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Okay, now I am definately out of ideas I pulled the chip and did the resistor jumpering as suggested. This GM "in line" IAC will not make all of the steps needed to completely close the pintle even though it is indicating as it is. When I start the engine first thing it will idle up fine. It does start to lessen somewhat but not to the degree that the indicated steps would or should make the idle speed at that given point in the time. Once the engine completely warms up the engine is still idling around 1100RPM (barely a change) and the IAC steps indicates the total amount of steps has been met which I have set to 220. Yet, if I shut the engine off and restart it, it will go to the preset afterstart fast idle (which I have set to 165 steps) and the after the 7 second delay I have set it will go to the normal idle of 700RPM.
I have 4 IAC's and they all do the exact same thing!
What in the world is causing me such grief with such a stupid item as this friggin' IAC?!!
Please help me to get to the bottom of this little aggrivation...it is making me more insane than I already am...and I have yet to start fine tuning!!!
Heres my settings:
Oops, I missed that. Can you post (repost?) your MSQ and remind me of you code version, and I'll try it on the bench.
One thing I am thinking is that since your cranking position (165) is higher than most of the warm-up steps (below 93.3 degrees), the code might not like 'backing up'. I don't know if that's the case, but can check it when I have loaded your MSQ. I'm not at all sure od that, but since it works the same on different IACs, and you have looked at virtually evry software setting, there may be an issue in the code.
Here's the latest .msq file.
I am running v2.35.abs.s19 code.
BTW, I had a much more linear step table as well that had a much more gradual curve to it and it was still the same way. See the attachment.
Thanx,
Bud
That looks more like the default settings than the settings you have above.
I have tried the file you posted on the bench a couple of dozen times - it always works perfectly (2.35 code, V3 main board, MS-II v2). I tried it with the CLT low enough that the steps (75) were less than the cranking setting (100), and the stepper simply went to 75 instead of 100 and then acted normally from there.
So ther's only two real posibilities:
- something is intermittent in your wiring/jumpers, etc.
- the stepper driver chip itself has problems. This isn't impossible, and I have had this happen on one of my MS-II where one of the 4 outputs had a problem causing the motor to act strange (but fairly consistently strange). On mine, a sure way to know it was having a problem was that the stepper chip on MS-II got very hot (180°-220°F). So you might check this on yours while the stepper is moving. Also, try setting the idle control to 'IAC - moving only' as a diagnostic (this passes less current through the chip, so it should get less hot....)
Well, I went out at lunchtime and tried it again. Before I started the engine I changed the algorithm to "Moving Only"I was starting the engine that was at around 100 degreesF coolant temp. This correlated to about 156 steps. It started right up and idled around 1100 RPM. I watched the steps go up in the gauge all the way until it was out of WAE. The steps read once again 220 and the idle was right exactly where it was when I started it...no change whatsoever. I shut the engine down and fired it back up and again it went to the start steps and all the way to 220 but this time, as all other warm re-starts, it was idling normally. I was playing around with stabbing the throttle and got the engine to stumble and fall below 300 RPM and the IAC picked up again at the cranking step as if I was starting the engine and then it would go immediately to another step (say 169) and just hold there. I could get it to do this repeatedly and it would always go from the crank step to an intermediate position (and it would indicate as well) and sometimes after a while at that "intermediate" step (wherever that is coming from) it would decide to step finally to 220. I will start the fully warmed up engine, it will step to the crank position, go to the "intermediate" step...and this will very in step number too, and then go to the final step...all within about 7 seconds but I have the "crank to run" set at 7 seconds but I do not know where this now "intermediate" step is coming from. The funny thing is that this "intermediate" step is also indicating in the IAC Steps window in MegaTune as well. I played with the step time and if I go too high it will miss alot of steps as well as if I go the other way to 1.5ms. The most stable, albeit still inconsistent operation, is around 2.8ms. It seems to work much worse in "Moving Only".
I cannot feel the chip getting very warm at all and the motor is also barely warm.
Ihave reached my upload quota so I will attach a log file of the last run when this frees up..
Thanx,
Bud
I am having what I think is a very similar issue. Seems the IAC goes to the cranking value then to the step indicated in the table but will not move from there unless I restart the vehicle. I am running a 1993 Jeep IAC motor, MSII code 2.34 MT2.25. I have tried moving only and always on. Also tried different cranking values and different number of steps.
MS2 v3.0 MT2.25 controlling 6 high z injectors and ignition
What I did over the weekend is start up my truck and just let it idle. It will follow the temp curve but not to the resolution that it should. If I leave it just running the idle will come down and once it reaches operatiing temperature it still idles faster then it should normally. The steps are indicating normally but it definately does not step to that number. Like you, if I restart the engine then and only then does the pintle completely close like it should. I have tried different curves, some of them with much more aggresive slopes, some nearly linear, increase/decrease step times, change algorithm's and all with like results. These "dumb" stepper motors (without positioning encoders) really never know where they are. If steps are missed they don't know it. I have put a very high end step number of 250 as opposed to the "home" number of 205 for the hell of it to see if this would help to recover lost steps and I still get the exact same behaviour. I even reversed that order making 250 the home position....same result. I decreased the hystersis to 2 degrees...same behaviour.
I take a spare CLT sensor and hook it up to the truck, put it in ice water, wait a few minutes and start it up. The steps and idle speed correspond to the change in temperature. I remove the sensor from the water and let it start to warm up. It will follow the temp change again. I can put it back into the cold water (all the while the engine is running) and the stepper will "back up" and the speed will increase. Warm it up and it once again starts to idle down yet as always, never to the same position and I still have to re-start the engine to get it to finally idle properly. Does the IAC work? yes. Does it work properly?..not really.
Maybe someone can see if possibly there is some 'padding" that needs to be added to the code or if there is some inherent issues with the GM "in-line" IAC's running with this version of code (V2.35).
I have tried everything that I can and it is all to no avail. The chip is not hot. The IAC it not too warm. The wiring as well as the integrity from MS to the motor are perfect.
I think I have covered all of the bases that I can. I have done all of the "things to check and try" as were suggested to me and all for naught.
All of you who are using Rochester TBI's with the bolt-on "in-line" IAC's please chime in and tell me your secrets to making yours work. I have run out of ideas.
Thanx,
Bud
I forgot to mention that with our GM IAC setup we went crazy for a time with almost similar situations. His truck stock wanted to idle at 900rpm which means the OEM computer was not fully closing the IAC.
I tried to duplicate this and get it to idle at 900 based on a certain number of steps once warm. I found that it was quite unreliable, I could never get a consistent 900rpm between starts.
I believe this is similar to what your talking about.
Finally we just said heck with it and set the final step position quite a bit higher than the start position, so that it would always be fully closed. I am not worried about inconsistencies while it is warming up now, just as long as once warm it is always the same. Now every time she idles at 750rpm because its always fully closed. In the end while its warming up does it matter if one day it idled at 1100rpm and another 1200rpm? My take is only once its warm it most definitely better consistently run at 750.
I too am wondering if there is some electrical or code issue that causes it to not always move the same steps, but for us it doesn't matter as long as we know its closed when its all said and done. Oh for the record we do have the resistors jumped, made no difference....
-- 86 monte carlo, vortec 355 xe268 cam, World Class T5, hedman headers with pypes x-pipe true dual exhaust system, still carbed NEEDS megasquirt bad
I pulled the chip and did the resistor jumpering as suggested. This GM "in line" IAC will not make all of the steps needed to completely close the pintle even though it is indicating as it is. When I start the engine first thing it will idle up fine. It does start to lessen somewhat but not to the degree that the indicated steps would or should make the idle speed at that given point in the time. Once the engine completely warms up the engine is still idling around 1100RPM (barely a change) and the IAC steps indicates the total amount of steps has been met which I have set to 220. Yet, if I shut the engine off and restart it, it will go to the preset afterstart fast idle (which I have set to 165 steps) and the after the 7 second delay I have set it will go to the normal idle of 700RPM.
