I have a 1973 240z manual trans, and I have 3 tachs that i am not sure if they work or not. One is the one that came off the car, the 3 prong tach, i also have a 4 prong from an earlier automatic 240z, and a tach from a 280z. Does anyone know how to test a tach to see if you can get any needle movemet with a battery or anything?

no, but I'm having the same problem. If you found the answer let me know.