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Home >> Chevrolet >> 1998 >> Forward Control P30, 6.5 F, Standard >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 4 (Engine Controls - Tests W/Codes - 7.4L) >> Diagnostic Trouble Codes >> DTC P1508 - Idle Air Control System Low Rpm >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different variant/trim than selected.
  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK , then go to next step.
  2. Turn ignition off. Connect IAC driver to IAC valve. Apply parking brake and block wheels. Ensure A/C is off. Operate engine at idle with transmission in Park (A/T) or Neutral (M/T). Install scan tool and display RPM. Using IAC driver, extend and retract IAC valve. Observe RPM display on scan tool. If engine RPM decreases and increases as IAC is cycled, go to next step. If engine RPM does not decreases and increases as IAC is cycled, go to step 4).
  3. Observe RPM display on scan tool. RPM should change smoothly from 700 RPM to 1500 RPM with each flash of IAC driver light. If RPM changes smoothly as specified, go to step 5). If RPM does not change smoothly as specified, go to next step.
  4. Check IAC passages. If IAC passages are okay, replace IAC valve. Perform IAC reset procedure if IAC valve is replaced. See ADJUSTMENTS - 7.4L article. Go to step 12).
  5. Install IAC Node Light (J 37027-A) in VCM harness. Cycle IAC driver and observe lights. As RPM is changed over its range, both lights should cycle Green and Red, but never off. If both Green and Red lights flash on and off, go to step 8). If both Green and Red lights does not flash on and off, go to next step.
  6. Check for the following conditions:
    • Faulty connector terminal contacts.
    • Open circuits including connections.
    • Circuits shorted to ground or voltage.
    • Faulty VCM connection.

    If a problem exits, go to next step. If no problem exists, go to step 11).

  7. Repair connections and terminal contacts as necessary. After repairs, go to step 12).
  8. Using other connector on IAC driver pigtail, check resistance of IAC coils. Check resistance between IAC coil terminals "A" and "B" and terminals "C" and "D". Resistance should be 40-80 ohms. If resistance is as specified, go to next step. If resistance is not as specified, go to step 10).
  9. Check resistance between IAC coil terminals "A" and "C" and terminals "B" and "D". Resistance should be infinite. If resistance is as specified, see DIAGNOSTIC AIDS. If resistance is not as specified, go to next step.
  10. Replace IAC valve. Perform IAC reset procedure if IAC valve is replaced. See ADJUSTMENTS - 7.4L article. After replacing IAC valve, go to step 12).
  11. Replace VCM. Program replacement VCM using required equipment. After replacing VCM, go to next step.
  12. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter this DTC. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
  13. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTCs are displayed, go to applicable DTC test.
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