I have an '86 Trans Am with a 305 TPI, and I'm having a big problem getting it to idle properly. I followed the tech article on how to set the idle and clean the IAC, but it seems that the IAC is not moving even though the computer seems to be trying to adjust it. I installed a new IAC thinking the old one went bad, but still no luck. I even pulled the IAC out and then plugged it in and turned the ignition on in diagnostic mode to watch what it does. Nothing moved, I could tell the IAC motor was getting pulses from the computer, but it wasn't moving the pintle out at all. I checked the voltage on each of the 4 wires in the IAC plug and got the following readings: .75V, 5.25V, 5.25V, and 10V, is this too low? What could possibly be wrong with it? Any help is much appreciated.

Thanks.

 

I had a similar problem, turned out to be the ecm.

 

You mean the ECM was just bad? Did it control everything else correctly, or were there some other signs it was not working correctly? From what I can tell, my ECM is controlling everything else correctly, the engine will run pretty good with just setting the idle set screw, but is a little rough when the engine is still cold, it would be running great if the IAC would take over. I'd like to find some other options to check out first, but if it turns out that it might be the ECM, do you know of any place where I could send it in to get checked out?

thanks

 

I recently got the winALDL application on my laptop and logged some data while driving. I have the raw data if anyone is familiar with how to read it, I can post it or send it to anybody that may be able to help me figure out why the IAC isn't working, and any other problems that may show up in the data.

Thanks.

 

Make sure you also adjust your TPS(throttle position sensor) to .54 volts...It could be off

 

I checked the TPS and adjusted it as necessary. I get a reading of .54V with a digital volt meter and WinALDL was showing it at .53 volts, so that is pretty much right on. I just can't figure out why the IAC pintle isn't moving when the computer seems to be trying to control it. Its a new IAC and I can feel the motor pulsing when the key is on and the computer is in diagnostic mode (ground pin B to pin A on the ALDL connector).

 

The driver transistors in the ECM may have failed, or the connections (including ground) at the ECM may be a problem. There might also be a solder connection problem on teh driver transistor leads, since they are more subject to vibration problems than other components. You'd really have to monitor the IAC outputs wioth a two-channel oscilloscope to see if the P-P voltage and phasing is correct.

 

I checked all the ECM grounds and they are fine, no resistance between ECM and battery, but unfortunately I don't have access to an oscilloscope to check the voltage. It sounds like I'll have to break down and get a new ECM? The best deal I could find is here: http://tpiparts.net/82_92_ecm_and_prom
Anybody else have a recommendation on where to get a new one?

Thanks for the help!