Re: High Freq MAF Problem

[Ludis Langens <ludis@cruzers.com>]

Scot Sealander wrote:
> 
> 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.

To see if the signal is getting through, hook your scope to pin 16 of
the '34984/'79435.  That's the output of an inverter.  Pin 17 is the
input.  The counting is done in a different chip.

> 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.

The frequency MAF in the 727/730/749 has an RC filter on the input of
the inverter mentioned above.  R is 100K.  The C looks like many of the
other surface mount capacitors.

The 8253 ECM (used with the 3800) has a different RC filter.  The R is
48.7K (1%).  The capacitor looks different from any other cap on the
circuit board.


Hmmm, is the high frequency MAF also used on the Buick 3300 V6?  This
would explain why it uses an ECM (1228706) that doesn't interchange yet
looks just like the 1227730.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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