[Gmecm] a question of timing

[David Allen]

  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




>
> Hi Andrew,
> I was hoping you (or anyone)  could expand on the "low" side of the
signal.
> am I understanding you correctly in that the square signal is between +5v
> and -5v?
> I was under the assummption it was +5v and  0v.
> thanks,
> MV
>
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