The IAC is very delicate. Don't mess with it unless you have to. Here's how it works:
Have you ever seen a stepper motor? The IAC works similarly:
You have two coils, A and B, each of which has two connections. You can apply a voltage to one of the coils (or both) in two polarities, positive and negative (reverse). This will result in a magnetic field of one direction, positive/negative, forward/backward, whatever you like to call it. That's why they didn't ground one of each coils' connectors. They would save a cable, but need a negative auxiliary voltage. Instead they use two electronic switches for each coil. In a motor, you use brushes to reverse the polarity of the coils depending on the position of the rotor. On the IAC, the ECM takes care of that. By switching the coils A and B on and off and applying the right polarities at the right times, it moves the pintle in and out.
It moves the pintle to the "fully closed" position by applying a fixed number of pulses, thus "calibrating" the system. It then opens the idle air passage by stepping the pintle out until the desired RPM is met. It then controls the idle to within 50rpm by continually stepping in and out as required. If it can't do that, it sets an error code. Usually the IAC is at fault, since the ECM is pretty bullet proof (as I was told by a GM research engineer in charge of ECMs), the wiring/connector and IAC itself is more prone to problems. For instance, if the IAC's tip is dirty, it can't fully close, but the ECM thinks it's closed, thus you have excess air and possibly a lean mixture or high idle. If one of the coils is shot, the ECM will think it moved the pintle a certain number of steps, but the pintle moved less or not at all. The pintle can also get stuck.
How to test it? You can safely measure resistance and compare it to the IAC in a good vehicle. You can also ground the diagnostic pin and check for voltage on all four connector pins, the ECM should energize all solenoids when in diagnostic mode. You can look up the exact procedure in the GM shop manual. (Yep, the $75 one, but it's worth it...)
From: Oliver Scholz
Don't run out and buy an IAC yet. If memory serves me correctly, the Code 35 is set when the idle is 50 rpm or more above or below the set idle speed for 30 seconds. This is most likely caused by a vacuum leak. I would carefully inspect all hoses for cracks, holes, loose connections, or any other sign of wear. Also check around the EGR valve for vacuum leaks. Oh yeah, check the PCV valve bye taking it out and shaking it. If it doesn't rattle, replace it. If none of these fix your problem, then go get an IAC valve.
1985-88 VALVE KIT, idle air control 17111288 (no price listed)
From: Aaron Smith
If you're looking for a used one, or live somewhere where Fiero parts are expensive and hard to come by, you might want to ask/look for an IAC for a Chevy 305 or 350 TPI engine. They use the same IAC valve.
The IAC valve is easy to replace, if you have a 32mm socket (V6, may be different for four-cylinders). All you have to do is disconnect the electrical connector and unscrew it. You MIGHT be able to get an adjustable wrench in there, but there's won't be much room to work with.
Before you install the IAC, you have to make sure the pintle (the part that sticks out of the valve into the throttle body) is fully compressed. Just push in on it and wiggle it around until it won't go in any more.
Just make sure you don't over-tighten it. I was installing a used one off of a Chevy 350 TPI (same part #), and it broke off in the throttle body when tightening. We had to remove the throttle body and drill it out with a dremmel tool. I got a new one and had no problems, but I was a bit careful with tightening it. You just need to tighten it enough to seal, so that air will not leak in.
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