP sensor, EGR valve and fuel tank pressure sensor. Turn ignition on. Connect DVOM between TP sensor 5-volt reference circuit and TP sensor ground circuit. If voltage is less than 4.5 volts, go to step 12). If voltage is not less than 4.5 volts, go to next step.
  10. With DVOM still connected to TP sensor 5-volt reference and ground circuits, reconnect (one at a time) MAP sensor, EGR valve and fuel tank pressure sensor. If voltage changes when one of the listed devices is reconnected, replace that component. After repairs, go to step 14). If voltage does not change, go to next step.
  11. Disconnect EGR valve. Connect DVOM between EGR connector 5-volt reference circuit and ground circuit. Reconnect TP sensor. If voltage measured on EGR connector changes, replace TP sensor. After replacing sensor, go to step 14). If voltage measured on EGR connector does not change, go to step 13).
  12. Turn ignition off. Disconnect PCM connectors. Turn ignition on. Check 5-volt reference "A" circuit for short to ground. Repair as necessary. After repairs, go to step 14). If circuit is okay, go to next step.
  13. Replace PCM. Program replacement PCM using required equipment. After replacing PCM, go to next step.
  14. 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 P1635. If scan tool displays DTC P1635 FAILED THIS IGN, return to step 2). If scan tool does not display DTC P1635 FAILED THIS IGN, repair is complete.
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