AFM Resistance Readings

[Champ51]

On a friend's '91 318i which won't rev over 2500rpm once warmed, I am trying to diagnose the problem before spending money for parts.

The vane type air flow sensor reads the following resistance between pins 1 and 2 as I test it off the car.
400 ohms with vane closed,   smoothly increasing to
750 ohms at 20 degrees open,
750 ohms at 50 degrees open,
750 ohms at 70 degrees open,  and then decreasing smoothly to
350 ohms when fully open.

What have others measured?  This doesn't sound intuitively correct, but I wouldn't put it past those Bayerischen engineers to do something unusual.

The temperature resistance was spot on at 70F, and I've got exactly 5v at pin 1.

[DesktopDave]

Mine runs smoothly from about 400 ohms up to about 800 ohms, but I haven't really tested it all the way open.  One of these days I'll take it apart and test it the right way.

I do have an M20 that's all apart, if I get to that AFM I'll let you know.  They're pretty much the same part with a different plug.

I'll bet if you take the cover off you'll see that the sweep arm has eaten a path through the resistor matrix.  I'm pretty sure you can adjust the sweep, but I'd leave the tensioner spring on the clock wheel alone.  I had a heck of a time with my old E34 M20 after I tweaked the door tension...

[Champ51]

Thanks much,

After posting, and then looking at the price of a replacement ($600+), I did open the top to examine the electronics.  Everything looked clean and good inside.  The swept area didn't look like it was too worn to function properly.

I sure would appreciate if you could check yours from the M20.  I must admit this one reminds me of my old E12 which must have also been similar.

[cristimm]

On a friend's '91 318i which won't rev over 2500rpm once warmed, I am trying to diagnose the problem before spending money for parts.

The vane type air flow sensor reads the following resistance between pins 1 and 2 as I test it off the car.
400 ohms with vane closed,   smoothly increasing to
750 ohms at 20 degrees open,
750 ohms at 50 degrees open,
750 ohms at 70 degrees open,  and then decreasing smoothly to
350 ohms when fully open.

What have others measured?  This doesn't sound intuitively correct, but I wouldn't put it past those Bayerischen engineers to do something unusual.

The temperature resistance was spot on at 70F, and I've got exactly 5v at pin 1.

I think it's broken if it has the same value at 20 degrees open as 70 degrees. This can't be right.

[Champ51]

To paraphrase Despair  Inc.'s great poster:  It could be the only purpose of your post is to serve as a warning to others.

Or in the words of the lesser Homer:  "Doh".  Why am I reading resistance across the pins, when with the cover removed, I can read the actual voltage going to the computer.

Swinging the vane open yields a steadily increasing output voltage on pin 2.

So now its back to looking for the true cause of no power over 2500rpm . . . and resealing the air flow sensor's electronics.

[DesktopDave]

Well put.  Very philosophical, but old cars will do that to you.

You did check the coolant temp sensor & O2 sensor?  Both of these will signal the DME for closed-loop, and that's where your problem lies.

The Motronic is quite a bit more advanced than your (or my) old e12's Jetronic CIS...try a stomp test & see what comes up.  It might signal a failing component that ends up being a chafed wire or loose ground.

Also, check very carefully for vac leaks.  They're hard to track down but they definitely cause a lot of grief.  Motronic won't run well with a vac leak or bad IAC valve.