At 09:10 AM 7/3/2014, Harold Teer wrote:
>Thanks, this makes sense in theory. � Can you 
>walk me through how to do this without the AFM 
>connected if I don't have any fancy equipment. � 
>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