I'm missing something...

[Chris Conlon]

At 11:30 PM 11/9/01 -0700, Diehl, Jeffrey wrote:

>This car is equiped with a flapper door AFM which I've been told are self
>compensating for air temp and elevation.

The flap-type AFM does not properly compensate for either of those
factors, by itself. The AFM responds to both air flow velocity and
air density, but the terms are different orders. So, you have a
single equation with 2 unknowns (density and velocity), and you
can't get a single-point solution to that. (If the density and
velocity terms had the same order it would not matter, since mass
flow is linear in both those terms, and their product would be
all that mattered.)

The AFM pretty much assumes some known density to start with (probably
sea level). The ECU can (presumably does) use the temperature sensor
to make a guess at the air density, based on that "known" starting
point. With a guess at the density you can solve for velocity. When
the actual baseline density gets too far away from the assumed baseline,
the meter reads incorrectly.

   Chris C.

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