High Freq MAF Problem

[Scot Sealander]

Well, been trying to get a high freq MAF (Buick 3800) working as
an additional sensor input on my Turbo GP.   It just did not want
to work!

Pretty sure the added code section worked, but DIACOM would never
display any readings for airflow.  (The MAF airflow was put in the 
DIACOM scan string as a substitute for the the normal calculated 
airflow.)

Found a snag for those thinking about adding a newer MAF to the
730 and/or 165 ECMs.  As you are aware, the MAF on the early P4
ECMs can be DC or freq input. There is a different input on the ECM
for each sensor type.  Both the MAF and the VATS would use the same 
counter input. (Any application can only use either the MAF or the VATS, 
not both at same time.)  The VATS input is low freq, (about 30Hz) and 
the low freq MAF is about 30 to 145 Hz....

I suppose because of the low freq, the ECM input is very heavily low 
pass filtered. The min freq of the high freq MAF (1 kHz) cannot even get 
through to the input of the counter!  With the 'scope attached to the 
input pin on the counter chip, even the VATS 30 Hz squarewave is very 
heavily rounded.  The high freq MAF just doesn't have a chance.

So, if a high freq MAF is going to be added in the future, a chip cap 
change is going to have to be made to the ECM to move the LP corner freq 
much higher.

Scot Sealander  (Off to change a chip cap in the '727...)

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