How do I determine which coil I need? The tractor is a 1969 3400. Everything is buried where it cannot be inspected. I managed to remove the coil working blind but there isn't any information on it. The parts store asked me if the coil needs an internal resistor. I told him the only thing in the maint manual is 5" and 5.5" coils and that stumped him. Both of his coils appear to be the same size and both are shorter than what came off the tractor. The part number in the parts book came up with a coil that has a resistor mounted to it but with such limited access I don't think it would work and the wire from the key has a strange cover on some of it. Could the resistor be built into the wire? IF that is the case, would it hurt to buy a coil with a built in resistor and replace the hot wire?