IAC stepper motor direction question

[tonkatoy]

After 2 years of research, fabrication and assembly I finally got to the big moment and test fired my engine for the first time. After some initial problems with the firing order and EDIS plug wire layout, it started right up Although I am configured for spark knock and Idle control I had them disabled for the initial firing.
When I enabled the IAC, I observed that the stepper would close fully at key on, and then retract (the number of steps I had programmed for the start position, I assume) about 1.5 seconds after I started cranking. The MegaManual is clear that the behavior of the IAC is to open fully and then close the number of steps required and is confirmed in the forums by Grippo's quote from a post elsewhere in this forum:
Your msq says you are using a stepper motor idle control, but IACStart is 0. This variable is actually the max number of steps when the motor is fully open - it should be a number like 160 - 200 or more. You need to read up on this in the megamanual so you will be able to tune the other parameters. At startup the IAC motor should immediately retract to full open (IACstart steps) then it should close to a position as specified in your steps vs coolant temperature table. You should adjust your throttle plate so that when the motor is fully closed, the engine idles where you want it. The IAC motor then just supplioes extra air when the coolant is below operating temperature.

I've talked to a couple of GM mechanics that confirm that the IAC should home by CLOSING at key-on, and have confirmed this on my current 4.3 engine. This seems the more logical operation to my mind, by having the Idle Air bypass wide open, wouldn't the engine immediately jump in RPM when first started and then come right back to idle? In my case, that would be very bad because the primary purpose of this engine is for my rockcrawler, and we regularly re-start in gear after an engine stall when negotiating an obstacle. I guess I would like to know the reasoning behind having the IAC home in the open position vs. the closed positon before I swap wires in the IAC plug to change polarity.

Alternately, could I accomplish the same thing by inverting the values in my cold start IAC position step table? Or would MegaSquirt still be trying to run the IAC the wrong direction after warm up has ended (e.g. if the alternator load kicks up or you are steering at idle)? Any insight would be greatly appreciated.

P.S.--guess I need to update my signature line...

[Matt Cramer]

The GM mechanics are probably thinking of what a stock GM ECU would do here - MegaSquirt opens the IAC at cranking to allow in more air.

[PSIG]

... I've talked to a couple of GM mechanics that confirm that the IAC should home by CLOSING at key-on, and have confirmed this on my current 4.3 engine. This seems the more logical operation to my mind, by having the Idle Air bypass wide open, wouldn't the engine immediately jump in RPM when first started and then come right back to idle? In my case, that would be very bad because the primary purpose of this engine is for my rockcrawler, and we regularly re-start in gear after an engine stall when negotiating an obstacle. ...

If it's any consolation, my L05 ('91 350/5.7) ECU was factory configured to go Home at open, then closes on start - the opposite of what your 4.3 does. It starts that way too, with a slight <2 second rev and immediately to idle. I see how that could be bad for you, but I don't think GM ever anticipated folks bypassing neutral safety or clutch switches to start in-gear, and I think does give a more reliable and solid initial start. Anyway, since some GM IACs do start in the open mode with somewhat better results, that's probably why they configured MS to do so as well.

David

[grippo]

To make it work the way you want I believe you can reverse the direction of movement by swapping each of the 2 pairs of wires, and in addition, because the initial command is to go to 0, you need to enter all the steps as negative, so they will go from, for example, -200 to 0 at warm temp, rather than now, where they go from 0 to 200 at warm temp. This way the steps still run increasing, which makes the code happy, but the motor runs backward. If the ini file won't let you enter negative steps, you can change the file so the min value is -large no. instead of 0.

You may also need to adjust the cranking steps so that doesn't do anything you don't want

[tonkatoy]

Thanks for the help, guys.
This may all be much ado about nothing--a friend who has already done a successful MSII install told me that IAC control as implemented in the base code is only for warm-up, after coolant goes higher (and stays higher) than the fully warmed up value, there is no further IAC function. Re-reading the quote I pulled from Al, I now realize he even mentions that in his reply I assumed that it operated closed-loop fashion to hold idle to a target RPM when alternator or power steering loads vary like GM does stock. My friend did tell me that there is a code variant that DOES allow for this but he hasn't messed with it yet. Guess I need to do a little more research before I start down the tuning path. If that code does exist, it only makes sense to use it from the start.

Al-- My IAC is running backwards right now (thats what made me start this thread); when MS commands the motor to open it is actually going closed. I just bought the tool to remove the pins from the connector so I can swap them in the plug housing to get the stepper coil polarity switched. That's the easy part. I wish I understood code better; I actually used to be pretty good with computers, but that was back when a Commodore 64 was hot stuff .
As a noob, I guess I've picked a daunting project to cut my teeth on, but in the end the satisfaction will be just that much sweeter.

[msuaet]

Our team for our school project just figured out the IAC mess we had going on. Make sure under TUNING/IDLE STEPS that you specify that you want the IAC to be fully extended at warm up temp, and a value of 0 (fully retracted) should be good for the super cold extreme temp. Generate the curve under TOOLS/CURVE GENERATOR.

Another tip, get the engine warmed up, and in the idle steps table, change the current temp value (green) [should be close to fully warmed up] and change the value until you get a nice air flow for idle. We had problems with too much air being allowed in by the IAC and it was raising our RPMS and wouldn't let us idle. Also, if you close it too much it will kill the engine and also give you start up problems.....

Good luck....