DIS conversion solution?

[Shannen Durphey]

The main problem when using a distributor code with a DIS system seems to be
getting the timing of the EST signal correct.  The DIS module seems to be able
to handle EST signals of unequal width, i.e. a signal conditioned for single
coil dwell control.  In the 749 (and likely others) it is possible to modify the
code to simply skip the dwell computation and store 1/2 the value of the
reference period counter as dwell.  The final effect is to deliver an EST output
which is the same as the DIS EST signal, 1/2 available time high, 1/2 available
time low.

I've spent some time thinking about possible solutions.  While the neatest
answer would be to find the "real" solution by fully understanding the
differences between DIS and distributor code, there may be a very workable
solution for 4 and 6 cylinder engines.

The problem is that the DIS module output signal's timing is difficult to
correct for distributor code.  For an engine which has a distributor or a
distributor-like oil pump drive one answer may be to incorporate a pickup coil
to generate reference pulses to the ecm.  For injector firing, rpm input, and
spark timing this signal will be to the distributor code the same thing it's
always been.  The ecm will have no trouble with this signal since it can be
delivered at #1 cylinder TDC or relatively close to TDC.  

The DIS module will need to be set up with a crankshaft signal similar to the
original, which is a 7 notch design in many of the GM engines.  For startup the
module will use the 7 notch signal (with the synch notch timed similar to
original design) to generate spark @ TDC and the ECM will use the pickup coil
generated signal for reference.  When sufficient rpm has been reached, the ecm
can apply 5V to the DIS module as if it were a distributor module, and the EST
signals generated by the ecm can then be recieved by the DIS module at the
correct time to fire the spark plug.

I cannot say yet that this is a guaranteed solution.  The DIS module does pay
attention to the crankshaft synch notch at all times, even in EST mode.  But
it's definitely worth a try.  For an engine like a 2.8 which came with both
types of ignition it might be an easy enough experiment.  For any of the guys
that have fabbed up a DIS crankshaft sensor wheel, I'd say this test is a good
next step.  I do recommend setting the DIS module off to the side, and looping
the plug wires from one terminal of the coil back to the other for testing.  A
timing light can be used to determine whether or not delivered spark is timed
correctly and is consistent.  The distributor can be left in place and
throughout this test actual engine timing will stay at module timing, due to the
lack of 5V or EST at the distributor.

Something to think about anyway.
Shannen
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