[alfa] RE: Swapping AFM's

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi George,

I just read your post on the AD about swapping AFM's for BMW types.

Although they might look identical they most certainly are not the same part.  This is
proved by the different part numbers.

The mechanical bits are probably the same, the vane, air channel, air temp sensor etc, but
the important item is the resistance track that the vane drives.  If you look at the
ceramic resistor wafer you will see that it has a number of tertiary taps and other
resistor portions which attach at various points along the main track.  With a powerful
magnifying glass you will see that the tertiary resistor taps have laser trim cuts.  This
is done on calibration to make the output voltage law correct for a particular engine.  In
all probability the ceramic wafer is a standard part before it is trimmed to suit a
particular engine, but once trimmed it becomes "engine specific".

The resistors are deposited carbon paints and these have a 20% tolerance when fired onto
the substrate - a thick-film process - after curing the laser is used to nibble away small
portions to bring the resistor to the correct value.  This is a process known as
"active-trimming".  The AFM wafer is probed across the resistor portions and the laser
cuts tiny lines until the resistor is within specification.

On some of the GM AFM's for example (used on the Opel Commedor) the output voltage does
not rise in a linear fashion as the vane is moved from closed to open.  Here a very
non-linear resistance track is used.  At low vane angles the voltage rises more rapidly
than when the vane is near fully open.

The point of all this is that the ECU uses the AFM voltage to interpret the engine load,
more voltage means more air is flowing and hence needs more fuel to give the correct
mixture.  Very few engine use a linear resistance law, some like the GM engines have a
distorted linear law to provide correct fueling.  Speed-Density EFI systems use a linear
resistance law TPS (throttle position sensor) and a MAP (Manifold Air Pressure) sensor to
get engine load information.  The Bosch vane AFM dose not use Speed-Density measurement
but instead it uses Mass-Air to derive engine load, the AFM has an internal air temp
sensor to measure the temperature of the air entering the manifold. With these two factors
the ECU can compute the required fuel.

So although you might be able to physically swap the AFM, the output voltage will probably
be incorrect for the engine and cause it to either run too lean or too rich at certain AFM
vane displacements.  Another factor is the vane spring tension.  Many uninformed people
mess with this to change the AFM characteristics.  Without a calibrated air flow rig to
set the air flow to a known value you are more likely to upset the final result than get
it better!

John
Durban
South Africa
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