I have a Mallory Comp 9000 electronic dist(series 91), a Promaster coil and a Hyfire box. The tach is a Stewart Warner with three connections: S=signal, Ign=Ignition, G=ground. The tach instructions say the ign. hookup goes to a 12-volt source when the key is on, the signal hookup goes to negative side of the coil, and ground is for ground. The hyfire instructions say if using a tach to hook to the trigger on the hyfire box. The instructions for both the tach and hyfire are not real clear which wire goes to trigger.