Ever since I got my Fiero, '85 notch, the volt meter would show about 13 v when running. When fan or lights turned on it will drop below 13 v. I've never had any dead battery issues or lack of electrical power for anything except the gauge. I just put a 100 amp alternator on it when I rebuilt my A/C. With the A/C turned on(the cooling fan also) there is a significant drop below 13 v. I've checked it myself and had it checked and it's always putting out 14+ v. Is this a gauge issue? My Sonoma p/u and my Blazer S10 don't show a dramatic drop like that when more voltage is required, they never go below 14.
If you've checked it at the battery and you have good voltage, I wouldn't worry about it. Looks like the gauge is off by a few volts. My 87 GT will drop below 12 when I run the AC but still doesn't drain the battery. Some of the reading error could be attributed to older and weaker connections throughout the charging circuit compared to a newer Sonoma or S10.