fluctuating alternator output.

to cut a long story short im trying to get my autometer tach working with no success but with the engine running the alternator output is not constant but jumping all over the place.should see roughly 14volts with the engine running but cant see what it is.could this be a bad earth to the whole system because when using a circuit tester on the fuses with the engine running its got the same symptoms.a little look underneath and i cant see an earth strap from the gearbox to the chassis.could it be just the regulator in the alternator? whats your thoughts any auto electricians on here?