[Gmecm] Re: Big cap HEI on 7747

[Andrew Gibson]

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.

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