IAC tuning. Am I crazy? (Solved!)

bluetrepidation · Post by **bluetrepidation** » Sat Apr 08, 2006 4:21 pm

OK I've been toying with this all day but here is the deal. I'm running MS II on a Ver 3 board. I'm using my stock IAC stepper motor in my 1999 Saturn SC2. This is what I have found:

Now if I set the start value to say 210 I can start the car and it jumps to my start step value of 165. Then it tapers off to my warm value of 210.

Now if I keep the same setting and restart the car sometimes it idles less. Sometimes it idles higher. I also noticed under these conditions is the stepper takes ONE step at a time rather than jumping to a value there is no change in RPM. This makes me wonder about my time step size value?

Now I slowly changed the start value by about 5 steps till I found out this. Once I get to 230 for my start and rebuild the graph's end point to match the new 230 start value (I do this for every new start value I try btw.) when MS goes from 165 steps to say 230 the engine rpm INCREASES rather than decrease. This is rather repeatable over several starts. Always an increase. This makes me wonder is the IAC motor is wired backwards?

OK guys am I thinking correctly here? I want to know I'm not going crazy?

Thanks,
A.J.

tebriel · Post by **tebriel** » Sat Apr 08, 2006 4:27 pm

just a note, not sure if it matters, but the stepper only takes it's beginning position when the car is started. so if it's a little open, and it thinks it's all the way closed it won't close all the way again until you restart the car.
-teb

bluetrepidation · Post by **bluetrepidation** » Sat Apr 08, 2006 4:29 pm

Yes I read this. If this occurs you need to increase your start step value correct?

69 1/2 Six Pack Bee · Post by **69 1/2 Six Pack Bee** » Sat Apr 08, 2006 7:57 pm

Hey, bluetrepidation,
You are'nt alone with the IAC woes.
I was having just about the exact same problems you were having (and unfortunately still have

).
Are you running the factory GM IAC stepper motor that the Saturn uses?
Maybe I can help.
....Bud

bluetrepidation · Post by **bluetrepidation** » Sun Apr 09, 2006 7:46 am

Yes I am.

Bernard Fife · Post by **Bernard Fife** » Sun Apr 09, 2006 7:55 am

bluetrepidation,

Did you jumper the IAC resistors on the bottom of MS-II as described in the manual:

For some IAC stepper motors, you might need some adjustment because the power resistor values may not be quite low enough for your application. This could cause sporadic problems. To see if this is the problem, cut the leads off a resistor or use a small piece of wire and solder it across the two power resistors on the bottom of the MegaSquirt-II board. These are the two large brown units on the BOTTOM of the board, on the end opposite the side of the board with the 2x3 pin header. These have large solder pads on each side - just jumper across each one so you get 0 Ohms and see if this improves operation. This is very likely to be the problem for those who see the proper step commands given in software but inconsistent response from the motor.

This is here: http://www.megasquirt.info/ms2/IAC.htm

You can increase the time step value as well, generally you want to keep it in the 2.0 to 4.0 range, ideally closer to 2.5 to 3.5 for most steppers.

Also, note that the step algorithm is complex, and depends on the time since start, the coolant temperature, and the rate of change of temperature. So be sure to let the engine fully warm up before drawing conclusions.

Lance.

Philip Lochner · Post by **Philip Lochner** » Mon Apr 10, 2006 1:54 am

69 1/2 Six Pack Bee wrote:Hey, bluetrepidation,
You are'nt alone with the IAC woes.....Bud

Same here. Although, I recon I'm making progress. I've got my IAC on "15minute IAC" as this overcomes to some extent what I believe is a sitcky IAC when cold. My starting value is 230 (although I could have sworn that 160 steps was enough on the bench to open if fully from fully closed...) I have DISabled the "Time based after start" by setting the "Cold temp to -20". (I'll try and figure this out later)

Then, I've got my "IAC steps" graph STARTING at the same value as the cranking position (130) which itself is a fairly "open" position as the engine will idle at about 1500rpm for one second (the "crank to run taper time") and the IAC will then jump to the position on the graph as per the CLT temp and keep idle speeds decent (around 1000rpm) until its fully warmed up(where it stops at around 175steps)

The above results in the IAC having to move ONLY in the closing direction after starting the engine

My ending value does NOT result in the IAC fully closing as my engine is designed to have the IAC control the idle at all time - which of course MS-2 does not yet do. It does seem as though I've managed to get pretty consitent idle speeds. AC makes idle speed drop much more than I'm comfortable with....

bluetrepidation · Post by **bluetrepidation** » Mon Apr 10, 2006 7:18 am

Thanks Lance I'll keep this in mind. Is this a common thing to need to do with a stock GM stepper IAC? Anyhow I got things pretty stable with my start value of 225 and a linear idle step from 90 to 200. Now should I be about to enter a new value in the idle steps section and see a result as soon as it is entered? Sometimes I can enter a new end value and I see and idle change, sometimes not.

A.J.

JSC · Post by **JSC** » Mon Apr 10, 2006 8:16 am

OK;

This IAC situation has been a thorn in my side. My MS friend here has the same problem. We are using GM IAC motors, using IAC always on, if the engine is up to temp and you have a short taper time like 1 sec. the IAC works fairly reliably. On warmup the IAC never works, have to let the engine warm up at high idle, turn it off and restart for the IAC to bring the idle down. (Sound familiar?) In an effort to resolve this situation, I purchased a UDN2916B motor driver, set it up on a breadboard with the same value components as shown on the MSII schematic, connected an IAC motor with a dial indicator on the pintle, and ran tests. RESULTS: 100% repeatability from around 2 ms to 1.5 sec step time. I did have 2 instances during some self induced eratic input pulsing where the 2916 went in to some sort of fault mode. (all input signals were normal but the outputs were turned off, power down to reset) I haven't been able to recreate this fault mode again, however, this led me to check the voltage to the IAC on the vehicle. Turned the ignition on, 11.8V on both coils, started engine (cold), waited until IAC took first steps on warmup, 0V on coils. (IAC set for always on) For some reason the 2916 motor driver is turning its outputs off. Don't know yet if it's going into some fault mode or if the MSII is turning off the IACEnbl. I'll try to do some more testing this evening.
I would be interested in some feedback. With IAC set to always on, start engine and when you think the IAC is not working, unplug the connector to IAC motor (or if you have a relay board, check at the TB) and check for voltage.

Scott

69 1/2 Six Pack Bee · Post by **69 1/2 Six Pack Bee** » Tue Apr 11, 2006 5:35 am

Please let me know what your results are as I am having those exact same issues with my IAC

If you have found a reliable "fix" then by all means PLEASE share it with me....(and the rest of the group) so I can put this IAC "thorn" in my side to bed as well.
Thanx,
Bud