MAF History

[Tom Hussey]

> I wonder if i can disable the obd-2 stuff in the 97 ecm?  The 94/95
Yes. There is an 18 byte table that is used to enable/disable each of
the supported DTCs. If no DTCs are set, the "normal" function is
enabled. If free tools are on the way 94/5 may be a better bet. The 96/7
ecm only programs through the class two vpw stream. I am also working on
free tools but I can't say they are close to done.

>Just curious... Is it possible to adapt an older style distributor
>(non-optical-pulse) for inputs to these PCM's?
This is possible but not easy. The Opti-spark has three functions. The
rotor and cap are built in to direct the spark to the correct plug. This
function could easily be done in a regular distributor. The need here
would be a large diameter cap to prevent cross arcing. Next function is
the high res signal. I believe there are 180pulses per crank revolution
such that the rising edge and falling edge can be used to determine
engine position to +- 1 degree. That is a bit tricky to reproduce in a
distributor. With the right angle gear at the cam, timing chain and
error produced as the cam "walks" forward and back in the bearings there
is several degrees of slop. This prevents accurate determination of
mis-fire.
Last there is the low res signal. The rising edge of this signal
indicates aprox tdc for each cyl while the width determines which one.
There are four low res with a short pulse and four with differing width.
This is used in the startup strategy of the ecm. I understand they do a
spray from all injectors and then fall into a pattern. After the first
1/4 of a revolution the ECM should have figured out the next cyl to
spray.

If you got rid of the OBD requirements you could make it work by putting
the optical sensors into a distributor. I don't think it would work as
well as opti because of the play in the timing. I understand that the
'98/9 ecm does away with all this AND offers independent timing for each
cyl. This means that if one is knocking not all need get spark retard!
Instead of setting spark off the weakest cyl, each is optimized.

-Tom