[Gmecm] a question of timing

[David Allen]

> My question below is about the reference signal.  The application is
>
> buick 455
> 90 corvette TPI (vin 8)
> electromotive hpv1 dis
>
>
> The tpi to buick is bridged by an adapter that connects the two.


PLEASE explain how this adaption contraption works?

  If the ECM is NOT a sequential injection system, and is NOT providing any
ignition timing function, it needs only engine speed information; the
position information is irrelevant.
  The reference signal from the factory HEI module or DIS unit is a 5-volt
signal. The ECM provides a 5-volt signal on the reference line and the
module pulls this to ground (momentarily) to indicate a trigger event. It is
a more or less square waveform (not sinusoidal like the raw pickup coil
signal).
  The distributor outputs 4 pulses per crank revolution, and if your HPV1
also puts out 4 pulses per rev, grounded, it should work.  You might need a
simple, passive device with a resistor and or diode at worst case.
  You're correct, the knock sensor is only needed if the ECM is controlling
timing. You can turn off the service codes for the knock sensor and EST
bypass mode in the calibration.
  Now, if the ECM is sequential (don't beleive yours is) it will require the
distribitor reference signal to be timed and you may get a code if there is
a mismatch between the distributor and cam sensor signals. This area is out
of my experience.
  Hope this helps!
David