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Griz, youre pretty much right on. The bottom line and limiting factor is the amount of current the points can switch so as to not burn up prematurely which is in the 4 to 5 amps range. Coils and the points which switch their current on and off are current devices, not so much voltage devices. In order to limit points current to the 4 amp or so range, the TOTAL resistance in the ignitions primary circuit needs to be, therefore, around 1.5 ohms on a 6 volt system or 3 ohms on a 12 volt system. Thats the reason why if you have 12 votls but use a 6 volt coil you MUST add an external series Ballast Resisistor as the coil is around 1.5 ohmes and the ballast near the same ohms so the points just switch 4 amps or so. However, if you use a full 12 volt rated coil that has 3 ohms of primary resistance, then NO external ballast is used. Nowwwww wwwww when cranking the battery voltage drops, so if the tractor is equipped with a starting ballast by pass circuit (as mnay are) ONLY while cranking the coil sees unballasted battery voltage fro a hotter startign sprak but then she runs with the ballst in series so a 6 volt coil just draws the 4 amps or so to keep it form overhearign plus extending the life of the points. However, if one uses a full 12 volt rated coil, the ballast by pass system isnt available as it is when you use a 6 volt coil plus an external ballst on a 12 volt system. Its true on a 12 volt using a 6 volt coil and a ballast, the coil starts on battery voltage (12 less the drop caused by starter motor) but then runs on 6 volts. John T
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