You can test the one you have (and I don't remember one going bad). There are two 12v+ wires going to the coil. One has power in ign. start position and goes straight to coil + terminal. The other has power in ign. run position, and goes thru the resistor and then to coil +. This give you a full 12v while cranking for the hottest posible spark, and a reduced voltage to the coil (less coil heating) when running. I think I remember seeing 12v in start and about 7-8v at run. Mine also have a radio noise suppression thingy on the coil +...

$40 coil just because if I tell the the $20 one, and it quits, ....