Date: Thu, 3 Jul 2014 17:08:37 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: AFM test readings
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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