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.
This is my problem. I'm having to do a custom setup on my spyder intake with a holly race tb. I got a fox style iac that insted of a second hole it has a 5/8 hose fitting then I tapped the intake elbow with a 5/8 hose end. I think the iac is from a newer buillet mustang witch looks the same as a fox iac but it has the hose hookup on it to bypass the air.
I got all of it hooked up but the iab I have the plunger is open and the other fox iab I have is the same way and i can blow air throug it with no problem. Now on my 94/95 iab the plunger is closed and i can NOT blow air through it. This is with bolt of them not installed and just holding them in my hand.
So when I install the setup I want with the fox style iab my rpm's wont go below 2-3k and thats with the tb blade closed all of the way.