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Your stated voltage measurements don't match how you report the circuit is wired. If the wiring is ignition switch -> coil -> resistor -> points, then one side of the coil should read 12 volts all the time, the other side of the coil should read 12 volts with points open and 6 to 9 volts with the points closed. The exact voltage depends on your choices in coils and resistors. The only point that should measure 12 volts with points open and zero volts with points closed is the points themselves, if the circuit is wired as you describe. (If the circuit is wired with the resistor upstream of the coil (correctly), then the - side of the coil is connected directly to the points themselves, and will naturally show the same behavior as the points.) Now if your ignition switch has gone bad, or there is a badly corroded connection to the coil, then both sides of the coil could read 12 volts with the points open, and zero volts with the points closed. If I ignore the reported voltage measurement, and look at the described wiring, I would then say this: Move the resistor to the plus side of the coil. I'm assuming negative ground, 12V system. This will probably fix the problem. Long explanation follows: The coil, condensor, and points form a "tuned circuit". When the points open, the DC current flow through the primary coil is suddenly stopped. This induces a large voltage "spike" on the primary due to L di/dt. This is multiplied by the turns ratio of the coil and creates a much higher voltage capable of bridging the gap in the spark plug. The condensor (a capacitor) prevents the burning of the points which would otherwise be caused by too fast of a voltage buildup there. Its value is sized to get an optimum voltage waveform out of the points acting as a switch. Too big a capacitor, and change in current flow is too slow (charge gets diverted to capacitor, and di is too small). Too small a capacitor, and the voltage rises so quickly that a strong arc is established across the points, establishing an ionizing path across the points, reducing the peak voltage (and also burning the points in the process). Putting the resistor downstream of the coil destroys the tuning of the circuit, by slowing the rate of change of voltage as the points open. It probably changes an underdamped RLC circuit with a sharp rise and desirable ringing to an overdamped one with a wimpy peak voltage. While the net DC current with the points open is the same with the resistor in either location, and is the same (zero) once the spark has fired, the shape of "di" in di/dt is much different with the resistor downstream of the coil primary rather than upstream.
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