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This could be wrong because I am operating from a memory of how the original Smiths tach worked but this is what I remember. There was a white wire that went through the tach and to the positive side of the coil. The tach picked up impulses from the wire (I think it had a couple of loops inside the tach). As I remember there was no physical electrical connection other than inductive. The Tack had power ground and lighting in addition to the inductive loop. I took mine apart and put Mustang tach guts in it because the Smiths had a big lag time. You might have a problem with an MSD because of the multiple pulses at lower rpm. MSD has a tach signal but it is not set up for an inductive pulse pick up. This information would be applicable to 1966-1968 cars newer tachs may be different.
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