I also wanted a console tach for my 64 Sport Fury, but I read that many people had trouble with those old sending units. So I bought the correct tach off eBay and it didn't even include a sending unit. I read that the gauge rebuilders can just put everything into the tach, so I had Redline Gauge do it. That way I would get the tach calibrated with
greatly simplified wiring (as shown). With their nice labeling, even I can figure it out. Also, when these tachs are rebuilt, they can handle more modern ignition systems.
I know this does not answer your question, as your sending unit may work just fine. I do know that on the 67 Barracuda I have, a grey wire does go to negative side of the coil. On the 67, there is no sending unit. I tried looking for your wiring diagram online, but I couldn't find it. Good luck.
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