IAC Valve ?

My daughter's 94 Ford Eplorer has too low an idle speed and stalls cold and when making left turns with the foot off the gas. I was told that the idle air control valve might be bad. I took it off and
it looked reasonably clean. Then I removed the solenoid and I could push the valve shaft and see it open. The coil measures 9.6 ohms. I assume it has to push rather than pull the valve. I put 12 volts DC on it didn't hear any snap as I expected with a solenoid slamming. After I did that I could shake the solenoid and hear something moving. How does this valve work and how does one test it? tnx
--

73
Hank WD5JFR



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The computer dithers the valve back and forth by small amounts to maintain correct idle speed. Problem happens when the valve sticks shut. Best approach IMHO is to go to a place such as Autozone, buy a new one and replace it. I cleaned this valve several times on my '99, but had longer-lasting results when I replaced it.
On Mon, 24 Oct 2005 16:34:05 GMT, "Henry Kolesnik"

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Hey Big Shoe My daughter says it hasn't stalled yet so it must have been the solenoid stuck or sticking because the valve itself was pretty clean and move freely with light pressure. I'll have to wait to see how long it lasts. Does the ditherer apply 12 volts to the coil? tnx Hank
wrote:

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Here's how it works.... but I have to warn up front that "duty cycle" and "pulse width modulation" may appear to be the same thing - they are not... It is nigh on impossible to accurately position the pintle of a solenoid anywhere in between all the way in and all the way out. To overcome this, the engineers "dither" the solenoid - turn it on and off rapidly so that the pintle moves around it's desired position. We call this "duty cycling".
Not all duty cycled solenoids are 12 volt solenoids.... some will burn out very quickly with the application of 12 volts.
Let's start with one cycle... the time period is set by the device (in this case it is the PCM) controlling the circuit. Just to make the math easy, let's say the frequency of the circuit is 100Hz (100 cycles per second). One cycle would last 10 milliseconds. At 0% duty cycle, the circuit is off all the time. At 100% duty cycle, the circuit is full on. At 50% duty cycle, the circuit would be turned on for one half of the cycle (in our example, 50 milliseconds) and off for the other half of the duty cycle.
More complications.... Some of these circuits are considered "full on" at 60% duty cycle... applying a steady 12 volts to one of these solenoids will kill the poor little beastie quite suddenly. Others that do get cycled to 100% wont last long with a steady 12 volts applied but, in their normal life, they will occasionally get 100% for very brief periods.
These types of circuits are extremely light duty.... It takes time for the magnetic field to build and decay yet we still need the action to be rapid to avoid a loping or porpoising effect (if one cycle lasts less time, it becomes easier to position the pintle more accurately but we have less time to build the magnetic field. The resulting weaker field can only move so much pintle). Since these circuits are light duty, it doesn't take much to "trap" the pintle.... very slight mechanical resistance can make these controls fail.
HTH.
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Jim Thanks for the explanation, I understand what you wrote but that leaves me with another question. The solenoid is pushing the valve stem against what I assume is a rubber diaphram at the other end. To get the proper duty cycle there would have to be some kind of feedback as I assume that the diaphram resistance could vary with temperture and age. I saw another device on the same manifold just a few inches in front of the IACV and wonder if its might be the feedback sensor? My daughter said the Explorer only stalled once and was so much better that she asked me what I did. I told her that I thought the solenoid armature was stuck. She said she could hear some noise when it wasn't working and now that it wasn't stalling she didn't hear anything. Is it possible to hear anything inside the vehicle coming from the IAC if its solenoid armature is stuck? tnx Hank

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