Date:         Sat, 12 May 2001 20:54:54 -0500
Reply-To:     Larry Alofs <lalofs@enteract.com>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Larry Alofs <lalofs@enteract.com>
Subject:      Re: AFM Testing?
Comments: To: Steve Gough <vw@emriver.com>
Content-Type: text/plain; charset=us-ascii

Steve Gough wrote:

> Dear List, I'm trying to tell if I need a new air flow meter for my
> '86 Vanagon. Can anybody enlighten me on how resistance should
> changeas the vane moves?  My manuals (even Probst) are vague.I have
> two AFM's in hand.  Both show similar resistance changeswith movement,
> but one seems to have some drop-outs. Couldn't locate anything in the
> archives. thanks, Steve

Steve:
  The AFM is not a typical potentiometer.  It is a rather unique
configuration of a resistive track for the sliding contact with other
resistors attached at various points.
  Measuring the resistance from the sliding contact to either the ground
or the positive end of the array does not give a reading that increases
or decreases continuously in one direction.  Complete drop-outs
(infinite resistance) are of course a bad sign.  Otherwise it is hard to
tell anything definite by checking resistance.
  A more meaningful test is to connect a voltage, perhaps 1.5 V from a D
or C cell, to  the ends and measure the voltage from the slider to
either end, as you slowly move the vane from one extreme to the other.
The voltage should vary smoothly between zero and the 1.5 V with no
drop-outs.  Perhaps you could even simulate engine vibration by tapping
on the AFM while testing to see if there are any glitches.

Have fun,

Larry A.