Date: Tue, 30 Sep 2003 14:59:54 -0700
Reply-To: James V <tornadored@EARTHLINK.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: James V <tornadored@EARTHLINK.NET>
Subject: Re: The "Other" Vanagon Syndrome, Part II
In-Reply-To: <001401c38773$47343b20$4f0afea9@pacbell.net>
Content-type: text/plain; charset="US-ASCII"
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
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