Could it be a Sun made tach? or is it AC. If it's a Sun, you need only 4 wires. The back of the tach has a positive and negative terminals which go the the same marked terminals on the transmitter. Take a good know ground, directly from the engine is possible, connect that the transmitter ground. The remaining wire goes from the transmitter terminal marked dist., take that to the distributer side of the coil, not the ignition side.
