Afm at idle?

[betsy325e]

Dear E30ers,

I am still trying to track my high idle. From what I understand there is an adjustment screw for idle mix on the AFM. Some people refer to this as a "bypass".

My question: what does the AFM do -- if anything -- at idle? Is it counting air and adjusting fuel mix, or is it just letting a predetermined amount of air through to the throttle body/ICV and letting the ICV handle the air bypass based on predetermined specs?

Also, does anyone have ohms/voltage specifications for a properly functioning AFM? The numbers I have seen on the web are all over the goddamned place.

Thanks,

J

[jlevie]

Don't touch the idle bypass adjustment screw in the AFM. Adjusting it requires that the engine be operating properly and the CO level in the exhaust monitored with a exhaust gas analyzer.

The first thing to do in the case of a high idle is to have a smoke test run to look for intake leaks. Next the ICV needs to be cleaned and verified as to operation. Then the throttle stop and TPS may need to be adjusted. It is critical that the idle and WOT switches in the TPS function as tested for their signals at the DME connector.

[betsy325e]

Didn't play with the idle screw on the AFM -- not going to, either.
TPS works.
ICV works.
No vacuum leaks.

Not that any of that has anything to do with what I asked...

I'm asking about what -- if anything -- the AFM is doing at idle in terms of communicating with the ECU and what the ECU does with the information supplied by the AFM, and what the proper voltage/resistance readings are for a properly functioning AFM. I want to know how the system works and what's in spec.

[betsy325e]

In case anyone was curious about this, I found that the way to test this thing is to find the ratio of voltage output to voltage input across the range of the wiper. Approximately (I'm going to try to nail down these exact numbers-- will post if I find the info) .05 ratio when closed and .85 when fully open.

Don't bother mucking with the ohms -- it's all over the place. "Resistance must vary" Bentley test isn't going to tell you much of anything without voltage supplied to the AFM. Don't freak out about having to hook up the 9 volt battery -- you aren't going to break anything.

[betsy325e]

Guy on here, gregs///m just sent me a message saying that everything I've said above is wrong. Apparently the battery thing is a bad idea, just ignore everything above haha

[dereliiict]

Didn't play with the idle screw on the AFM -- not going to, either.
TPS works.
ICV works.
No vacuum leaks.

Not that any of that has anything to do with what I asked...

I'm asking about what -- if anything -- the AFM is doing at idle in terms of communicating with the ECU and what the ECU does with the information supplied by the AFM, and what the proper voltage/resistance readings are for a properly functioning AFM. I want to know how the system works and what's in spec.

Haha! OP you know more than jlevie. Keep giving us your golden advice!

[EatsHondas]

The E30 throttle position switch has 3 "settings:" idle, everything in between, and wide open throttle. In the "idle" mode, the pulsewidth-modulated IAC valve continuously changes the amount of air it lets bypass the throttle blade in order to hold a smooth idle. I'm not sure if the AFM signals normally at idle or if the ECU is programmed to ignore it.

When you're off idle but not fully WOT, the AFM sends a continuous voltage to the ECU that changes based on the amount of air flowing past the spring-loaded door. The IAT sensor is integrated into the AFM and functions 100% of the time.

At wide open throttle, the ECU ignores the AFM data from the spring-loaded door and follows a preset fuel and timing curve. The temp sensors will provide the necessary situational compensation to these curves, but since the ECU knows you have your foot to the floor, it knows almost exactly how much fuel and timing it's going to need to keep running efficiently at full load.

This is my basic understanding, if I am wrong or missed anything important, please share.

[betsy325e]

I didn't say I know more than JLevie -- I obviously don't. However, he didn't even remotely answer my question.

Do you even reading comprehension, dereliiicht?

[betsy325e]

Thanks eatshondas,

Do you know where I could read up on this?

[packratbimmer]

325e?

Check temp switch in thermostat housing:

http://www.realoem.com/bmw/enUS/showparts?id=1554-USA-11-1985-E30-BMW-325e&diagId=13_0337

15 TEMPERATURE SWITCH 0CEL 1 12631279720 $13.42

[TobyB]

the ECU ignores the AFM data from the spring-loaded door

That's what I thought too, but:

http://www.r3vlimited.com/board/showthread.php?t=361511

Varg disagrees. And I haven't dived in deep enough to see what's true.

t

[digger]

Move the flap when it's idling, if nothing happens then it's notbdoing anything and the position of the flap doesn't matter

[EatsHondas]

Thanks eatshondas,

Do you know where I could read up on this?

Haven't found many slick articles in my travels, but the E30 Bentley service manual would be a fantastic place to start. It certainly explains the systems well enough to troubleshoot and maintain them, but it doesn't get into the engineering side of things much.

I did find this old school video of K-Jetronic fuel injection, which is the predecessor to the more "modern" Motronic systems. The concepts are very similar.

Early Bosch Fuel Injection

[betsy325e]

Thanks Eats,

Definitely going to check out this video. I'm definitely interested in the engineering side of it -- I hate trying to troubleshoot when I don't know exactly how something works (especially because the test for the AFM at ECU harness is el-wrongo).

Also, just so everyone is clear, I wasn't dissing jlevie -- he's one of the few on the interwebz who really knows wtf he's talking about. Frankly, I want him to respond to all of my questions haha.

I just see a lot of people answering the question people "wished" had been asked rather than the one posed...and it makes these otherwise great bmw forum resources a little tedious to slog through.

Someone told me there's a "bosch motronic" book out there, but I haven't been able to find it. Googling yields like three pages of "download this bosch motronic engineering manual" subtext: "if you never want to be able to use your computer again."

[jlevie]

I have a PDF, written by a Porsche guy I think, that describes the operation of the AFM and how to test it. Because it is a PDF I can't post it on this site. But if you will send me a PM with your email address I will send you a copy.

I have used the test procedure in the past and it does work.