Date:         Sun, 23 Jul 2000 17:45:25 -0400
Reply-To:     David Beierl <dbeierl@attglobal.net>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@attglobal.net>
Subject:      Re: High altitude power loss
Comments: To: The Gunnings <ngunn@landmarknet.net>
In-Reply-To:  <000f01bff4e5$7bd7eda0$738c0b0c@ngunn>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 16:33 7/23/2000, The Gunnings wrote:
>but, I recently sprung for the price of a "wiring harness upgrade, genuine
>VW" from the Bus Depot.  But w/ a price of $119.95 I do not believe that I
>would recommend it to you.  It's basically nothing more than a very
>expensive 6" extension cord.  Despite it's claim to the contrary, I do not
>see how this item can be of much real use expect maybe having to do with a
>very specific problem of a lack of length on your original  wiring harness

Now, now, let's not be hasty here.  That six-inch cable contains an active
low-pass filter which limits the rate at which the AFM signal to the ECU
can change.  It was made to alleviate a bizarre scenario where the AFM
would start mechanically oscillating on a long straightaway, sending a
varying signal to the ECU which eventually would decide the AFM was nuts,
and stop listening to it.

It also can alleviate a problem with similar results, that is a noisy
(worn) AFM which sends voltage spikes to the ECU.  It will not pass the
spikes, and the result is that the ECU doesn't write the AFM off as a bad job.

*If* you don't have either of these problems, then as you say it's an
expensive extension cord.

A less elegant solution to the same problems is to put a capacitor across
the AFM signal leads, which is in fact a basic (passive) low-pass
filter.  It seems to work pretty well, and the part costs a couple of bucks
(22uF 25WV tantalum cap, available from Radio Shack special order or any
real electronics supplier).  Combined with cleaning and possibly relocating
the carbon track in the AFM, can string a worn AFM along for quite a while.

A major tipoff that this might be the problem is that it's instantly fixed
by turning key off and back on.  If it's not so fixed, then it undoubtedly
is not the trouble -- if it is, then it's a good possibility.

david
David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"