IAC

That should be 12 volts, however it is a "special electricity" called "PWM" which is rapidly switched on/off to move a stepper motor (IAC) to a desired position.

And you are best to not test it removed because the pintle (thing which moves back and forth) may go flying off somewhere.

You can check to see if the IAC is getting a signal from the computer with "noid lights". Search google.com for the words...

IAC noid lights

You can also listen to it with a doctors stethoscope. The times it would move would be when first turning on the ignition, after the vehicle has warmed up, and when placing a load on the engine like turning on the A/C. Maybe turning on headlights. And possibly when turning off the engine.

Basically it would never move except to adjust the idle or to reset itself to a "known position" by moving all the way forward, then back. That would be done when first turning on the ignition or turning off the vehicle. (If the IAC is removed, then there would be nothing to stop the pintle from flying off into space during a reset.)

IAC's are "dumb" devices in that they have no way of telling the engine computer where they are positioned. So for the computer to get it to where it should be for starting the engine, it moves it all the way forward, then back so many steps.

One problem with these is the air openings around the pintle in the throttle body get clogged. Use throttle body cleaner to clean those openings out.

Check the wire going to the IAC to be sure there are no bare spots and that it is not shorting.

Also the throttle position sensor tells the engine computer when it is at idle (foot off accelerator) and the computer then takes control of idle via the IAC. If the throttle position sensor is worn and not telling the computer when it is in the idle position, then the computer would not adjust the IAC.

But on my 92, it always repositions the IAC when turning on or off the ignition (I forget which). I imagine it should still do that even if there is a problem with the throttle position sensor? I don't know?

Thanks Bill. I had read somewhere that if you took it off and just turned the key to on you would see it move. I no longer have an oscilloscope so I might have to try the noid lights.

Being as how you know how to use an oscilloscope...

The signals might come quickly, but try a digital meter on AC or better yet would be an analog meter on AC or DC.

Note the + - may reverse depending on direction.

And here is some technical info on how they work in general...

Sorry it took so long to get back to you. In the meantime I got the engine check code and it was a 43, knock sensor, so it may be something to do with that as well. The analog meter would probably be better as you could easily see the needle sweep. Thanks for the info.

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