Any ideas on what is causing the tach to work part time?

What year car? A 240z? If so, is it hot out? The tach in 240zs are known to malfunction when it gets hot (95+) outside. I believe it is the resistance (?) loop on the back of the tach which causes the problem. Under normal operation, this loop heats up due to the current passing through it.

When it gets hot outside, the heat is unable to dissipate as well and the tach starts to creep up over time. Eventually it will just peg at redline.

there might be a short in the wiring, check out were all it conects to and just trace it out. it might be as simple as a loose conection, i had that problem with my 77 280z, it would come on, till i touched what it was in link to then shut off. but in order to diagnose it properly we need the info asked by (ktm) what year, moddle, and such.

SGT Johnson

USMC

according to mholmes profile, we are talking about a 280z here. bigjohnson, you may be on point with your suggestion, I was about to suggest the same, something sometimes on, sometimes off sounds like a bad connection of some sort. mholmes, find your tach line and start going backwards, fun!