Date: Tue, 30 Sep 2003 09:52:48 -0700
Reply-To: Pensioner <al_knoll@PACBELL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Pensioner <al_knoll@PACBELL.NET>
Subject: Re: The "Other" Vanagon Syndrome, Part II
In-Reply-To: <200309300404.h8U44d64147868@vmj-ext.prodigy.net>
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Date: Mon, 29 Sep 2003 18:45:51 -0600
From: tom ring <taring@TARING.ORG>
Subject: Re: The "Other" Vanagon Syndrome, Part II
I am less experienced than many on the list when it comes to the AFM, but I
am
experienced in electronic parts wearing out in general.
~~~~~~
Tom's pretty much dead on track with his analysis below. You can induce VS
by injecting noise on the AFM harness. If you have the capacitor (read
noise filter) installed it just takes more ( higher amplitude ) noise to
cause the "problem".
The AFM has a limp mode. When critical signals are too noisy or out of
range it defaults to a 'limp' rather than a 'run' Since it samples the
signals quickly it will self correct if the signal "gets better" and go into
'run' again. Hence the stumblestutter.
You can generally correct VS by gently moving the contact to a less worn
part of the resistive surface even if you don't have the factory capacitor
installed.
The track changes resistance as it wears eventually having minute gaps as
Tom mentions. The evidence can be captured on a storage oscilloscope set to
trigger on a fast rise time waveform.
~~~~~~~
The AFM doesn't have anything magical in it as far as I can tell. It is a
fairly interesting version of a variable resistor, commonly called a
potentiometer or "pot", with an airflow measuring door and a spring with an
adjustable preload. Add the temp sensor to the mix and that's it.
The pot is probably the only thing that wears significantly in any of them.
It
has two connection points on the pickup, which presumably the designers did
to
make it more reliable. It probably doesn't, as both points will wear at the
same rates.
The problem with all these pot based systems is that we tend to run the
engine
at just a few airflow rates for most of our driving, which causes most of
the
wear to occur at just a few fairly narrow points on the range of pot. For
the
type of driving vanagons drivers do, mostly at one or two speed points, I'd
bet.
When visually inspected, the pot may look good, but can really have very
small
dropout points within the most used areas. I think, personal guess here,
that
one thing the capacitor fix does is allow the AFM to hit a bad spot, have
the