Date:         Thu, 21 Oct 1999 08:21:26 -0700
Reply-To:     "Tom L. Neal" <jneal@NETCOM.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Tom L. Neal" <jneal@NETCOM.COM>
Subject:      air flow meter resistance measurements
Comments: To: Pabst Michael <Michael.Pabst@VT.SIEMENS.DE>
In-Reply-To:  <BB54B4ABA1FED011A54100805FBECCCB0110DD0D@bwga522a.vt.siemens.de>
Content-Type: TEXT/PLAIN; charset=US-ASCII

Am testing two AFM's right in front of me.

Pretty easy.
Remove connector.
Get any simple ohmmeter (I use Radio Shack $10 version analog so I can
watch the needle for spikes. My 5 digit digital meter isn't as good.)

On a new, assumed good, digifant AFM:
Alligator clip to Pins 4 (starboard) and 2 (two over from starboard)
Slowly push the gate in.
Starts close to 0 ohms.
Moves quickly up to about 1200 ohms
Varies up and down from about 1200 to 1400 ohms about 8 times.
Ends at about 1000 ohms.
(The travel is NOT monotonic--one direction--as I indicated in "AFM
Pathology".  Time for a new revision. This actually makes sense due to
the internal circuit construction.)

Between pins 3 and 2:
Starts at about 500 ohms.
Moves quickly up to about 1500 ohms
Varies back and forth, slowly dropping down to about 500 ohms at the end.
There is a dip down to about 200 ohms right before the end.

On a known suspicious junkyard digijet AFM:
Pins 4 and 2:
Starts about 0
Moves quickly up to about 1000
Varies back and forth, dropping slowly
Has a couple of spikes up to about 3000-5000 range (clearly intermittant)
   about 1/3 up the scale, probably the highway cruising point.
Varies up and down a little above 1000
Drops down to about 500 at the end

Pins 3 and 2
Starts about 500
Moves quickly up to about 1500
Varies around 1500 with a couple of spikes up to 3000-5000 range (same
places as above)
Ends at about 200

The digijet one looks pretty worn.

The spikiness appears to be worse when the unit is colder (40 F verses 80 F)

One experiment to do with van that hesitates at about 70 is to run it at
a very different speed away from the worn spot, 30 or 90 (!) and see if it
still hesitates.  If it does, the worn spot is probably not the problem.

Cheers, Tom Neal

On Thu, 21 Oct 1999, Pabst Michael wrote:

> Hi everybody,
> did anybody ever check the afm resistance change when opening the flap (engine
> off, connector disconn'd) ? My today measurement was far from linearity and I
> wonder if a new / good afm behaves like this.
> TIA,
> Michael (still looking for mpg/emissions of an 86 7-seater)
>