[Gmecm] Big cap HEI on 7747

[bcroe at juno.com]

I'm going to add to this.  That second image isn't the whole 
story, because there is no flat horizontal line at left except 
when the shaft first starts turning.  More importantly, the 
waveform is balanced above and below ground, or + and 
- portions if you like.  The very sharp edge occurs when the 
magnetic field stops building, and starts declining; that 
is about as the pole pieces are closest.  This always 
occurs the same spot, though velocity may change the 
amplitude and period of the waveform.  

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

9 Jun 2006  Andrew Gibson <andrewsharyn at yahoo.com> writes:
> I have some info that I think will be useful to all out there 
> concerning triggering of the gm Ignition modules. The distributors 
> don't produce an occilating wave form. (AC Sine wave) I dont know 
> what it is called but here are pictures of both styles so you can 
> see the difference. I also plagerized these from elsewhere so please 
> don't send me nasty notes about not getting the exact patterns or 
> going to the work of creating a website about it.
>    
>   The sine wave is produced from square teeth. Most mag pulse 
> generators make one like this: 
> www.picotech.com/auto/graphics/fuel_pump_waveform.png
>    
>   Triangular teeth (GM distributors) make this wave form (Please 
> keep in mind this is only the positive side or upper half. The lower 
> half or negative is a upside down mirror opposite) 
> www.krchealth.com/images/waveform.jpg
>    
>   The vertical slope is what triggers the module. The module is 
> looking for a specific voltage point to trigger the internal 
> transistor. Too little voltage and nothing will happen. The vertical 
> slope is used because it is always in the same place. If you wire 
> the pickup coil backwards or the magnet is on upside down, the slope 
> will show up first. The timing will be off by about 40 degrees and 
> will change drastically as the engine rpms change (The pattern 
> changes size as the magnetic field speeds up.)
>    
>   Since I know there will be questions and challenges about timing 
> on a conventional crank sensor with a sine wave: Anything with a 
> crank sensor requires an analog to digital converter. (AD converter) 
> It is additional electronics which were not available in the 70's 
> when gm started using modules instead of points. They also require 
> more space than is available inside a distributor. And also cost a 
> lot more.
>    
>   In short, the polarity is vital. Triggering off a slope will get 
> you an injector pulse, but will never get you an accurate timing 
> control without an AD converter and a lot of extra homework.