Author Topic: AFM Output Voltage Curve<=>Idle (Read 4435 times)

Asterian · « **on:** May 12, 2012, 03:08:00 AM »

Hi everybody.
Yesterday took measurements of AFM Voltage output (pin 2 and ground).
Looks like:

In Values:
RPM   U(V)
0   0,25
800   0,88-1,0
1000   1,06-1,07
1500   1,4-1,43
2000   1,62-1,64
2500   1,8
3000   2,05
6000   4,25
Reference Voltage - constant. IGN - 4.7V, when running - 4.95 V.

Does anybody have an idea if its correct?

And the major question - on idle voltage constantly pulsating from 0,88 to 1,0 V. Can it be cause of unstable idle (pulsating 750-850 rpm)?
Thanks.

DesktopDave · « **Reply #1 on:** May 12, 2012, 07:32:01 AM »

I'm no expert, but that looks like what I've been told it should. I'd say that's good enough for the computer.

In my experience a pulsating idle tends to be a dirty ICV or a vacuum leak in the intake system.

Asterian · « **Reply #2 on:** May 12, 2012, 11:57:02 AM »

Quote from: DesktopDave;112494

I'm no expert, but that looks like what I've been told it should. I'd say that's good enough for the computer.

In my experience a pulsating idle tends to be a dirty ICV or a vacuum leak in the intake system.

Thanks.
ICV was cleaned with carbcleaner, but if I would have a chance - will try to check with surely working one.
Vacuum leaks - I found just 2 small cracks on vacuum hoses - was temporary fixed with silicone.
Would like to add - Idle 750-850 - can presume, cause tacho showing just 800, but per engine sound constant change I'm sure that rpm drops/increases.

DesktopDave · « **Reply #3 on:** May 12, 2012, 01:33:49 PM »

I've noted that many M42s are somewhat noisy and rough-running, at least as far as BMW goes. I also have other theories, but I have no real proof for any of them. The first is the AFM resistor tracks and air leaks. As that resistor media wears out and the tolerances open up a bit, the DME gets inconsistent signalling and RPM wanders like that.

Secondly, M42s tend to be just as noisy as high-mileage VW & Toyota motors and a bit quieter than equivalent Hondas. I think it has something to do with the chain drive and high frequency vibration transmitted via worn hydraulic motor mounts.

bmwman91 · « **Reply #4 on:** May 12, 2012, 03:54:45 PM »

Your AFM output curve looks fine.

The M42 runs rough. If the tach needle isn't moving up & down, you are fine. When people have issues with vacuum leaks, the O2 sensor & the ICV, they are usually seeing RPM swings of +/- 100 RPM or more. If you really want to be sure of the AFM, all you need to do is open the cover & move the two little copper contacts a fraction of a millimeter so that they are wiping on a fresh part of the carbon track. You can clearly see where the track is worn and easily move the contacts to a little to get them out of there. Unplug your battery to reset the ECU, and you would be good to go. It is probably unnecessary, though.

Asterian · « **Reply #5 on:** May 13, 2012, 01:25:36 PM »

Thanks for advice. Probably I will live it like it is.
Just interesting - if we'll have AFM in 100% working condition - will voltage be constant on idle..

Asterian · « **Reply #6 on:** May 30, 2012, 11:14:03 AM »

Yesterday made test to look inside AFM when can is running at idle. In fact AFM "spade" was twitching with 2-3 mm amplitude. For sure it causes voltage change.
But question again - is this twitching normal?
Means unstable air flow (which probably normal)
or maybe something wrong with return spring...

bmwman91 · « **Reply #7 on:** May 30, 2012, 01:19:18 PM »

How fast was the wiper twitching?

Asterian · « **Reply #8 on:** May 30, 2012, 03:57:29 PM »

Relatevely "fast". Maybe few "movements" per second.

..designed to prevent vibration of flap.. - seems not fully preventing.
Will be great if somebody can have a look how it is on his engine.