The ECU in most Bosch systems doesn't look directly at the resistance of the air flow meter. That would give a very dirty, noisy signal.
Instead, they apply a constant voltage to the airflow meter, in series with a current measuring resistor. As the resistance of the air flow meter changes, the voltage drop across the second resistor changes, giving a nice clean signal for the ECU to use.
A multimeter will pass only about 5 micro-amps through the unit under test. This small current is extremely easily upset by dirt, moisture, corrostion, wear and even humidity on and around the contacts. The larger current passed through it by the ECU, usually around 50 milliamps, is much less susceptible to such variations.
So the best way to test your air flow meter is to set up a resistor network with a regulated power supply, and then measure the voltage drop across the second resistor, and calculate the resistance by subtracting the voltage drop seen from the voltage applied to the network, then transposing the V=IR theory.
Scott
- Original Message -----
From: Brian Holm
To: august ; Bob Bruce
Cc: Peugeot List
Sent: Wednesday, December 14, 2005 9:15 AM
Subject: Re: [Peugeot-L] Re: 505 Turbo AFM checks
I have tested about 20 N9T AFMs and found the same thing. Also the
same with fuel level senders. I think that an ohm meter simply isn't
the instrument to use where the conductive characteristics of the
swept surface may mask the fundamental resistance, due to dirt,
corrosion, and minute pressure fluctuations of the slider. A log
scale is not linear, it just has the greatest sensitivity with lower
values, but it does not reverse itself!
Brian Holm Parts for peugeots
At 01:38 PM 12/13/2005, august wrote:
>Interesting Bob, thanks for that info, your totally right.. I did a
>little research and found the same thing:
>
>Right from the Bosch book of Automotive Fuel Injection Systems
>Technical Guide:
>
>Page 100
>"The relationship between the mass airflow and the flap angle is
>logarithmic, which ensures a non linear characteristic in the
>potentiometer."
>
>i take it logarithmic means its not linear (obviously), but it will
>fluctuate during the complete sweep of the flap.
>
>I asked another friend / peugeot homie who said:
>"it should read the highest value when the flapper reaches to most
>deflection it can attain with airflow pushing on it"
>
>Bob, is the AFM you have for sale new? Would you be able to test it?
>
>On 12/13/05, Bob Bruce <bobbruce@mts.net> wrote:
> > Bosch info states that these AFMs are logrithmic rather than linear
> > if anybody knows what that means
> > Apparently this explains to the whizes why the resistance readings are
> > not what you might expect as in a light dimmer
> >
> > Bob
> >
> > PS I have a spare turbo AFM forsale with airfilter and box
> > ----- Original Message -----
> > From: Bill
> > To: peugeot-L@yahoogroups.com
> > Sent: Tuesday, December 13, 2005 12:07 AM
> > Subject: [Peugeot-L] Re: 505 Turbo AFM checks
> >
> >
> > I've seen the same behavior on different units.
> >
> > -Bill
> >
> > --- In peugeot-L@yahoogroups.com, august <august.macbeth@g...> wrote:
> > >
> > > There's one test shown in the N9T/N9TE/N9TEA for checking the AFM:
> > >
> > > Connect an ohmmeter between pin 5 and pin 7 of the AFM. Then move the
> > > flap door slowly to full open.
> > >
> > > It says it should be between 60 and 1000 ohms.
> > >
> > > Now, I've done this test on 4 AFM's. You'd think that whatever the
> > > value the book was calling for would be a clean sweep from low (full
> > > shut), to high (full open). Not the case, on all FOUR that I have.
> > > They seem to jump around a lot. As the doors opens, the value rises,
> > > but can go up and down, and full open on all isn't the peak value.
> > >
> > > (more like, 60(door closed), 100, 300, 150, 500, 800, 200, 900, 400
> > > (full open))
> > >
> > > Is this normal, or do I have a bunch of bunk AFM's?
> > >
> > > Here's a sample reading:
> > >
> > > full shut: 80
> > > peak: 1000
> > > full open: 450
> > >
> > > Do any of you have experience with this? Or would any of you be
> > > willing to test the AFM's you have? I've been looking around the net
> > > to try to educate myself on exactley how these things work, but
> > > haven't found anything on super bouncy readings.
> > >
> > > thanks!
> > > /august
> > >
> >
> >
> >
> >
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Brian Holm, Parts for Peugeots
at Peugeot Holm, since 1969
2120 Maple Hill
Plainfield, VT, 05667
802-454-7132, fax -1310
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Received on Tue Dec 13 15:19:21 2005
