I have a 1998 Z-28 and am wondering if the voltage gauge is working properly. The needle reading varies between 14-15 when the car starts up and hovers around 13 for normal city driving. It usually dips down to 11.5/12 when I put on the defogger or A/C -- are these readings normal??? Should there be so much variance, if not, do I have a potential problem with the alternator or charging system? What are the warning signs of a "bad" or dying alternator??

Thanks