I just had my clutch replaced in my Z31. Now my volt meter is fluctuating. It is going from 11 volts to 15 volts then dropping to 12 volts then back again. Thought it was because the battery wasn't strong enough with the new sound system i installed and was finally giving up the ghost. So i put in a new optima yellow top and it is still doing it. I don't know if my mechanic forgot to connect a wire from doing the transmission or what might be going wrong. Any thoughts. Possibly the voltage regulator on the alternator might me going. Not sure. But the thing is that it was working right before i brought the car in to get the clutch replaced. It was a a steady 13-14 volts.
Any thoughts?

 

you are perhaps focusing in on the clutch but have an electrical mystery to solve.
One possibility is your ground cable. It should go from the battery to a bolt into the frame about a foot below the battery shelf. then on to the starter. If your car was running by grounding through the bell-housing (which is an iffy connection) then the clutch replacement might well have disturbed that tenuous connection which would probably give you the symptoms you describe.

another possibility would be the alternator went out coincidentally with the clutch.

Those yellow top batteries might look Kool but they are poor gel cell technology. Let it run down to 0 twice and it is a throwaway.