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Discussion Starter #1
Hey guys, been thinking about this one for a while, but I can only think so much.

Problem:
IAC stays wide open when plugged in, like the control wire to the IAC (grouded by IAC) is shorted out. BUT, when tested for continuity between the connector and any ground there is no connection. the IAC meets the specs that I found searching around FSB. With the IAC plugged in and the ignition on the IAC control wire shows no continuity to ground. I'm stumped on this one, but have a sneaking suspicion the EEC may be the culprit.

Solution:
Considering manual control of the IAC for user selectable idle speed, will allow for switched high idle (Good for crawling?) but, the question is how? If I run a toggle switch we have the on/off option which means all the way open or all the way closed (this I know we can do, because that's what it's doing right now) but, if the IAC can be controlled by varying the voltage (this is my question) with a potentiometer, then the driver can select the idle speed by a knob located on the dash.

So, what kind of signal does the IAC use? I read somewhere that it was frequency based, determining speed by pulses in voltage, but to me it seems like varying the voltage would be the way to go. Any ideas?

Cheers,
Mike
 

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yo Mike,
Here is some info that you probably know, but will post it up for basics...

Overview & Testing; "First let me say this little thing has many names. But they all talk about the same item under the hood. Here all the names I’ve had the torture of learning throughout the years: Idle Air Bypass, Idle Air Control, Idle Speed Control, Throttle Bypass Air, Idle Bypass, Inlet Air Controller, Inlet Air Bypass, Intake Air Bypass, Intake Air Control; This is really easy to test. First as with all problems you should gather the trouble-codes from the computer. Follow the codes for testing and repair. If you get a code that points to a problem with the IAB start the vehicle and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection. The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced
...
The IAB is not opened like door half way, its either open or closed completely. The EEC sends many pulses per second to flutter the IAB open and closed. It flutters so fast that you should (miesk5 Note, Ryan omitted, Not) notice any RPM fluctuation with a functional IAB. This fluttering is called duty cycle, when its calculated a percentage of on time. So when the IAB is off it is at 0%, when it is fully open it is at 100%, and any RPM in-between." Source: by Ryan M (Fireguy50)
http://web.archive.org/web/20101201013829/http://fordfuelinjection.com/index.php?p=39

some duty cycle info from the Web;
"...duty cycle controlled valve. This type of IAC is fairly simple in operation. It consists of an electric motor coil winding and operates by receiving continuous rapid pulses by the ECM having a specific duty cycle (ON TIME). The longer the duty cycle (ON TIME), the more the IAC valve opens and hence the more bypassed air that the valve lets through, therefore raising the idle speed. This type of IAC operation closely resembles that of an injector. It is referenced on a scan tool as IAC %, not count. A high percentage indicates a longer duty cycle (ON TIME)..."

"...I used a scanner to read the IAC duty cycle PID but that was verified using a Fluke 87-III duty cycle function and later with a OTC DAT. Later I bought a Fluke 189 which doesn't read DC below 10%..."



Ford IAC Bad Diode Waveform; http://web.archive.org/web/20140714133502/http://www.omitec.com/en/support/waveforms/
http://web.archive.org/web/20101201013829/http://fordfuelinjection.com/index.php?p=39

How to Test and Clean your Idle Air Controller (IAC) by Seattle FSB
http://www.fullsizebronco.com/forum/showthread.php?t=206960
 

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Yo, since you are still here; in lieu of editing my reply and you may not see it until you refresh screen; here is some more info;
"...Point of clarification: the IAC always has Vpwr (+12V) on one side. That's done so the PCM can use pulse width modulation (PWM) on the other side to augment plunger position. This type of signal is NOT readable with a traditional (D)VOM; you'd need an oscilloscope to see the pulse width to know for sure what it is. Sometimes you can get lucky and approximate the duty cycle with a voltmeter, but it's really not a good measurement.
Also, with the key on and the engine off, the PCM holds duty cycle @ 100%, i.e. it grounds the side opposite Vpwr so the valve is 100% open to aid starting...
The only reasons I can think it'd do that is either a loopy TPS, sticky throttle bores/linkage, or an IAC with broken internals (dashpot, return spring, etc).

When you tested the TPS, was it with the key on, and did you sweep through the entire range, closed to WOT? Did the voltage increase smoothly? What's the voltage when the engine's idling? Have you cleaned the throttle bores and linkages lately? SeaFoam is great for that, btw.

Also, I'm with Ryan on a lot of the diagnostic routines in the Haynes. However, the 10.5V test (KOEO) just verifies that the PCM is commanding the IAC 100% open: Vpwr on one side, ground on the other. The reason that it's not specifying ~12V is because 10.5V is the published number Ford came up with; it accounts for variants such as low battery, bad ground connections, poor wiring, corrosion in the connector, or a high-resistance connection between the DVOM and the connector & wiring. After all of that is taken into account, dropping a couple of volts isn't all that much..." by SigEpBlue
http://www.fullsizebronco.com/forum/showthread.php?t=88799
 

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Discussion Starter #4
Thanks, miesk5.

That's what I needed to know, the IAC is controlled by PWM, So now I wonder if a controller could be built with a potentiometer and a PWM module that would still allow user selectable idle?

I know enough to get myself in trouble here, anyone with knowledge of these micro electronics have any ideas?

Cheers,
Mike
 
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