Last night my town cars belt snapped and I was driving the trans am out and around. I pull in to our station to go put up some things and I come out and smell something terrible. Low and behold it's not charging and smells like rotten eggs. So I shut it off. I looked under the hood, and didn't see anything. The alternator was hot and so was the battery. So I drove it home. When I got home the fan kept running with key off. My mechanic had it hard wired to come on with ignition on because it wouldn't come on and the computer didn't work. So I disconnect the ground for it and I take the battery to the store and replace it with a new one. My interstate was three years old but has seen a lot of use. So I put a new one in and started it. Now the fan comes on as it should and the battery seemed to be charging. However now my issue is: when driving the gauge reads low. Moves, but never hits the 13 mark (where it always sits before, sometimes below). And with all lights on and Ac it still runs but does read low. However I drove it for 30 minutes yesterday and today and it started and ran no problem both times. Is my gauge busted or could the alternator be cooked. There's a fain whine in the engine bay when I drive too. Alternator was replaced 6 months ago

tldr: battery was changed after going bad and now gauge reads low

edit: smell is gone and battery and alternator aren't very hot

 

What does a multi meter actually read for voltage sitting and charging?
Does it change with RPM?
Have you load tested the alternator?

Finally unwire the fan from ignition and never go to that mechanic again. After that wire up a fan properly with a thermal switch

 

He offered me the thermal switch option but with the heat we get here I had him do it when ignition is on until I can get dual fans. The parts store said the battery needed some charge and the alternator was bad. But couldn't tell me how many volts it's was outputting. So afterwards the alternator and I had a meeting with a hammer and the volts came back and hasn't gone down since. So I know the alternator is bad but at least I band aided it for a little bit while I drive my second car