tach problems

Hi; I have a little problem with my 1978 280-Z. I won't say that my tach won't work because it dosen't matter if it's running or shut off it reads 3000 RPM and that's where it stays. I think I have checked out the wiring and can't find anything visable wrong. Could this be terminal for the tach or does some one have a fix. The car only has just over 39000 kilometres on it (24000miles). Mel