Date: Sat, 6 Mar 2010 09:40:18 -0600
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: AIR FLOW METER
In-Reply-To: <4b92554d.02c3f10a.1b12.fffff38d@mx.google.com>
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David,
The fact that the carbon resistor is laser trimmed is what I was eluding to
when I stated that relocating the wiper isn't a 100% fix. The problem is
that the missing carbon track across the carbon resistor changes the value
all the way across the sweep even when you relocate the wiper.
Another list member mentioned a megasquirt option as a total FI replacement
and I believe this to e a good option.
Thanks,
Tom Hargrave
256-656-1924
Our Web Sites:
www.kegkits.com
www.stir-plate.com
www.andyshotsauce.com
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
David Beierl
Sent: Saturday, March 06, 2010 7:15 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: AIR FLOW METER
At 02:57 AM 3/6/2010, Arkady Mirvis wrote:
>Yours is one describing the AFM eventual failure in simple terms. Thanks. I
>never looked inside the part and would think that the manufacturer had an
>obligation to have the part ( wiper ) which wears down to be a replaceable
>part. I understand that no two variable resistors can be absolutely
>identical, but I was involved in sophisticated production and dealt with
>resistors with no more than 5% difference. The design was allowing perfect
>adjustments. Hope that some heads on the list will eventually find the best
>and economical solution.
Ark, the part that wears is actually the carbon material
itself. It's a carbon film on a rectangular ceramic plate perhaps
4x6 cm, made by Bourne. Each one is laser-trimmed, so I suspect the
accuracy is 1% or better. It's possible to move the plate very
slightly (having due care for the screws which are very tight), if
necessary elongating the mounting holes a bit. This will allow the
wiper to track along an unworn path perhaps half a millimeter away
from the original and seems to work acceptably. The device functions
as a potentiometer, but the construction is more complex, with the
track being fed by multiple laser-trimmed resistors along its
length. I strongly suspect that the plates are interchangeable
amongst different AFM units of the same type, but I don't know for
sure. There are two internal adjustments in the AFM -- wiper
position and spring tension, corresponding to zero and span; but
Bosch clearly did not contemplate adjustments in the field.
The early AFMs maintained contact to the wiper by means of a
spring-loaded carbon button; later ones still have the button but
actual contact is made by a loop of flexible copper braid attached to
both parts -- apparently they had problems with the original setup.
Yours,
David
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