missing IAC signal

Hi, I've got an 85 se 2.5 that's been giving me some trouble. It keeps setting code 22, but the tps is fine. I've tracked the problem down to the IAC, apparently one of the four wires is not giving a correct signal. When I go into diagnostic mode all terminals should be energized, and I have twelve volts on all except that one. I've traced the wire down to the ecm and found that there is only 850mV when there should be 12V. I replaced the ecm today and found my new one had the same problem, is this a common problem? And what can I do to fix it? I would greatly appreciate any help you guys can offer.

Thanks in advance,
Greg Mullis

Did you check the other obvious faults, such as a bad wire. If you're checking for voltage at the source (i.e. the contact on the ECU) then I would say its an OBD1 issue.

A lot of OBD2 ECUs take about 40-50 miles of driving to preperly calibrate themselves. At that point everything starts working "as it should."

I can't get a straight answer from ANYONE about OBD1 ECUs. However, I would imagine you'd have the same latency. Drive it for 40-50 miles, and recheck your source voltage.

Just a guess.

Well, I fixed the code 22, turned out the tps was working fine, but was just out of adjustment. I'm still not getting any signal from that one terminal to the IAC, the wire's good and I'm only getting 850mV from the ecm. I was under the impression that upon entering diagnostic mode the ecu energized all solenoids, no matter of relearn procedures. I drove it a good 50 miles anyway and retested. I'm still not getting the signal I'm looking for. Has anyone else experienced this problem? Is there anything I can do?

Thanks in advance,

Greg mullis

is the cars idle speed kind of high? cuz if so u would have a vacuum leak which could fire that 22 it has happend to me before

I fixed the code 22, the tps was out of adjustment. The Iac still isn't getting the right signal on one terminal, but it isn't setting a code. My idle is normal.

Thanks for the suggestion,
Greg M

Greg,

The IAC isn't a solenoid, it's actually a stepper motor - it actually adjusts in steps rather then just a flat on/off. The IAC has four wires .... 2 power and 2 ground. If both circuits were fully energized at the same time, the IAC would be trying to go 2 directions at once and burn out.

Ok, so why do I have 12V at three terminals and 800mV on one in diagnostic mode?

12+ is always present at the 2 positive wires, one for each direction. The ecm grounds one of the 2 other wires as needed to extend (for less air) or retract (for more air) the IAC valve. In diagnostic mode, the IAC is held fully in one direction (I'm guessing rich) Hence 12V at 2 of the wires. The 3rd wire with 12v is for the other direction, and the 4th wire the ground for that winding. The 800mv at that wire is probably no more then stray voltage picked up by the meter.

Ok, that makes sense. I had read in a shop manual that all terminals should be energized in diagnostic mode, I guess it was referring to key on engine off, out of diagnostic mode, or something like that. I don't think it would fry it if it had 12v all the way around, as long is both circuits weren't grounded at the same time.

Thanks for the help.

peace,
Greg M

Good ? for you all. Where is this mystical diagnostic port that every one is talking about.

It's right behind the panel with the cigarette lighter on the center console. You have to remove two torx screws.

peace,
Greg M

Useless trivia - The only Pontiac (with electronic fuel injection) ever built that did NOT have it's ALDL (Assembly Line Diagnostic Link) located under the driver's side dash. :)

ppreciate you.