Unable to make car idle properly with PWM IAC
 

I've copied DIY's idle air control valve settings number for number, tuned and tuned and tuned and tuned. Under no circumstances with their settings will my car idle at < 1500rpm. If I start to alter the settings and change the idle valve frequency from 26 to a value of 62 the car will try and idle lower. After about 5 minutes of idle, the valve starts making a very loud and progressively louder purring buzzing noise and I don't think thats good. Under the value of 26 it idles high, but doesn't make strange noises.

Both settings of 26 and 62 I have found on DIY's website I might add, thats where I got those two numbers from.

Unplugging the idle valve all together has netted me the best result so far, where I've managed to get the car to idle at a solid 930rpm with only an occasional 'bump' from a misfire.

The other problem I have is the car doesn't want to idle very well if the AFR is anything other than like 10 - 11.5:1. Starting the car and letting it complete warmup, if I start data logging and let it just sit at idle for like 20 minutes doing nothing else, run that log and burn the new map it idles like complete shit. Is that normal?

I was having the same issue with my built megasquirt controlling idle. With it controlling idle i would idle about 2400 rpms and nothing I changed lowered that. If I unplugged the wire and let nothing control idle it would idle around 900. IT really confused me. So I gave up and will keep letting the stock ecu control idle/

Well Crap. Nice to see I'm not the only one with the problem, but doesn't help me fix it. Thanks for the post anyway!

Honestly the car idles perfect without the use of the control motor anyway, which makes me wonder if I might have a slight vacuum leak? Already backed the idle screw on the throttle body 100% away from the throttle.

Really the only reason I'm interested in using idle control now that I've seen how it idles with nothing at all is my air conditioning circuit that I'm building, it'd be nice to use the control motor to help compensate for extra drag created on the engine by the compressor, and when I turn my headlights on sometimes the MASSIVE current drain from the HID lights actually ends up killing the car.

Not sure if it's relevant, but today while answering a PM I realized then when I did my original install I forgot to wire a diode across the IAC lines. I just put one in, and while I had to readjust my IAC settings somewhat, it has markedly improved the quality of my closed-loop idle. The car finally idles as stock.

!?!?

A diode across the +/- input of the valve?! Shit..

The purple box under Step 62 of the assembly instructions, entitled "For PWM Idle Valve Users Only": http://www.megamanual.com/ms2/V3assemble.htm

Quote:

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From The MegaManual:
... Do not install Q20, do not install D8, and jumper R39 as well. You will also have to put a 1N4001 diode across the PWM Idle valve itself - the banded end goes to the 12 Volt supply, the non-banded end goes to the lead that goes to MegaSquirt

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For your '95, the white/red wire is the +12 supply to the IAC, and the Blue/Orange wire goes to the ECU. The striped end of the diode would thus go towards the white/red wire.

Joe is a genius.

i ran my diode across the harness itself. It just serves in the flyback circuit. Mine goes from the idle wire on the harness to 12V on the harness. That way you dont have to worry about wiring into the factory wiring.

You are a freakin Genius Joe.

Megamanual doesn't mention anything about that in the 2.2 assembly, and I'm blind as fuck it appears..

http://www.jugrnot.com/tip120.jpg

This is the page I got the instructions from to do PWM mod.

Lemme ask you this, did you use 62 or 26 for the frequency? Not a big deal, I can experiment either way..

i am at 62 on a v3... but it needs tweaking

EDIT:

That diagram shows the diode

Didn't realize that you had a 2.2 board, my experience being only with the 3.0 and different in this regard the two would seem.

Curious that the MSNS-Extra documentation gives the advice shown in that illustration. While the 3.0 board lacks such a damping diode, a 2.2 board assembled as per instructions would already the diode have in position D9, at least insofar as this schematic is concerned: http://www.bgsoflex.com/v22/megasquirt_ShemV2.2.pdf

It seems odd that a second diode should be called for, electrically in the same position, when the larger transistor is swapped in. Yet no harm may come of trying, should the diode you install.

In my own MS I run 62 as the frequency divisor, having verified with my oscilloscope that 160 Hz (being 10000 / 62) indeed the frequency is with which the stock ECU runs the IAC. Yet I note that DIY's defaults for the '94-'95 place 26 in that position. That it might be a transposition I know not, and for this should other 1.8 owners answer.

Sorry about the lack of information, normally I'm pretty specific about everything I have when posting these questions.

Looking over the wiring diagram I'm honestly pretty confused about that myself. For shits and grins I'm gonna go ahead and install the diode on my boomslang harness and play around to see what happens. According to the wiring diagram for the car Key-on gives it +12 volts then the computer grounds it to trigger the valve action. Maybe Im misunderstanding the diagram for the megasquirt?

D9 installed should direct +12 volts away from the IDL output, if I'm reading the diagram correctly? So maybe the diode across the idle motor is for dampening purpose?

No, that's correct. The IDL solenoid is wired and driven much the same as the fuel injectors. +12 is supplied directly to one side of the solenoid, the other goes into the ECU and is switched to ground via the transistor. When the transistor is on (having a positive voltage on the base, which is pin 2 of Q5 on the R2.2) then it conducts to ground, turning the solenoid on.

Well, that's what bothers me. The function of D9, and of the other diode shown, are absolutely identical.

If we assume that current in a circuit flows from + to - (which is technically wrong, but adequate for our purposes) then power can flow through a diode only in the direction shown below, with the stripe on the symbol corresponding to the stripe on the case, both indicating the cathode side of the semiconductor:
http://img358.imageshack.us/img358/5199/diodeje4.gif

Now, take a close look at this excerpt from the R2.2 schematic:
http://img397.imageshack.us/img397/2...hematiclg1.gif
The anode (non-striped) end of the diode is connected to the collector pin of Q5 (and also the pin to which the IDL solenoid connects) and the anode end is connected to +12. This is exactly what is also shown in the TIP222 modification.


Very odd indeed. Still, no harm in fitting the external diode. Directly in the wiring is the better place for it, that voltage spikes be kept out of the case.

So I was in fact reading it correctly. Well if I get some spare time tomorrow I'll go ahead and throw that diode in and let you know what happens. Honestly I still fail to see how or what difference it'll make, but I'm willing to give it a shot! :)

Assuming that D9 is present and the circuit is otherwise properly constructed I cannot see how it would. Like I said, I brought it up assuming you had an R3. Still, there may yet be some unforeseen justification for its presence in the mod. I eagerly await news of the experiment.

The other thing to consider on copying our MSPNP settings, is you're using a different transistor to control the valve than we are with the MSPNP, and I've seen firsthand that yes that effects these settings so you won't just be able to plug in our numbers and have a near perfect idle that at most needs find tuning.

FoundSoul: I'm pretty sure the FET I ordered was the TIP120 from DIY...

So I'm curious, on the topic of idle.. If I do get the idle control working properly do you suppose it will compensate for the extra drag on the engine from running the A/C?

If not, How much software work would be involved in adding a 2 step idle control? Basically idle at like say 900 normally, but when A/C switched on it gives a switched signal to an MS Input that would switch to like a 1200rpm idle..

I noticed that you guys are talking about changing the idle valve frequency. Why would you do that? Isnt that specific to the valve itself irrespective of what transistor is being used to drive it?

I am definitely not in a position to give MS advice, but I've spent upwards of 4 hours just getting the car to idle over the weekend. I think I am close to stock quality. Here's what worked for me (car is a bone-stock 92):

Warmup only

Frequency 62

Cranking 12

Duty @ 130 F : 12

Duty @ 180 F : 22

I still havent tuned cold start so that 12 number is still as per DIY.

Another thing I noticed is that removing the entire charcoal canister and plugging the vacuum bung on the manifold helped a lot. I am running standalone with the EBC hooked up to the purge solenoid, so that was causing me some issues before I decided to ditch the entire charcoal setup altogether.

I assumed you probably were using a TIP120 as most would, and as we recommend for DIY builds generally, but that's not what we use in the MSPNP where the unit is based on the MS1v3.57 board and has a PWM capable driver on the board.

It will compensate for AC drag in a reactive fashion-- not proactive. It will wait to see RPMs fall (when the AC comes on) and then open the valve in response.

As for software work required to make it proactive-- you'd need to determine what pin you wanted to use as an input, which might (don't quote me on this) require sacrificing some other feature as the MS1 is pretty much maxed out, and add the code to control this to the firmware, and update the MT ini file to allow you to tune it.

In my experience it would be a 'nice to have' but properly setup I've always been able to get a car to idle very well with/without AC. The transitions aren't exactly like stock, but they're fine, rpms drop bit, MS notices, opens the valve a couple DC and smooths things out.

Yes. The contention seems to be whether the 1.8 NA, which has a different IAC valve than the 1.6, requires a PWM frequency of 384Hz (as indicated by the 26 value in the PNP config) or a frequency of 160Hz, as we 1.6'ers are running. It's easy to determine by looking at the valve when the stock ECU is driving it using an oscilloscope- problem is most folks don't have one. Beyond that, Jerry is correct in that having established that, the particulars of duty cycle will vary somewhat with the circuit in question. Merely adding the diode to mine brought the idle DC down from ~40 to ~23.

In a roundabout way. Assuming it's set to closed loop, when the A/C comes on it will note the drop in RPM and increase DC to compensate.

edit- looks like Jerry beat me to it. ;)