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e30_afm_ljet_testing From digest.v9.n1664 Sat Dec 9 06:29:00 2000
From: Dog-n-Hog <>
Date: Sat, 09 Dec 2000 08:09:07 -0600
Subject: [Fwd: L-Jetronic (E30) Air Flow meter]

Attached is a response I sent to the original poster. I believe all info is correcte (or as close as can be expected without enough coffee in me this early in the morning:)). Hopefully this ends up in an archive for later retrieval .. ASCII art is a real pain :(.


Date: Sat, 09 Dec 2000 07:40:52 -0600
From: Dog-n-Hog <>
To: Bruce Rehm <> Subject: Re: L-Jetronic (E30) Air Flow meter


Sorry, I wasn't real clear on the topic. The four terminals are:

  • AFM "potentiometer" (+) reference terminal. This is one of the outer pins of the pot, and is supplied with power (typically some positive reference voltage) by the vehicle wiring harness.

Connect 1 side of your 1.5V battery to this terminal.

  • AFM "potentiometer" (-) ground terminal. This is the other of the outer pins of the pot, and is typically connected to ground by the vehicle wiring harness.

Connect the other side of your 1.5 battery to this terminal.

  • AFM "potentiometer" wiper terminal. This terminal is the one that supplies the flap position information to the ECM.
     Connect your (+) DVM lead to this terminal.  Connect the (-) lead
     of your DVM to either of the two terminals listed above.
  • The fourth AFM terminal is used for air temperature indication to the ECM (the AFM housing contains a temperature probe as well).

The ASCII art below details the internals of the AFM (according to the 1985 E30 BMW Electrical Troubleshoting Manual). The pinout designations are as follows:

              318 (Early)   318 (Late)    325
               Wiring        Wiring      Wiring
               Harness       Harness     Harness
               -------       -------     -------
Gnd (-)           5             5           5
Ref (+)           1             1           7
Wiper             2             7           9
Air Temp          4             4           8
Not Used          3             3           X
  Gnd (-)         Wiper          Ref (+)          Not Used
    o              o               o                o
    |              |               |                |               
    |              |               |                |       /             Air
    |              |               |                +----/\/\/\/----+---o Temp
    |              |               |                |     /         |
    |              |               |                |               |
    |              |               +----/\/\/\/-----+----/\/\/\/\---+
    |              |               |
    |              v               |
            AFM Potentiometer

I've got a 325, and I discovered that the pinout information in the Bentley manual
was incorrect (the correct wiring harness info is listed above). If you have a 318, I'm not sure what is meant by "early" vs. "late" production in in the '85 ETM. I also have not looked into the associated Bentley info, but do make certain
you are addressing the correct sections in the manual (L-Jectronic vs. Motronic).
I cannout say whether or not the listed 318 pinsout info in the Bentley or ETM manuals are correct.

The most certain way to check this is to open the cover of the AFM and check the mechanical connections/layout (the cover can be reinstalled and held down with some

Good luck, and hope this helps.

Bruce Rehm wrote:

> Many thanks for the reply. Question though: the AFM has four terminals.
> Which one are you referring to as the center terminal?
> Bruce
> -----Original Message-----
> From: Dog-n-Hog []
> Sent: Friday, December 08, 2000 12:35 PM
> To: Bruce Rehm
> Subject: Re: L-Jetronic (E30) Air Flow meter
> The Bentley test for the AFM is bullshit. The AFM uses a
> bridge network, not a simple potentiometer.
> To test it:
> o Disconnect the AFM from the vehicle's wiring harness.
> o Apply a whetting (sp?) voltage across the outer terminals

>     of the AFM "potentiometer".  A simple 1.5 volt AA battery will work
> just fine, and polarity is not important.

> o Connect the (+) terminal of a DVM (set to voltage, not continuity)

>     to the center terminal of the AFM "potentiometer", and the (-) terminal
> of the DVM to same outer terminal of the AFM to which you connected the > (-) terminal of the 1.5V battery.

> o Move the flap of the AFM from one extreme to the other, and your

>     DVM should show a smooth voltage sweep from 0.0V to 1.5V (or
> whatever voltage your small battery is).

> Norm
> Bruce Rehm wrote:
> >
> > Hi,
> >
> > I was wondering if anyone had comments/experience with the Bentley bench
> > test regarding the air flow meter for the L-Jetronic.

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