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Jason -- The points of a conventional Kettering-type ignition operating as part of a 6 volt electrical system switch full battery voltage without too much trouble, but they don't live long when trying to switch the doubled voltage of a 12 volt electrical system at high engine speeds. Engineers added a resistor in series with the points to drop the switched voltage enough that the points would survive for an acceptable length of time. Some added the resistor as a separate part in the battery-to-coil-to-points-to-ground circuit, making it a battery-to-resistor-to-coil-to-points-to-ground circuit, while others chose to build the resistor into the coil. The coil design that built the resitor into the coil was termed an "internal resistor" coil, while the conventional coil became known as a "non-resistor" coil. Using the wrong coil type causes problems. Adding a separate resistor to an internal-resistor coil cuts the primary system voltage, which leads to a weak spark; not using a separate resistor with a non-resistor coil leads to burnt points. Eventually the coil makers started labelling their non-resistor coils "For use with external resistor" to emphasize the difference between the "internal resistor" and no-internal-resistor coils. Interestingly enough, the adding resistance to the ignition primary circuit made engine starting harder, especially when abnormally-high starter motor current draw caused battery voltage to sag. So engineers added a starting bypass circuit that allows full battery voltage to the points when cranking. So why do your Farmall points hold up on a 12 volt system without a resistor in the ignition primary? I don't know, but I'd hazard a guess that the Farmall engines run at much lower RPMs than the engine in a typical car or truck crusing down the highway. John
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