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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 370 (Engine Control System - Self-Diagnostics - 4.2L) >> Diagnostic Tests >> DTC P1635: 5-Volt Reference "A" Circuit >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: If a cranks but will not run condition is present, proceed to next step.
  2. Observe the Freeze Frame/Failure Records data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate vehicle within the Conditions For Running DTC or as close as possible to the Freeze Frame/Failure Records data that you observed. See CONDITIONS FOR RUNNING DTC . Does the DTC fail this ignition cycle? If yes, go to next step. If no, problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 4.2L BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article.
  3. Turn ignition off. Disconnect the throttle body electrical connector. Turn ignition on, with engine off. Measure the voltage from the 5-volt reference "A" circuit of the Throttle Position (TP) sensor 1 to a good ground with a DVOM. Is the voltage about 5 volts? If yes, go to step  5. If no, go to next step.
  4. Is the voltage more than 5 volts? If yes, go to step  8. If no, go to step  6.
  5. Connect the throttle body electrical connector. Disconnect the AC pressure sensor. Measure the voltage from the 5-volt reference circuit of the AC pressure sensor to a good ground with a DVOM. Is the voltage about 5 volts? If yes, go to step  10. If no, go to step  9.
  6. Monitor the DVOM while disconnecting all other devices connected to the 5-volt reference "A" circuit one at a time. If voltage changes when one of the components are disconnected, replace component. See REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. Was a component replaced? If yes, go to step  12. If no, go to next step.
  7. Turn ignition off. Disconnect PCM. Test the 5-volt reference "A" circuit for a short to ground or for any sensor low reference circuit. 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  10.
  8. Turn ignition on, with engine off. Test the following circuits for a short to voltage:
    • The 5-volt reference "A" circuit.
    • The FTP sensor signal circuit.

    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  10.

  9. Replace throttle body assembly. See REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. After repairs, go to step  12.
  10. Test the 5-volt reference "A" circuit for a short to the TAC motor control 1 or 2 circuits. Did you find and correct a condition? If yes, go to step  12. If no, go to next step.
  11. Replace PCM. See REMOVAL & INSTALLATION - BRAVADA, ENVOY, ENVOY XL, TRAILBLAZER & TRAILBLAZER EXT article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  12. Clear the DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Operate vehicle within the Conditions For Running DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  13. Observe the stored information, Capture Info with scan tool. Does 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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