Hey Len

SO, you checked the grey wire that goes from the tach to the terminal block on the engine and that wire and its connections at both ends are good, right? Good start.

Now, check the little "dip" switches on the back of the tach itself. There will be settings labled 4, 6, 8, etc. These indicate the number of poles on the alternator. The tach must be set to match the number of poles for your engines alternator. You can look that info up, or just systematically try all the different settings to see if it works. ( Those swicthes can be inadvertantly swicthed if you were working on some other wiring, rooting around for something behind the gauge panel etc)

If its not the dipswitch setting, it may be your charging system. The tach, after all, reads pulses from the alternator--via the rectifier, actually. Are you charging. At cruise, is your voltmeter reading 13.5 or better? Its should be. (Actually, you should connect a multimeter to the alternator with the engine running to check this, as your older guages and wires can be suspect as diagnostic tools)

Try the first two tips, and make a note of your charging then get back to us,

G'Luck

Kevin