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Two types of tach. Binary and inductance.
If your ignition wire runs from the switch through a loop at the tach and then to the "+" side of the coil you have the inductance tach and it is needing a rebuild.
If you just have an "extra" wire either from the "-" or "+" side of the tach it is binary (on-off) and you should check that the wire is still attached at both ends. It may also require a rebuild if a wire has not come loose where you can get to it.
Both diagnoses assume your ignition is otherwise working properly (i.e. the car runs).
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Mike!
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