Just ohm the coil with a multimeter. Across the terminals (+) to (-) it should read 1.5 ohms. If it reads 1.5 ohms, you need a 2 ohm ballast resistor on the firewall in line with the ignition switch.
If it reads 3 or 4 ohms, it is what they call a Resistor Coil and has that ballast resistor incorporated in the coil.
Hope that helps. Either way, you need 3.5 or so ohms so that you have approx 6 volts to the points. Like Jerry said, it approximate depending on charging, etc.
Another thing.. most 12v conversions in these trucks do NOT have a secondary wire for better starting during cranking. That is because we use foot operated solenoids and an ON/OFF ignition switch. So there is normally only one wire to the coil that goes through a ballast resistor. It seems to work just fine even during starting.
