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Date:         Sat, 4 Nov 2006 13:00:04 -0800
Reply-To:     jon <jon@KENNEKE.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         jon <jon@KENNEKE.COM>
Subject:      Re: AFM Voltage Ratios
Comments: To: Mike Bucchino <mbucchino@CHARTER.NET>
In-Reply-To:  <019601c70044$445907d0$6501a8c0@mike>
Content-Type: TEXT/PLAIN; charset=US-ASCII

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 > > >


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