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.

Signal is the wire that sends the pulse info to the tach from the ignition box.

From trigger on hyfire box to signal on tach.

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Steve Strasemeier (70SS 396, Fathom Blue/White Stripes)