DIY - the RPM change is due to the PCM noting the change in the oil temp and is 'expected'. Haven't found a "published" value but I'm pretty sure that's why the pot is in there (within easy reach).

 

I just located the relays up under the dash. I used some 18 gauge cable I had laying around for this mod and left the leads on the relays fairly long so that if I ever needed access to them (to replace them for instance) I could just pull them out from under the dash and work on them that way. I taped the two relays together with several wraps of quality electrical tape and then wire tied the cable bundle to a point behind the ash tray in the dash. This way if I ever need to get the relays out of there all I have to do is pull the ash tray out, clip a wire tie and pull them out to work on them. I actually took the more permanent route to mounting the switch and pot and very carefully drilled holes into the blank panel next to the diesel engine indicator light panel that houses the wait to start light and other diesel system indicators. I'll try to remember to snap a couple pictures tomorrow while it is light outside and post them up here. I am really pleased with how it turned out.

And yes, makomark, that was my assumption as well that the PCM was fooling with the injector mapping as the engine comes up to temperature. Like I said, it is really not a big deal to me to sit in the truck for a minute until things reach a reasonable temperature and the RPMs stabilize. Even on a cold cold morning like this one was (7 degrees when I fired the truck this morning) it only takes a minute or so for things to warm to the point where the RPMs stabilize. Albeit, the block heater was plugged in as well to preheat things.
All in all, I'm really pleased with the results. Not bad at all for a mod that costs around $30.

 

I was just curious if it changes a little like 200 rpm or a rediculous amount like a 1000 rpm.

 

If I were to start the truck up cold and set the idle to about 1000 RPM I am going to guess and say that if I were to leave it alone and not adjust the AIC again it would probably run the engine up to around 1500 RPM. Not high enough that I am concerned about damaging anything on a warm engine, but that's higher than I want it to run when cold. So I just sit there for 60 seconds or so and slowly back the pot off until the RPMs stabilize. Once it has stabilized, it really doesn't seem to waver at all.

 

Here is a picture of the finished product -- finally.

 

Has anyone tested the resistance on the pot as the RPM increases?.. It may change as it warms up too. Or is it simply that the PCM isn't in charge when warming up ?

Maybe just measure the pot resistance when set to high idle & use resisters & an on & off switch ...

TPS voltage across the rheostat is .37 volts at idle position and no more than 4.5 volts at full throttle.

THe 5 volt veh ref should never change

 

You are essentially correct in your description of the function of the circuit. .37 volts (or there about)=idle and 4.5 volts (approximately)=full throttle.

I think the problem you will see just using resistors and a switch is that you will not be able to adjust the idle speed at all. I can guarantee you that without the adjustability that the pot provides you will not be able to regulate the idle speed effectively as the engine warms up.

I am all but positive that the increase in idle speed as the truck warms up is only due to the PCM changing injector mapping as the oil temperature increases. This can be proven by the fact that after the truck is warmed up, I can switch off the AIC at a set speed, say 1000 RPM and then re-engage it later and have it go back to exactly the same idle speed as it was before. If the change in idle speed was being caused by a change in resistance in the pot as it "warms up" it would vary again after being allowed to cool by being switched off for a period of time.

Another thing is that the current being passed through this pot is such a low current that there is no "warming up" of the circuit. The way this circuit is designed the pot is essentially clamping the 5 volt input down to the point where it is putting out a much lower voltage output to regulate the idle speed. This is the .37 volts referenced in the above quote. The lower the resistance in the circuit (closer to 0 ohms which is no resistance and essentially a dead short), the higher the output voltage (or the closer to the 5 volt input voltage) and as a result, the higher the idle speed. The more resistance that is added to the circuit (the closer to the top end of the pot's 2000 ohm adjustment), the lower the resulting voltage through the circuit and thus the lower the idle speed.

The other relays in the circuit are simply there to toggle control of the engine speed between the AIC and the go fast pedal in your truck. The POT is the only thing in the circuit that determines the idle speed when the AIC is engaged.

A simple way to illustrate this is to set the AIC to a given speed and then just barely crack the accelerator pedal. by doing so you are combining the resistance of the two systems and you will see your engine speed climb accordingly.

 

I was just wondering why the high idle on my chip stays the same RPM ,all the time ?..EOT is changing , timing is changing ,Idle is not... It must be the PCM is left out of the loop ...

Or ,,the resistance is changing ...

If you were to put a broom stick on the go pedal when cold at start up . I'd bet the idle won't increase measurably ...

Some one needs to test the resistance & post the results ..

We also need to know if the timing advances when warmer or retards ...

 

I am gonna go with pcm being left out.

 

I suspect ,if that were the case , the idle would decrease .. (warm oil , slower timing )

I Also suspect , that is why our trucks stink on cold start ,too short pulse width ...(just a guess )...

 

thats what iam saying. is that the chip being installed in the pcm makes up the difference for oil temp and prevents the idle from raising. that is one reason i herd the EOT mod is a bad idea. i do not belive that there is enough resistance going through the pot of the aic mod to cause the windings to expand/contract enough to cause a rise in idle. this is just my opinion, waiting to see the outcome of this

 

My guess on the high idle on the chip is that it is working with the PCM to maintain a consistent speed. The PCM is certainly smart enough that if the chip (or the Ford AIC) instruct the chip to return an idle speed of 1000 RPM the PCM can handle that without even looking at the TPS for a pedal reference.

Remember, just about every engine parameter possible is available to the PCM and all of the injection parameters are being adjusted by the PCM. That is why one chip with a display can read out oil temp, water temp, fuel pressure, etc, etc. That's also why adding a chip can instantly increase horsepower by such a large margin. The PCM is the be-all end-all on the injection of fuel.

As "smart" as this PCM is I am sure it can maintain a set idle speed when it is told to by either a chip or a Ford AIC. The issue with the DIY AIC, if you see it as an issue, is that we are not talking to the PCM we are just fooling it into doing what we want. There is no data exchanged, we are just fudging one (well, two) of the inputs.

 

... Just wanted to make this show up in my history for future work on my 7.3... Good writeup... Does anyone know if changing the resistors ever ended up changing the Original Posters RPM level??? Sure would like to lower the rpms down to 7-1200rpm if I could.... 1200rpm seems to be the max i am comfortable (my 6.0 High Idle Mod is set to 1200 and works great!!)

Thanks and good writeup!!!!!

 

If you use the 10 turn pot that is referenced you can adjust the speed of the idle from base idle to just about whatever you want it to be. That's the route I went with mine and it works pretty well. I may be deleting the mod in the future if I get a chip. I'll just put a high idle tune on it. The idle speed on mine changes as the truck warms up which is a little annoying...

 

The instructions state to use a ten turn 20k OHM pot, I have access to lots of 10 turn 50K OHM pots at the shop I work at, will the 50K OHM just end up making it a "VERY FINE" adjustment, or will i be oprning a wholoe other can of worms...???

Also, on the 50k OHM pot, I am having isses trying to figure out which terminals will give me that 5000 OHM reading on the multimeter.. Any pointers???