Date: Sat, 4 Nov 2006 11:41:02 -0800
Reply-To: Mark Drillock <drillock@EARTHLINK.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Mark Drillock <drillock@EARTHLINK.NET>
Subject: Re: AFM Voltage Ratios
In-Reply-To: <vanagon%2006110413151141@GERRY.VANAGON.COM>
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What standard test equipment was available at the VW dealer in 1986 that
could have been used to calibrate the AFM? They were never intended to
be field adjusted and they last a very long time. Testing them on the
engine relies on other engine parameters that can not be controlled for
with any real accuracy.
There is no reason for the Bentley to provide that information since
there is nothing one could reasonably do to accurately check it and
little you could do to change it. How would you construct a test and
what would you adjust?
While I have compared the voltage readings of several 2.1 AFMs on the
same engine to try and determine why they ran each differently, there is
not much you can do other than adjust the spring tension so they read
the same at a certain rpm while the engine is not under load. I have
done this and improved the apparent functioning of AFMs with respect to
throttle response. If the circuit board swiped surface is bad,
calibration won't help but moving the board so it swipes a different
track might. If the response rate of the AFM spring is wrong you can't
change it, only adjust it so it is closer to correct at a certain region
of operation.
Mark
Geza Polony wrote:
> This is a question I've been asking for a long time, both here and on
> westfalia.org, and no one seems to know. You're right, there must be SOME
> concrete relationship between manifold pressure and AFM output voltage, but
> no one seems to know what it is. The Bentley doesn't help, either. I took
> mine off and measured the voltage using a hair dryer to simulate the air
> flow, but that doesn't really tell you anything except that the voltage does
> change, and pretty much evenly.
>
> Without knowing what the voltage should be for a given set of AFM
> conditions, it's impossible to calibrate the AFM. And you can bet that, with
> most of these AFM's having 20-odd years on them, the voltage output "ain't
> what it used to be." That would mean that at some point in the scale of air
> flow, the engines are running too rich or too lean.
>
> Unless there's something we're not getting. Why wouldn't the voltage data be
> available?
>
> ??
>
> Geza
>
> On Fri, 3 Nov 2006 18:22:51 -0800, jon <jon@KENNEKE.COM> wrote:
>
>
>>Does anyone have data on AFM voltage (out of the resistive divider) versus
>>manifold pressure? I would think they would directly correlate in some
>>linear fashion.
>>
>>Yes, I think of these types of things on a Friday night. ;)
>>
>>Jon
>
>
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