David,

Thanks for a great and detailed explanation on the AFM that even I
understand.  Between the information from you, Karl, and Neil, this thread
will be a wonderful resource in the archives for understanding the AFM
better.

Have a great weekend and 4th of July,

Harold

Harold Teer
1991 Westy -- VANGAUX
Harrisonburg, VA

On Thu, Jul 3, 2014 at 5:08 PM, David Beierl <dbeierl@attglobal.net> wrote:

> At 09:10 AM 7/3/2014, Harold Teer wrote:
>
>> Thanks, this makes sense in theory. Â Can you walk me through how to do
>> this without the AFM connected if I don't have any fancy equipment. Â Do
>> have analog and digital meters so have that part covered.
>>
>
> To do it with the AFM in the van, follow the procedure in tencentlife's
> video.  Pull the boot off the connector, stick your meter probes in as he
> shows, turn on the ignition and follow his procedure of very slowly pushing
> the AFM vane open with a stick of some sort.  You have to be slow enough
> that the meter has time to respond to any dropouts.  Notice he's using a
> fairly small meter with a fairly quick-responding needle.  If you have a
> larger one that responds very slowly (like a couple of Radio Shack meters I
> have) you have to move the vane slow enough that you can see the needle
> dip.  Any dip at all means a problem at that point on the track, you don't
> have to wait for it to fall to zero; you should see the voltage only rising
> as you push the vane open.
>
> To do it using dry cells or a power supply (and having been reminded by
> Neil's photo of the resistor plate itself, I expect a nine-volt square
> battery would be fine and maybe easier to hook up to), hook up the battery
> to the same pins the ECU would use.  I don't have access to a Bentley right
> now but according to the video the AFM pins for the waterboxers are:
>
> 1 - Temp I hot
> 2 - AFM wiper
> 3 - AFM + supply
> 4 - System ground for both AFM and Temp I
>
> So you'd hook B+ to pin 3 and B- to pin 4.  You'd hook meter + to pin 2
> and meter - to pin 4.
>
> Don't use your digital meter, it will only cause you grief.  It probably
> only takes 2-3 readings per second, and the reading is averaged over the
> sampling time which is less than half of the half- or third-second cycle
> time.  If your meter has a quick-responding bar graph display you might
> manage; but the analog meter responds in real time and is always reading
> the line.  Even if the needle response is slow it will be easier to deal
> with than the digital (may or may not be easier than the bar graph display.
>
> If for some reason you do use the digital, set it on a fixed range, as you
> don't want it switching ranges while you're testing.
>
> Set your meter range as low as practical that's at least half the input
> voltage.  As tcl says that will cover the part of the AFM track that gets
> the wear.
>
> FYI, if you look at Neil's photo of an uninistalled resistor plate:
> https://lh6.googleusercontent.com/-c5axVMck2Ks/T-tDy8qL_MI/
> AAAAAAAAF2o/Cly2OSeHmUk/s912/AFM%2520Board%2520Wear.jpg
>
> At the bottom there's the Bourns name and Bosch symbol, indicating a
> proprietary part made for Bosch.  8712 means it was made in week 12 of
> 1987.  The two crosses and two triangles I imagine are registration test
> marks for the several layers of stuff that get printed onto the ceramic
> substrate.  The spot on the left-hand triangle may be an inspector's mark.
>  The long number is Bourns' part number for the plate, the 1 probably means
> revision 1 of the device, and the reversed C looks like a test resistor to
> validate the resistor printing process for quality control
>
> Above that is the wiper track. The right side is the ground side and the
> left side is hot, corresponding to the rotation of the vane in the AFM.
>
> Now look at the very top.  The two bright dots are where the spring
> contacts have burnished the silver paint that forms the conductors.
>  Theleft one is + and the right one -.  Following them, you'll see that
> each connects to a green patch with lines on it.  Those are resistors, and
> the lines are where the trimming laser burned away parts of the resistor to
> increase its resistance to the correct value.
>
> From the other end of the resistor conductors go to the two ends of the
> wiper track.  If they were the only connections you'd be able to do this
> test with an analog ohmmeter instead of a voltmeter.  But as you can see
> there is also a resistor that stretches the entire length of the track,
> with nine connections into the track and laser trims between each pair of
> connections.  That's what's driving your ohmmeter nuts.  It's a voltage
> divider that's feeding an offset voltage into the track at those nine
> points to adjust the wiper response.  When Bourns builds the plate the
> laser trimmer has point contacts that leave the tiny bright mark on the
> little side tabs coming away from the various resistors, and it trims the
> resistors in a particular sequence until all the values are correct.  It
> looks as though it does a roughing pass and then a finishing pass to take
> care of interaction between the voltage divider and the two main input
> resistors.  Then they ship it to Bosch who glue it onto the metal mounting
> plate.
>
> While I'm on the subject, in tcl's video the long copper arm is carries
> the contact from the AFM connector to the wiper.  It has a carbon button in
> the round end ant there's a flat spring with a bump on it that comes up
> from the vane/wiper assembly and presses on it.  However apparently there
> were difficulties with this, because by at least late '83 Bosch had added a
> braided copper flex cable to bypass this button and make a solid connection.
>
> Yours,
> David
>