Volts to the IAC

[FordRanger1991]

Hey could someone tell me what volts the ECM should be sending to the IAC during idle? Also, what kid of voltages the IAC will receive while opening the throttle and then immediately after letting the throttle return to the closed position?
I have heard around 7 volts for idle before but just was wanting to see if someone knew off the top of there head.
My IAC is only receiving 1.5 volts at idle and like 3 volts when giving it a little throttle... Seems way low...
I am trying to figure out an idle problem...
My bros truck with a 2.9 gives the IAC about 6 volts at idle and the volts will only increase after letting the throttle drop not while holding it open like mine...
Should the 4.0 IAC be receiving those same volts? Seems like it
Thanks for the time!!

 

[martin]

I don't know how you're measuring it but according to the schematic the iac receives 12v and the computer sinks the signal to control it. The iac is just a solenoid so I don't believe it's a constant variation, more than likely pulse width modulation.

 

[FordRanger1991]

Ya that makes sense. All I was doing was back probing the IAC with a voltmeter. The red wire seems to splice in with the red wires running to the injectors and some other relays. ( what the EVTM manual shows) so that would be a constant voltage supply. And the W/LB I imagine would be getting the pulses to make the IAC move from the computer. Does that make sense? I am just trying to eliminate or find the computer at fault for the idle problem. Thanks for the time martin

 

[martin]

I chased a problem with the iac on my explorer for quite some time (two or three years). After the first few weeks I removed the solenoid from the valve (leaving the solenoid plugged into the harness) and made a mechanical bye-pass valve to control the idle. I eventually nailed down the cause when the truck started throwing up the cel, it was strange really as when the light was out it would run fine but when the light was on it would run like crap. When I tried to pull codes it would do it so I decided it was time to start tracing wires and find out where the problem was. First thing was to remove the cover to access the computer, what a mess. The whole thing was covered with corrosion and upon opening the cover it was the same or worse inside, its amazing it was running at all. I replaced the computer (covered in plastic) and upon replacing the solenoid back on the iac it was back to working again. I'm not saying yours is the same but it might be an idea to take a look at the computer, the top is not sealed so any water can easily enter.

 

[FordRanger1991]

I chased a problem with the iac on my explorer for quite some time (two or three years). After the first few weeks I removed the solenoid from the valve (leaving the solenoid plugged into the harness) and made a mechanical bye-pass valve to control the idle. I eventually nailed down the cause when the truck started throwing up the cel, it was strange really as when the light was out it would run fine but when the light was on it would run like crap. When I tried to pull codes it would do it so I decided it was time to start tracing wires and find out where the problem was. First thing was to remove the cover to access the computer, what a mess. The whole thing was covered with corrosion and upon opening the cover it was the same or worse inside, its amazing it was running at all. I replaced the computer (covered in plastic) and upon replacing the solenoid back on the iac it was back to working again. I'm not saying yours is the same but it might be an idea to take a look at the computer, the top is not sealed so any water can easily enter.

Ya martin I think your onto something. I am starting to think the computer is just old and done. I looked at it and opened it up and it seemed really clean but i guess in this case there is really no way to tell if the comp is working just by looking at it...when examining the comp I looked very close at the circuit board and all the pins and fuses/relays and couldn't find any possible break or anything but i dont know..

Just for fun I will tell you what the actual problem is and what i have checked/ done to fix it (with no solution)... Also the 4.0 just got put in after a 4.0 swap.

Problem: Idle seems to be high at most times (1200-1500) and after a short drive on the freeway or highway the idle will stick at 1700...
Today for some reason it was actually idling at 700 at one point I laughed because it was so random lol

Things checked/changed...

All grounds accounted for/checked with the evtm
New fuel filter
pretty new o2 sensor
New TPS
New IAC (motocraft)
Checked the resistance from comp to the IAC 0 OHM
New pcv valve
checked voltage output of engine coolant temp sensor (good)
checked voltage output of air charge temp sensor (good)
checked voltage of the new tps (good)
checked voltage of maf (good)
cleaned the throttle body/ lubed the the springs to make sure it wasn't sticking open.
new lower middle and upper intake manifold gaskets (also new head gaskets just a side note)
the vacuum pressure is at a steady perfect 19-20 in of Mercury (i really doubt that there is a vacuum leak because that is so good but i also threw some propane in there for fun with no idle change.
The last thing I can really think it could be is just a weird/bad computer
I think I will probably go to the local junk yard and snag one for cheap and plug her in to see what happens unless anyone else has an idea? lol
Thanks for the help martin I am really thinking that is what it is...

 

[martin]

Here's a thought. You changed the iac so you should still have the old one hanging around. un-plug the one that is mounted on the motor and plug the old one in (so the computer sees an iac connected). With a warm engine the idle should be around four hundred. May not be a lot of help but if nothing has changed it would seem the problem is elsewhere.

 

[FordRanger1991]

Here's a thought. You changed the iac so you should still have the old one hanging around. un-plug the one that is mounted on the motor and plug the old one in (so the computer sees an iac connected). With a warm engine the idle should be around four hundred. May not be a lot of help but if nothing has changed it would seem the problem is elsewhere.

Well at this point anything is worth a shot lol
I will give it a go and let you know
Thanks again!

 

[FordRanger1991]

Here's a thought. You changed the iac so you should still have the old one hanging around. un-plug the one that is mounted on the motor and plug the old one in (so the computer sees an iac connected). With a warm engine the idle should be around four hundred. May not be a lot of help but if nothing has changed it would seem the problem is elsewhere.

So after the engine was warm I un-plugged the IAC and the idle did not change at all. I plugged the old IAC in and the idle didnt change.

The engine seems to be running better now for some reason but there is sill a problem. After completely warm, the rpm will sometimes actually drop down to 700 which seems to be sweet. But after giving it some gas or driving on the freeway the idle will stick around 1500 still.
I have an extra fuel pressure regulator sitting around so I think I am going to throw it in for fun since it is the only thing on the engine that I haven't changed/checked yet lol
I really cant think of anything else to try besides changing the comp... I checked all the voltage outputs from the sensors again and they seem to be perfect still.

 

[FordRanger1991]

Well changed the fuel pressure regulator for fun and no change...that was expected
I threw a different computer in there... no change.
I am out of ideas haha

 

[SenorNoob]

Just a thought, but what computer are you running? Auto or manual? I've heard that could cause it to hang high if an auto computer thinks you're in gear all the time. Reminds me. I bet in my case it's cause I don't have the neutral switch connected correctly. Computer thinks I'm in gear till I press the clutch. (I know this because I have to hold my clutch in during a codes run to not get a code.)

 

[FordRanger1991]

Just a thought, but what computer are you running? Auto or manual? I've heard that could cause it to hang high if an auto computer thinks you're in gear all the time. Reminds me. I bet in my case it's cause I don't have the neutral switch connected correctly. Computer thinks I'm in gear till I press the clutch. (I know this because I have to hold my clutch in during a codes run to not get a code.)

Well ya that was a good thought. I never even thought about that. But so ya both the computers I have tried are from manuals which is what I have.
I haven't gotten any codes with the new comp. When I run the scanner on it I don't get that code for the tranny being in gear. That seems to work fine for me.
Bringing up the Computer idea makes me wonder about some possible wire miss-placements that could have happened during the swap though... I might read through the evtms for fun this weekend just to check some of the wiring. But I doubt anything is wrong there.
Local Ford Dealer said they would run diagnostics on it and tell me what I need to fix for $185. If my problem is still here by the time summer comes around I might take them up on the offer lol
It is just funny because it seems like this problem should be so easy to fix... I have the know how to swap engines I just cant fix an idle problem

Thanks for the comments though Noob! Always appreciated

 

[cvar]

Every time my A/C compressor kicks on, my idle speed changes. Perhaps yours is sensing some perceived load that isn't really present? This post suggests A/C and Power Steering pressure switch as possible culprits.

http://www.explorerforum.com/forums/showpost.php?p=2748551&postcount=12

Hope it helps?

Also, I thought that IAC was a stepper motor, not a solenoid. So the computer has to send the IAC an exact number of pulses, to turn it an exact measured number of degrees (in tiny steps) to open/close the air-bypass port by that much.

 

[SenorNoob]

^ That's actually the most likely thing I've seen so far.

BTW, If the IAC is a stepper motor then why does it close when the power is removed from it?

Edit: Why is it most of my posts are the first one on the second page?

 

[cvar]

BTW, If the IAC is a stepper motor then why does it close when the power is removed from it?

Dunno, cuz I didn't take my old ones apart. But now I'm curious! I figure the IAC can't be a simple solenoid, cuz those would act like a toggle switch (ie, totally OPEN or totally closed). A stepper motor can be controlled by counting short pulses or by the duration of a long pulse. Isn't that how IAC is controlled?

Edit: See diagram at http://www.ford-trucks.com/forums/136188-idle-air-control-valve.html#post848276

P.S. Sorry for the thread hijack. (oops!)

 

[FordRanger1991]

Hey guys!
Thanks for the replies I have just been so busy I haven't been able to write back.
So but ya i think you guys are onto something with there being something telling the computer to idle up. This weekend I will be able to check over my evtms too look over all the items that communicate with the comp.
Last weekend was totally devoted to finishing up some work on my suspension lol