There are a few types... most commonly found on Volvos would be what you describe... a hard wire connection to the negative on the coil or at the points. there are also tachs with an inductive pickup, such that the wire between neg on coil and the points is routed through the dash, into the tach at the iductive pickup, then onward to the distributor. Either of these types should work fine... both were used stock in 1800s depending on the year.

Then there are totally mechanical tachs... cable driven. On my Jaguar, the tach is cable driven off the exhaust cam. But... you don't need to worry about that. Its not what you've got, and it won't work in a Volvo...

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-Matt I ♥ my ♂