I just replaced my alternator with an optima lifetime 120 amp alternator from oreilly's and it charges about 13.5-13.7 but when I turn on the ac it barely gets over 12 volts. It says ac delco on the back of the alternator but just doesn't perform like I think it should. Should I go with a different alternator or did I just get a dud? I know its hard to find good alternators for these cars.