Hy Kato, you didn't say if the truck was converted to 12 volt, but I'm going to assume it is, if your coil is marked "no external resistor required" then no you don't need a ballast resistor. There were no ballast resistors in the 6 volt trucks. If you haven't been having any symptoms with it wired that way then I would say it isn't doing any "harm", but your spark energy will definately be lower than if you took the ballast resistor out of the circuit, hope that helps.
