[Gmecm] Big cap HEI on 7747

[Steve Ravet]

> The module senses the crossing of voltage from one polarity 
> to the other; the voltage it switches at is zero, position of 
> which is not affected by amplitude.  The module is supposed 
> to fire on the very rapid transition, which will be very accurate.  
> If the wires are reversed, the module will fire on the very 
> slow changing part of the waveform crossing zero, which is 
> way off in time and not so accurate.  My recollection is that 
> proper firing is on the negative edge (relative to ground).  
> 
> Bruce Roe

My recollection is that the module fires on a threshold voltage, not at
zero.  The threshold voltage will vary in time from the zero crossing
depending on RPM, meaning that the module has some RPM based advance
built into the triggering circuit.  Somewhere in the archives is a
patent (and SAE article number) that describes this.  In fact I think
the datasheet of the triggering IC also has a good description.

For Robin's question, millions of TBI computers are triggered by this
HEI setup.

--steve


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