[Gmecm] Big cap HEI on 7747

[Robin Handley]

Please excuse my ignorance.

Is the upshot of this discussion that dizzy triggering does or does not
provide adequately accurate timing (for ignition/injection), IFF the two
wires of the reluctor trigger are connected the correct way around to the
HEI?

Robin

----- Original Message -----
From: <bcroe at juno.com>
To: <gmecm at diy-efi.org>
Sent: 10 June 2006 05:02
Subject: [Gmecm] Big cap HEI on 7747


> 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.
> _______________________________________________
> Gmecm mailing list
> Gmecm at diy-efi.org
> http://lists.diy-efi.org/mailman/listinfo/gmecm
>