I assumed the AFM was set up like a voltage divide, with +12v on one side,
wipe, and ground. But, it looks like it's just a variable resistance
(there may be a resistor to ground in the ECU, though).

Really, the big question is: is the resistance linear over the "scale"
(idle to max)?

J

On Sat, 4 Nov 2006, Mike Bucchino wrote:

>   AFM's don't 'put out voltage'. So they don't degrade in output strength.
> The wiper contacts inside wear, and degrade the quality of signal; IE; spots
> worn bare send jumpy/unreliable signals to the computer.  They vary a steady
> input to send a changing output to the computer, based on flap position
> changes.  The flap position varies according to air flowing through the AFM,
> based on engine demand.
>   The correlation between CFM airflow to the range of voltage signal output
> is detailed engineering data that's not published by the manufacturer,or
> obtainable by the average user.  Like a fuel or ignition map program in the
> computer chip.  Why would you need to know, and if you did find out, what
> would you do with this data?
>   You could bolt an AFM to a flowbench and run tests to determine these
> voltage changes for a given CFM, but you'd need to know how CFM equates to
> the engine's rpm, to get any usable information.  A hair dryer isn't capable
> of giving accurate, measurable CFM readings, so what do you gain by trying
> to use it?  You can move the flap with your finger to see the voltage or
> resistance changes.  This proves the internal contact wiper as healthy or
> not, which is really all you need to know to determine if it needs to be
> replaced or not.  Also, how would one go about calibrating the AFM?  Or why
> even try?  It works or it doesn't, just like every other sensor in the
> system.  You test according to Bentley and replace the malfunctioning parts.
> These parts are not able to be rebuilt or calibrated by any mechanic. R&R
> only.
>   If you're a DIY engineer attempting to re-invent the wheel here, you're
> pretty much on your own.
>
> Good luck from a fellow DIY engineer,
>
> Mike B.
>
> ----- Original Message -----
> From: "Geza Polony" <gezapolony@SBCGLOBAL.NET>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Saturday, November 04, 2006 1:06 PM
> Subject: Re: AFM Voltage Ratios
>
>
> > This is a question I've been asking for a long time, both here and on
> > westfalia.org, and no one seems to know. You're right, there must be SOME
> > concrete relationship between manifold pressure and AFM output voltage,
> > but
> > no one seems to know what it is. The Bentley doesn't help, either. I took
> > mine off and measured the voltage using a hair dryer to simulate the air
> > flow, but that doesn't really tell you anything except that the voltage
> > does
> > change, and pretty much evenly.
> >
> > Without knowing what the voltage should be for a given set of AFM
> > conditions, it's impossible to calibrate the AFM. And you can bet that,
> > with
> > most of these AFM's having 20-odd years on them, the voltage output "ain't
> > what it used to be." That would mean that at some point in the scale of
> > air
> > flow, the engines are running too rich or too lean.
> >
> > Unless there's something we're not getting. Why wouldn't the voltage data
> > be
> > available?
> >
> > ??
> >
> > Geza
> >
> > On Fri, 3 Nov 2006 18:22:51 -0800, jon <jon@KENNEKE.COM> wrote:
> >
> >>Does anyone have data on AFM voltage (out of the resistive divider) versus
> >>manifold pressure? I would think they would directly correlate in some
> >>linear fashion.
> >>
> >>Yes, I think of these types of things on a Friday night. ;)
> >>
> >>Jon
> >
>