Check your AFM boys and girls (re. hi rpm miss)

#21

Both of the units I purchased are rebuilt/reman as Jim says. I got them from Pelican Parts. So far no issues on either one.

Julio

#22

The below thread is from e30tech. The start is a duplicate of this thread, but then it goes into some interesting insights re. fighting engine management problems.

http://www.e30tech.com/forum/showthread.php?t=110486

#23

I got a new AFM from Bimmerparts.com a few years ago to fix a lean issue. Call Chris there. I think it was $300ish but cant remember for sure. Fixed my issues.

[quote=“theShoe” post=60970]Very cool info and test Ranger. I am going to check mine. I have 3 AFMs and have seen big differences from one to another, even before they let us open them. I am going to look at this. Thanks for posting!

I heard it is impossible to find a new AFM, true? If not where can I get a new one?[/quote]

#24

It says I don’t have sufficient privileges to post/access the forums. I registered and confirmed it last week.

#25

I might have been wrong about this. The explanation is going to be tricky tho.
Cliff notes:
Bentley’s test is flat-ass wrong because it references the wrong pins.

cdonnelly’s link (www.frwilk.com/the944/afm.htm) might have the right answer, and if it does than I’m wrong too. This would mean that my AFM was ok all along. And that’s bad for me fixing the high rpm lean issue. But there’s another possibility there and I’ll get to that in another post.

I thought that Bentley had the right test idea, but had the pins wrong. The 944 guy that cdonnelly linked to said that, in contrast to the Bentley test of measuring resistance, you had to set up a reference voltage.

Using the Bentley test w/ Ranger pins resistance seemed ok until the last bit of swiper sweep so I fixed it by eliminating the last couple mm of sweep and resistance behaved predictably. It just didn’t seem necessary to set up a reference voltage and make the test needlessly more complicated.

But just for the hell of it tonight I did the 944 guy test on one of the spare AFMs that flunked the RangerBentley test. And to my surprise it passed the 944 guy test just fine. I had to fall back and think about it a bit. If you read the 944 guy’s info carefully and ponder the circuit a bit it becomes clear that it’s not about absolute resistance between a couple pins, it’s a relationship with the reference voltage that is key.

I unded the work I’d done on my AFM swiper the other day. It now flunks the Bentley (Ranger pins) test again, but it passes 944 guy’s test fine. I should have been more suspicious when all my AFMs failed the RangerBentley test.

There’s some assumptions here tho. We don’t know for sure that the AFM board and DME logic in the late 944 is the same as ours. We don’t know that 944 guy has correctly explained how to test the 944 DME. We have to assume that the Bentley not only got the pins wrong, but got the whole test protocol wrong. Maybe the Bentley test protocol works for the e model? God knows the pins specified in the Bentley aren’t ever going to be right.

Ok, so what does this mean?

  1. Print out 944 guy’s web page. It might be the right test protocol.

  2. Test your own AFM and see if it seems to adhere to the Bentley test protocol (using my pins) or the 944 guy protocol. Do this while your AFM is “known good”.

  3. 944 guy says that the laser cuts are made in the resister substrate after assembly as part of the calibration process. If this is correct, than changing the sweep range like I did hoses the calibration. I poopoo’d this based on absolute resistance, but with this voltage ratio idea, it might be right.

  4. My excitement over the possible end of high rpm lean might have been premature. And that’s a bummer.

#26

There’s another issue that I’ve come across in this project. Cliffnotes: Adjusting the AFM spring tension might be counter-intuitive. 1) AFM changes that you make at the dyno might be meaningless because you’re not giving the DME a chance to adapt to the AFM change. 2) If you want a richer mixture at WOT, the solution might be to tighten, not loosen, the spring.

Go back and read this thread. Pay particular attention to post #20. It’s the one that got me thinking. http://www.e30tech.com/forum/showthread.php?t=110486

I think that the only assumption one needs to make in order for everything else to follow is that the DME does it’s fuel trim learning at partial throttle and low-mid rpm. It is under those conditions that the DME is listening to all inputs. At WOT and high rpm the DME isn’t listening to most of the inputs so it seems to me that it would not attempt to learn under those conditions.

Hmm, ok, maybe there’s a second assumption…DME fuel trim applies at WOT and high rpm. I’ve read this a time or two in books, I’ve heard it stated by tuners and IMO it makes sense.

The purpose of the fuel trim adaption is to adjust for situations like a vacuum leak. If your car has a vacuum leak than the engine is getting more air then the AFM indicates, that is to say “unmetered air”. The O2 sensor would spot the lean mixture and increase the injector pulse width.

Here’s what might be happening in the conventional case where a well-intentioned and dashing SpecE30 type loosens his AFM spring a couple clicks thinking it will richen his mixture. God knows our engines tend to run lean.

When the SpecE30 guy was doing this he was thinking “loose spring makes it easier for air to get in, therefore increased intake flow”.

Sure, a teensy bit more air gets in. But there’s a secondary affect too, so my conjecture goes…If the AFM door’s spring tension is reduced than the AFM opens more for a given amount of air flow. This tells DME that more air is coming than is actually the case. The DME responds by providing more fuel. Then the O2 sensor sees a rich condition and the DME reduces fuel trim. Learning has occured.

From then on the A/F ratio is normal at partial throttle because the DME has adjusted fuel trim to be lean to compensate for the DME signal associated with the loosened door spring. It’s at WOT the problem occurs because the lean fuel trim is still applied.

Therefore all the attempt to richen the mixture has done is to lean out the A/F once the DME goes to internal map.

The opposite should occur if you tighten the spring. Again, it won’t be apparent at the dyno because the DME takes time to learn. But once the DME does learn that the AFM signal has become a little low, it will richen the mixture so A/F will be correct. Once that occur, the A/F at partial throttle will be right and then at WOT and high rpm the + fuel trim will carry on.

Maybe if you want to richen the mixture at WOT/high rpm, tighten your AFM spring. Personally, I wouldn’t screw with the AFM spring at all.

I think.

#27

Your AFM spring theory makes sense, now go test and report! How long does it take the DME to “learn”? Does the DME forget after losing electrical power?

#28

I don’t see how this is related to the original problem of your resistance dropping within the last few degrees of movement. Resistance and voltage are inversely related so how does your resistance drop go away when you hook up a 9v battery to it. The resistance your meter tells you is effecting voltage to the dme.

#29

With respect to adaptation…

My understanding is that fuel trim adaptation occurs when the engine is operated at part throttle (WOT switch open). From watching this on OBD II cars it takes a while, perhaps as much as 100 miles of driving. I can’t say whether an E30 takes longer or shorter because we can’t interrogate the DME for that data.

I have some evidence that suggests that learned adaptation does apply to all modes of operation. That makes since when you consider that it was designed in to account for manufacturing variations and minor wear. So when you are tweaking things on the dyno to richen up the mixture you will be fighting the desire of the DME to get back to 14:1 operation. Using an aged O2 sensor helps in this regard because it falsely indicates a leaner mixture than actually exists.

The DME in an E30 was built before the days of flash memory, so learned adaptation is lost if all power is removed from the DME.

#30

The swiper and circuit board are not just a simply potentiometer like a temp sensor. The circuit acts like a variable bridge. Look up “bridge circuit” if that doesn’t make any sense. Whereas a simple variable resister could easily change value over time, the voltage ratio is much more stable. This is because changes in resistance effect both reference voltage and measured voltage.

Go to 944 guy’s website. Measure the resistance the way I did in the first post, and then test the AFM the way 944 guy does it with a reference voltage. You’ll see the same behavior I did.

#31

I have an extra BRAND NEW afm, if anyone wants it. Shoot me a PM.

#32

Would you be willing to do both the RangerBentley test and the 944 guy test on it and tell us what you get?

#33

So first weekend in my car and it was doing exactly what Ranger described–missing from 5-6k. Unfortunately, I forgot this thread and listened to the experts who all told me it was the ignition system. After replacing every part, I decided to throw a new AFM on and problem gone. Is the old one repairable, or is it junk now?

#34

You should be able to tell if it’s working or not using the test that the 944 website talked about. It’s getting one that hasn’t had it’s spring adjuster dicked with that’s the trick. I wish I had one that I knew for a fact was never dicked with. I think that getting the spring adjuster back to it’s orig position would be hard. What we need to do is to get out hands on a new one and see if there is a way to positively identify the OEM location of the spring adjuster wheel.

I think that the Palacios have new AFMs. At CMP next month I’ll ask them if I can’t pop the top off of one and look at the adjuster wheel. There’s a little tab that holds the spring in place. By noting the location of that tab we might ID where the adjusting wheel is set at the factory.

#35

Well, don’t I know its not working already? It was clearly the problem. Can I refurbish it?

#36

Nothing is for certain. I’d test the AFM and see what happens. Whether or not you can fix it depends on what’s wrong with it I suppose. If it’s an obvious problem it might be fixable. If it’s a subtle problem that was screwing up the DME’s fuel trim then you might not even be able to detect that it has a problem.

#37

I think I’ll just trash it and buy a rebuilt spare.

#38

[quote=“cosm3os” post=65211]I think I’ll just trash it and buy a rebuilt spare.[/quote]Give it to someone coming to the SE. I’ll test it for you.

#39

[quote=“Ranger” post=65204]You should be able to tell if it’s working or not using the test that the 944 website talked about. It’s getting one that hasn’t had it’s spring adjuster dicked with that’s the trick. I wish I had one that I knew for a fact was never dicked with. I think that getting the spring adjuster back to it’s orig position would be hard. What we need to do is to get out hands on a new one and see if there is a way to positively identify the OEM location of the spring adjuster wheel.

I think that the Palacios have new AFMs. At CMP next month I’ll ask them if I can’t pop the top off of one and look at the adjuster wheel. There’s a little tab that holds the spring in place. By noting the location of that tab we might ID where the adjusting wheel is set at the factory.[/quote]

I bought the reman units that Pelican sells (over a year ago…). I think they’re rebuilt by Python. Lower cost if you send a core back so keep yours if you go that route. We haven’t touched the covers, they’re still sealed with silicon.

#40

[quote=“tpr jules” post=65218][quote=“Ranger” post=65204]You should be able to tell if it’s working or not using the test that the 944 website talked about. It’s getting one that hasn’t had it’s spring adjuster dicked with that’s the trick. I wish I had one that I knew for a fact was never dicked with. I think that getting the spring adjuster back to it’s orig position would be hard. What we need to do is to get out hands on a new one and see if there is a way to positively identify the OEM location of the spring adjuster wheel.

I think that the Palacios have new AFMs. At CMP next month I’ll ask them if I can’t pop the top off of one and look at the adjuster wheel. There’s a little tab that holds the spring in place. By noting the location of that tab we might ID where the adjusting wheel is set at the factory.[/quote]

I bought the reman units that Pelican sells (over a year ago…). I think they’re rebuilt by Python. Lower cost if you send a core back so keep yours if you go that route. We haven’t touched the covers, they’re still sealed with silicon.[/quote]
I just reread the AFM page on the 944 website and if that guy is right then it won’t do us any good to check the OEM position of the wheel. He says that the AFM’s circuit board is calibrated to the specific spring wheel setting on that AFM. That being said, if you buy my theory that started this thread, then a lean mixture at high rpm would indicate that the spring is set too loose. With a bit of trial and error, one could get their AFM spring adjuster dial in the right ballpark if they had F/A ratio info and gave the DME time to adjust fuel trim after each change to the spring adjuster.

Of course getting F/A data for most folks means multiple trips to the dyno. Or, if you’re looking for a project, you could copy my F/A meter-on-the-dash goodness.