[Gmecm] a question of timing

[David Allen]

  Andrew,  maybe some ECM's are different,  but when expirimenting with my
boat EFI (1227132 ECM from a Chevy Astro van 4.3 TBI) it only needed the
'reference' and 'reference low' to count engine RPM and provide fuel
delivery.  ( Ref low is simply a "clean" ground reference grounded
internally at the module).
  The engine would crank and run normally but with no control of timing; and
with the MIL illuminated.  The EST line is an output sending data from the
ECM to the module.  Once again talking about my boat ECM - it didn't care if
this signal was going into a module or a capped off wire - as long as it
could see reference pulses it would fuel.
  I agree if he has a standard distributor module, by all means use all the
connections! But he is using an integrated stand-alone distributorless
ignition system, and wants to use it's engine speed output as reference
pulses to the ECM, which is doing fuel control only. His DIS has a built-in
advance curve and needs no ECM input for timing control - It was originally
for a carb engine.
  I tried this expiriment because at first I wanted to keep the marine
distributor and build an interface box which would provide reference to the
ECM.  It worked but then I noticed the Astro van distributor was identical
except for the computer-controller HEI module (instead of points) and lack
of centrifugal advance. With it I could use the ECM's mapped ignition timing
control which is light-years ahead of a cruddy marine points distributor!
Later,
David

> I concur with David on the voltage/amplitude being a 5 volt signal. One
thing to note on these conversions. You need BOTH high and low reference
signal as well as the EST wire or the ecm will not fire the injectors. The
Bypass wire is optional but I reccomend it to prevent that pesky check
engine light, and it's easier to hook up than it is to disable the flag in
the prom. I don't know why the ecm wants all those but it does.
>