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Discussion Starter #1
'90 LS with the 2,400 RPM idle problem.

My Chilton's directed me toward the IAC valve and then doing a search here shows that it could also be the TPS.

Chilton's shows a simple ohmmeter test for the IAC, but a more complicated test for the TPS. So I have decided to test the IAC first. Problem is Chilton's has a warning that states:

"Due to the diode in the solenoid, place the ohmmeter positive lead on the VPWR pin and the negative lead on the ISC pin."

OK. Which is which? Don't want to damage the IAC if it is in fact good. Any idea which pin is which?

Also, before the high idle popped up this morning, last Thursday the car started occasionally doing a kind of bucking bronco thing when I had a load on the engine. Almost felt like it was rapidly downshifting & upshifting a few times before I could back off the gas. Only did it probably 12-15 different times before that problem went away. Now the high idle problem popped up. Any correlation ya think?

Thanks for any ideas or tips.

Scott
 

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High idle is probably not the TPS. Easiest way to test the IAC is to put power and ground to the terminals and see if it moves all the way to one side, then reverse polarity and it should move to the opposite position.
 

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Discussion Starter #3
Thanks for the tip, Mike. With your suggestion I am guessing that I would have to remove the IAC valve from the car in order to run the test. Is that correct?

Also what you said about reversing the polarity so that it (whatever "it" is) moves to the other side makes me wonder about the Chilton's warning. If the valve can take 12 volts from either side, surely it could handle a resistance test even if I hook up the ohmmeter to the terminals incorrectly. Btw, I know basically squat about electricity/electronics.
 

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Yes the IAC will handle a resistance check. If you check it backwards, it will just read opened, but then if you check it the correct way, it will read properly. The reason I suggest using 12V to test it is that the resistance check is inconclusive. The IAC could pass a resistance check, but still be faulty. The 12V check will tell you for sure whether it is bad or not. You will have to remove the IAC to check it my way. I guess you could do the resistance check first, and if it passes that, pull it off and do the 12V test. As for what "it" is, when you pull the IAC off, look inside and you will see a little valve. 12V one way, the valve will be completely opened. Reverse polarity, it will be completely closed. The computer pulses the voltage going to it to keep it somewhere in the center. More on time from the computer will result in the valve being opened further and a higher idle. Less on time and it will close off some and lower the idle. Make sense?
 

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Discussion Starter #5
Very nice explanation, Mike. Makes a lot of sense and I do appreciate it a lot.

So I pulled the valve off the car and saw what "it" was. Once I saw it and looked at the manual again I figured that it was some sort of solenoid, which makes sense also. Not having read your last msg, I took a $59.99 gamble and did a resistance test not knowing if I would hurt a potentially good valve. Ended up with a 0.00 reading on the ohmmeter. I reversed the probes and got the same 0.00 reading. I haven't done your 12V test yet but I am guessing that there will be no movement of the valve/solenoid either way.

So I am guessing (tend to do a lot of that with these new fangled contraptions) that the IAC valve went completely dead and that my Cougar was probably idling and the maximum setting that the valve is mechanically calibrated for.

Let's hope this is the problem.

Man, that IAC valve has got to be the easiest part to replace on these cars. One of FoMoCo's better ideas.

I can't pick up a new valve until Thursday, but I will post my results here.

Mike, thanks again for all of your help. Where is that thumbsup smiley when I need it.

Later,

Scott
 

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If you are reading 0 ohms across the IAC, then the motor inside is shorted, and the 12V test will confirm that it won't move anywhere. Get the new IAC in and you should be good to go.
 
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