[Gmecm] a question of timing

[David Allen]

  If memory serves me correctly yes it is +5V.
  The tach signal has to be a clean, electronic signal. I tried to use
several different circuits to make a points system work in this manner and
it took some very complex filtering circuitry to prevent multiple triggering
as the points "bounced." What sort of ignition setup do you have that will
provide the signal?
  Later,
David

> Thanks David,
> the reason I ask, is I have a 12v tach signal available in a swap I'm
doing.
> Using a transistor, I was hoping to create the needed 5 volt square-wave
> for a 7749 to manage only the fuel.  Thanks for the detail on the low side
> of the signal.    I'm correct in the"high" being +5 (ish), yes?
> Thanks
> Mike V
>
> At 07:45 AM 3/13/2006 -0800, you wrote:
>
> >   Ok- The ref low signal is a ground reference inside the HEI module.
The
> >Ref and Ref Low inputs to the ECM and differential and the input
circuitry
> >in the ECM looks for voltage difference between the 2 wires.
> >   When the module is preparing to fire, it provides a high-current
ground to
> >the ignition coil.  This may create an elevated voltage on the module's
> >ground.  Under this condition, when the module creates a grounded
reference
> >pulse it is pulled to the slightly elevated ground voltage, instead of
0V.
> >This may not be below the threshold voltage where the ECM sees "a ground"
> >and therefore the ECM may miss a reference pulse.  SO to prevent this the
> >ECM compares the reference voltage not to it's internal ground, but to
the
> >module's ground.  It does this using the Ref Low circuit. Neither of
these
> >circuits should ever go below 0V relative to chassis ground.  If the
> >module's ground is elevated to 1.0V, the ECM detects this on the Ref Low
> >circuit and will then consider the Ref circuit to be "ground" at 1.0V
> >instead of 0V. It uses a special IC designed for this purpose - this is
in
> >hardware not in the code.
> >   This same scheme is used in lots of electronics, really.
> >Hope this fully declarifies things!
> >David
>
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