[Gmecm] Re: Big cap HEI on 7747

[Andrew Gibson]

>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.  

  You nailed it there Bruce. I had to "borrow" some pics from the net 
  since this is a text only mailing list. And I couldn't find the exact one I wanted.
   
  >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).  

  Sort of. It doesn't look for the voltage transistion. It's looking for a threshold volatge.
  If you manage to get your hands of a schematic of the insides of a gm hei
  module (4 or 7 pin doesn't matter) the system is controlled by a transistor. For those 
  who don't know transistors are a voltage gate controlled by a lower voltage. They can 
  either be positively or negatively controlled. In the case of the module they are positive 
  controlled. To open they need a minimum threshold voltage applied to the gate. This 
  voltage varies from transistor to transistor. Transistors don't care about opposing 
  voltage applied. (aka: the bottom half of the scope pattern)
   
  Knowing this the hei module is looking for a trigger voltage. Since the pickup produces 
  about 800-900mv it could be anywhere up to about 700 mv. Although it is more likely 
  about 250mv. When the input signal reaches the trigger voltage the transistor is 
  activated, signal strengthened (amplified), run through a flip flop circuit and the coil is 
  turned of which then makes spark. The vertical line in the signal is always centered so 
  it is ideal for that control.
   
  If you wire the pickup backwards, the module is still looking for the positive trigger 
  voltage. Since it will now be on the slope of the pattern and the slope is always 
  moving due to production defects, magnetic field variences, temperature, etc.  timing 
  become uncontrollable.
   
  So yes to Robin, the polarity of the pickup coil is vital to the operation of a GM efi 
  system with distributor. It's pretty darn important on the carburated ones too.
   

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