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Home >> Cadillac >> 2003 >> Seville STS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System & Component Testing >> Idle Control Systems >> Idle Air Control System Diagnosis >> Diagnostic Procedure
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Diagnostic Procedure

NOTE: To identify circuits, see WIRING DIAGRAMS  .
NOTE: Idle Air Control (IAC) Motor Driver (J-37027-1A) is required for this procedure.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 4.6L DEVILLE & SEVILLE article.
    2. NOTE: Ensure that the engine speed parameter stabilizes with each commanded RPM change to determine if the engine speed stays within 100 RPM less than the commanded RPM or within 100 RPM more than the commanded RPM.
  2. Set parking brake and block drive wheels. Install scan tool. Start engine. Turn all accessories off. Using scan tool RPM control function, slowly increment engine speed to 1700 RPM, then to 600 RPM, then to 1700 RPM. Exit RPM control function. Did the engine speed parameter stabilize within 100 RPM less than the commanded engine speed or within 100 RPM above the commanded RPM? If yes, see DIAGNOSTIC AIDS  . If no, go to next step.
  3. Turn ignition off. Disconnect IAC valve harness connector. Connect IAC Motor Driver to IAC valve. Start engine. Using IAC Motor Driver, command IAC valve in until about 600 RPM is reached and then command IAC valve out until about 1700 RPM is reached. Return engine RPM to desired idle as indicated on scan tool data list. Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1700 RPM when the IAC valve was commanded in and out? If yes, go to step  5 . If no, go to next step.
  4. Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver during the last test? If yes, go to step  11 . If no, go to step  12 .
  5. Using a test light connected to ground, probe one IAC valve circuit at IAC valve harness connector. Start engine. Using IAC Motor Driver, command low RPM while observing scan tool until IAC counts start to increment, then command high RPM until IAC counts start to increment. Observe test light while incrementing IAC counts. Return engine to idle as indicated on scan tool data list. Repeat procedure for other 3 IAC valve circuits. Did the test light remain ON, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the test? If yes, go to step  10 . If no, go to next step.
  6. Did the test light remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the previous test? If yes, go to step  9 . If no, go to next step.
  7. Connect a test light between IAC coil "A" low circuit and IAC coil "A" high circuit at IAC valve harness connector. See Fig 1 . Using IAC system motor driver, command low RPM while observing scan tool until IAC counts start to increment, then command high RPM until IAC counts start to increment. Observe test light while incrementing IAC counts. Return engine speed to desired idle as indicated on scan tool data list. Repeat procedure on IAC coil "B" low circuit and IAC coil "B" high circuit. Did the test light stay illuminated, never flashing, while the IAC counts where incrementing during the test? If yes, go to next step. If no, go to step  17 .
  8. Test for an intermittent and for a poor connection at the IAC valve. Repair as necessary. After repair is completed, go to step  18 . If no problem is found, go to step  13 .
  9. Turn ignition off. Disconnect PCM harness connectors. Using a DVOM, test circuit where the test light remained off for the following:
    • An open circuit.
    • A short to ground.
    • A short to another IAC valve circuit.

    Repair as necessary. After repair is completed, go to step  18 . If no problem is found, go to step  13 .

  10. Turn ignition on. Using a DVOM, test the circuit where the test light remained illuminated for the following:
    • A short to voltage.
    • A short to another IAC valve circuit.

    Repair as necessary. After repair is completed, go to step  18 . If no problem is found, go to step  13 .

  11. Visually and physically inspect for the following:
    • Throttle body damage and tampering.
    • Skewed TP sensor signal.
    • Throttle lever screw tampering (if equipped).
    • Vacuum leaks.
    • Faulty, incorrectly installed PCV valve and hose (if equipped).
    • Throttle shaft binding.
    • Throttle linkage or cruise control linkage binding (if equipped).

    Remove IAC valve. Inspect for the following:

    • Debris in IAC passage.
    • Excessive deposits on throttle plate.
    • Excessive deposits in throttle bore.
    • Excessive deposits on IAC valve pintle.

    Repair as necessary. After repair is completed, go to step  18 . If no problem is found, go to step  15 .

  12. Visually and physically inspect for the following:
    • Throttle body damage and tampering.
    • Throttle lever screw tampering (if equipped).
    • Restricted air intake system for the following conditions:
      1. A possible collapsed or clogged air intake duct, before and after the air filter element.
      2. A restricted air filter element.
      3. Restriction at the throttle body intake screen, if equipped.
    • Excessive deposits on throttle plate.
    • Excessive deposits in throttle bore.
    • Remove IAC valve and inspect for excessive deposits on IAC valve pintle and in IAC valve passage.

    Repair as necessary. After repair is completed, go to step  18 . If no problem found, go to step  15 .

  13. Check for poor PCM harness connections. Repair as necessary. After repair is completed, go to step  18 . If no problem found, go to next step.
  14. Disconnect the PCM harness connectors. Using a DVOM, test all IAC valve circuits for high resistance. Repair as necessary. After repair is completed, go to step  18 . If no problem found, go to step  17 .
  15. Check for poor IAC valve harness connection. Repair as necessary. After repair is completed, go to step  18 . If no problem is found, go to next step.
  16. Replace IAC valve. After repairs, go to step  18 .
  17. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. After repairs, go to next step.
  18. Reconnect any disconnected components. Use the scan tool to clear any DTCs that may have set. Turn OFF the ignition for 30 seconds. Operate the vehicle within conditions to verify the repair. Did you verify the repair? If yes, go to next step. If no, go to step  2 .
  19. Using a scan tool, observe the stored information in Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.6L DEVILLE & SEVILLE article. If no, system is okay.
Fig 1: Identifying IAC Valve Connector Terminals
G00115030Courtesy of GENERAL MOTORS CORP.
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