my 1965 Mustang not charging

Hello. I must be missing something here. :headscratch: What was the car doing that caused you to believe that there was something wrong with it in the first place? If you drive the car around some, shut it off, and have a battery that shows 12v, that's what's supposed to happen. :gringreen If that happens, but, you come back the next day and the battery is low, that's either the battery is unable to hold a charge, and should be replaced, or, you have a draw somewhere, in which case, reading this might help.
The Care and Feeding of Ponies: Finding an electrical draw
For other charging system issues, this might help.
The Care and Feeding of Ponies: Alternator and charging system

I agree with Veronica. A 1-volt fluctuation is inconsequential, but you still haven't explained why you changed it in the first place. What was the charging voltage before, and what is it now? Changing to a different alternator may or may not fix your problem, but it's a shot in the dark because you really haven't narrowed down just what the problem is.

sorry,maybe idid explane the whole problem very well(sorry)with the old regulator and old alternator,when the car was running,i had just barely 12 volts at the battery. when i gave it gas there was no fluctuation in the reading. which lead me to believe i have a problem. then i took the alernator to the auto parts store and they tested it. said it was good. so i re[place the regulaor. now i have a fluctation when the car is runnning. but not about 13plus volts(hope this helps you help me.lol)

Okay, now we're getting somewhere! Assuming you do not have two bad regulators in a row and your alternator is good, you have some bad wiring somewhere between your alternator, regulator, and starter relay. Check the link to the schematic at the top of the main page, and verify your wiring. Make sure nothing is broken, and nothing is shorted to ground that shouldn't be a ground.