GM Full Size Vans 1987-1997 Repair Guide

Idle Air Control (IAC) Valve

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OPERATION



See Figure 1

The engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, (called counts).

This movement controls air flow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a scan tool. Zero counts corresponds to a fully closed passage, while 140 or more counts (depending on the application) corresponds to full flow.



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Fig. Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor

TESTING



See Figure 2

  1. Disengage the IAC electrical connector.
  2.  
  3. Using an ohmmeter, measure the resistance between IAC terminals A and B. Next measure the resistance between terminals C and D.
  4.  
  5. Verify that the resistance between both sets of IAC terminals is 20-80 ohms. If the resistance is not as specified, the IAC may be faulty.
  6.  
  7. Measure the resistance between IAC terminals B and C. Next measure the resistance between terminals A and D.
  8.  
  9. Verify that the resistance between both sets of IAC terminals is infinite. If the resistance is not infinite, the IAC may be faulty.
  10.  
  11. Also, with a small mirror, inspect IAC air inlet passage and pintle for debris. Clean as necessary, as this can cause IAC malfunction.
  12.  



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