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Thread: Anyone have information to bench test the AFM?

  1. #1
    Join Date
    Jun 2015
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    Wilmington, NC, USA
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    91 735, 84 733, 84 633

    Anyone have information to bench test the AFM?

    '84 733 AFM was replaced with a new one many years ago but only has about 20K on it.
    The only test I can find in Bentley is for two of the harness connections.
    Surely there is a way to test the AFM with a meter at the connector on the AFM with 4 - pins?

    Thanks, Kenneth aka John
    '84 733
    91 735

  2. #2
    Join Date
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    1983/84/85 745i
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  3. #3
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    Testing the AFM

    Quote Originally Posted by Hit Man X View Post
    Excellent! Thank you. Hit Man X This is exactly what I need. Also:

    " UP must never decrease as the door is opening not even for a moment. This is called non-monotonicity and can drive your DME into oscillations "

    Part of the problem I'm having is some kind of oscillation at idle so the AFM could be the culprit.

    How does the black plastic cover on the bottom come off?

    Thanks, Kenneth aka John

  4. #4
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    You have to break the seal, pry it open gently.

  5. #5
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    A few comments about this test procedure

    Looking at the test procedure, (see picture) the battery looks like it is connected: –ve to terminal 6 (ground) and +ve to terminal 9 (input voltage from DME).
    So if you connect your black voltmeter probe to ground and red to terminal 7 (output voltage). it looks like you can test your AFM without removing the black plastic cover.
    Not saying I wouldn’t remove the cover to see how everything looks inside but, this looks like a useful quick check on the car.

    Interesting . . . . Bentley says you can’t test the AFM other than the harness terminal test.

    Haines manual says to check the resistance which this Bosch procedure says doAFM.jpgesn’t work.

  6. #6
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    Here is how I tested my AFM

    Picture shows my test setup. (See the link above from Hit Man X) which is excellent.

    It is not necessary to find 2mm spade terminals. The colored test leads shown in the picture have small insulated alligator clips and connect with the terminals in the AFM just fine. They are available from Radio Shack for a few $ (if you can find a Radio Shack).

    The most difficult part of the procedure is moving the "door" in the AFM smoothly through the whole range. My fingers are not long enough to do it and when you push it with a pencil, it is likely to slip. The signal appears to be damped because the volt meter reading was a lot smoother than my input.

    Result was fine, no irregularities in the voltage which was somewhat expected as the AFM does not have a lot of miles on it. So my quest to keep this car out of Pick and Pull has reached another dead end.

    This test could easily be done on the car but be careful. The pictures in Hit Man's link and in my picture attached all show the AFM upside down. If you test in the car it will be the other way up and the terminals will be in the opposite order.

    John aka Kenneth
    AFM Test 03-17.jpg

  7. #7
    Join Date
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    1983/84/85 745i

    Thumbs up

    Hey, some progress is good progress.

    Have you ever pinned out your engine harness?
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  8. #8
    Join Date
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    91 735, 84 733, 84 633

    I agree, here is some more recent information . . . .

    People on the boards are probably fed up of seeing my ramblings about this problem. Since the earlier cars have no on board diagnostics I am trying to use the few clues I have to figure this out. Here are my recent thoughts, we seem to be thinking in along the same track and your encouragement is much appreciated.

    Summary with update:
    1984 733 Auto 125K Car had been running OK for about 12 months.
    Driving normally around town then suddenly started running very rough at stop light.

    Car will start with clean plugs, idles very rough (see video) and fouls all 6 spark plugs.
    If you accelerate to about 2000 rpm it will run on all 6. Plugs are reasonably clean if you shut it off immediately. Car will actually drive if you manipulate the gas pedal. Runs rough at lower RPM then smooth and quiet with a light load at about 2200 rpm. Won’t accelerate much faster than this, “misfires” and “pops” and runs rough, my guess is now too lean at higher RPM.

    Summary:

    • Way too rich at idle with no load.
    • OK at 2000 – 2200 with light load.
    • Too lean? At higher RPM and/or heavier load.

    So fuel delivery is “stuck” for some reason and only correct for throttle position at 2000 – 2200 and light load. So what does this mean?
    The ECU is not getting the information?
    AFM mechanicals seem to be clean and smooth operating. I replaced AFM with new one probably over 10 years ago but only about 20K on it.
    The car leaks (I think they all do) so the environment is not healthy for electronics. I have yet to check ECU (Have one, NOS to swap out) connections and harness.

    Thanks for all the help, John aka Kenneth

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