IAC Function , Testing ?

I understand that the ECU takes signals from other sensors and then gives the IAC a signal for setting the idle. The situation is i am having an idle issue and today i disconnected it while the jeep was on (park-idle) and nothing happened. I did this to see if it was actually actuating. I was under the impression that if i unplugged it that it would die or would die as soon as i ran the AC. Anyway no matter what i did with it disconnected it my idle was unchanged.

So my question is What is its function ?, when does it move ? and can it be tested ?

pete

anyone..

I'm not too experienced in this area, but it seems like it's not doing its job... maybe replace it?

Have you tried a search here (IAC etc.) or other sites/forums?

jeepsrock said:

I understand that the ECU takes signals from other sensors and then gives the IAC a signal for setting the idle. The situation is i am having an idle issue and today i disconnected it while the jeep was on (park-idle) and nothing happened. I did this to see if it was actually actuating. I was under the impression that if i unplugged it that it would die or would die as soon as i ran the AC. Anyway no matter what i did with it disconnected it my idle was unchanged.

So my question is What is its function ?, when does it move ? and can it be tested ?

pete

Click to expand...

The IAC is a stepper motor, it needs a up pulse to go up, or a down pulse to go down.
If you unplug it it will stay where it is.

That i understand... but if i unplug it and stress the motor then shouldnt the engines condition change ??

pete

jeepsrock said:

That i understand... but if i unplug it and stress the motor then shouldnt the engines condition change ??

pete

Click to expand...

Nope it's only for Idle, after you open the throttle it has no effect.
The throttle should be closed at idle, so idle air is controlled by the IAC, if you have the throttle open at idle then the IAC can't control the idle.

Exactly so the only time the IAC has affect is when the engine is idleing with no throttle. So if i have turned on the jeep and disconnect it and turn the AC, turn the wheel (PS pump) shouldnt the jeep die since the throttle is at 0 and its at idle being controlled by the IAC.

pete

to skip all this hypothetical stuff...

"Does anyone know how to test an IAC?"

Jesse

There is an "IAC Exerciser" that can be had, and I'd like to see just how it works so it could be duplicated cheaply. I'd imagine it would be done in a manner similar to controlling CNC stepper motors - the key would be figuring out voltage/current and pulse width specs. Then, you either supply "extend" pulses or "retract" pulses, and monitor changes in idle.

However, the IAC ceases to be a factor when the throttle valve is opened. I don't recall if it closes entirely when the throttle opens, but I don't see why it should. After all, there would be no sense in letting the throttle slam shut when you have to "panic stop." That's why the "dashpot" was added to carburettors in the late 1970's - so the throttle wouldn't slam shut and stall the engine.

With the idle air passage remaining open, you eliminate the dashpot (which makes one less part to watch and repair,) and you can simplify the system. That would also reduce the wear on the IAC, and reduce maintenance there as well.

Also, loading the engine while it's idling may stall the engine, but it's not likely with the 4.0. Many smaller engines have a pressure valve on the power steering line to "bump" the idle when you're turning without the throttle down, but the 4.0 has enough torque at idle that this isn't a problem. Turning on the A/C might present enough load to notice, but I don't think that would stall the engine either.

Make sense?

5-90

5-90 thanks for your explanation.. i guess my next question would be what happens when the IAC decides to weaken or go bad ?

pete

Lack of response, I'd guess. A "wandering idle" caused by a failure to change to respond to temperature, load, or other possible conditions.

Actually, stepper motors usually hold up very well - I don't often hear of them failing. It's the intermittent operation that saves them.

5-90