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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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