IAC problem.. Whats wrong???

First of all, the idle needs to be adjusted to where the speed is below 600 RPM with the IAC disconnected. Then the current through the IAC can vary the airfflow through it under computer control.

The IAC depends on the computer to provide a ground to make the IAC work. Bad wiring, dirty connections or a failed IAC circuit inside the computer can keep the IAC from adjusting the Idle like it should.

IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.

UnderPressureGT said:

Could it have occured during a mass air conversion???? The wires that you tap and pins that you change, do any of them have to do with the IAC???

Since iim out of sunlight over here, ill pickup on it tomorrow, but for a fact, the red wire IS putting out 12 volts to the IAC.... thats all i know for now.. What would be the enxt thing you would go about doing????? Keep in mind im not very knowledgeable about electrical soo whatever you tell me, thatd be great to explain it like you're talking to a 2 year old...LOL

thanks for the info by the way

Click to expand...

Mass air conversion has no effect or connection to the IAC circuit.

Make sure your mechanical settings on the idle speed stop screw allow the engine to idle below 600 RPM. Once you have established a baseline, we will chase other problems from that point on.

I had a similiar problem with my iac and it was caused by the blower blowing air by the valve. I drilled a 1/8" in the throttle body blade and made a block off plate for the iac.

UnderPressureGT said:

i gotcha.... i honestly cannot get it to idle that low.. 750-800 is as low as it will go without the idle starting to surge till it dies.......

with that said, i set it to 800 prior to this thread, and plugged the IAC back in and it made no change whatsoever in idle...

Click to expand...

At 800 RPM. the computer has quit providing ground signal to the IAC. It thinks the idle is high enough. Remember that the IAC adds air to increase idle speed, and cannot take away from the air that is already there. Since it cannot take away air, it cannot slow the idle down below the speed the mechanical adjustment has set.

Do the mechanical adjustment first. It may surge & try to die, but that is what the IAC is for, to add air to increase the idle speed & smooth out the idle quality.


If that does not fix it, then look for small vacuum leaks. The small hoses are the prime suspects.


Once you have eliminated vacuum leaks by checking all the connections, set the TPS.

Here’s a TPS tip I got from NoGo50…

When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks. Set it at .97v-.99v, the closer to .99v the better. Just don’t go over .99, or you upset the fuel calibration and idle quality will suffer.

(copied from MustangMax, Glendale AZ)

1. Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

2. When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

3. Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

4. Check the procedure for your year, on my 90 I have to turn the idle screw until it just touches the tab, then insert a .010 feeler gauge and give it about one more turn. Then you adjust the TPS voltage to .98v, reset the computer. Start it up, if the idle is to low then turn the screw in until it is just right, then readjust the TPS voltage to .98v and reset the computer and start it up. The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.