Thanks for the explanations.  The decriptions as a potentiometer makes it
clearer to me.  What I still don't understand is, why the problem only
manifests after sustained cruising.  Why doesn't the van always stumble when
the AFM crosses the "bad contact" point?  I cruise control to work for 20
minutes and have never had the problem.  Yet, after a few hours of similar
cruising on the road trip, it happened.  Not that any of this theory
matters, really - just curious.

James
90 Carat

On 9/30/03 9:52, "Pensioner" <al_knoll@PACBELL.NET> wrote:

> 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