Date: Thu, 3 Jul 2014 16:56:15 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: AFM test readings
In-Reply-To: <CAGwfHKScqApT+Pgs7o-OQZmHysU-0aGx=mP5Gd1Go-JE-EKRgg@mail.gmail.com>
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David's suggestion to use an analog voltmeter is a good one. Analog
meters catch glitches and dropouts that digital meters don't.
The question is -- who has a good value in an analog multimeter these
days? Simpsons were my go-tos in Ye Days of Olde when Knights Were Bold
and I had a budget.
--
Jack "Rocket j Squirrel" Elliott
1984 Westfalia, auto trans,
Bend, Ore.
On 07/03/2014 04:12 PM, Harold Teer wrote:
> 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
>>
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