Date:         Tue, 31 Dec 96 09:47:19 -0600
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Rocko Flamefart <-@--.-->
Subject:      Air Flow Meters <sigh>

here's a copy of a private post to Herb... edited for artistic sake.


don't be too quick to condemn an AFM...  i've got several posts on 
these in the archives but i'll relate a little to you now.

it is my experience that one *cannot* use an ohmmeter to check the
afm, despite what Bentley says.  there are surface mount capacitors
in there that charge and discharge as the wiper arm moves and the
resistance jumps all over the place.  even on a brand spanking new
one, which i did buy :(

use an oscilliscope or a fast acting DVM and measure the key-on 
voltage on the wiped output.  look for voltage dropouts or non linear 
response.  each afm that i have tested with resistance alone, has tested 'bad'!

also, it is very typical for the carbon track to be worn down so one 
sees the fiberglass board.  one can squeeze down the wiper arm to 
have it ride on a new area, but even this doesn't seem totally 
essential.  what DOES make a difference, it cleaning the carbon track 
with a clean pencil eraser and then running a heavy weight cotton bond 
cardstock, eg a 'rough surfaced' business card, under the wiper and 
burninshing the wiper contact.

let me know if you need clarification on anything.

-dan