High Freq MAF Problem

[Scot Sealander]

Ludis Langens wrote:

> 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.

Yes, I did not explain exactly when/where I connected the probe very 
well.  I was at both the counter and the fuel chip.  No pulses at all on 
the counter.  The input conditioning expained the problem when probed at 
pin 17 of the 9435 chip.

 
> 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 C is .01 micro farad.  

 
> 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.

The C is 120 pF (measured in-circuit).

 
> Hmmm, is the high frequency MAF also used on the Buick 3300 V6?

Yes.  The 3300 is just a "batch" version of the 1228253 code.
The 1228706 also uses the 48.7k resistor, but the C would not
read right in-circuit.


> This would explain why it uses an ECM (1228706) that doesn't 
> interchange yet looks just like the 1227730.

Exactly. I did not understand the difference either, but now
it is clear. So if one wants to use the high freq MAF in
a 1227730 app, you have to find the 1228706 ECM.  This
might turn out to be the easiest way to get the high freq
MAF in an earlier application, changing chip caps is not
for the timid.  ;-)

Scot Sealander
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