Have had this problem a few times now....
I start the car and the voltmeter in dash reads 15. After driving for a while it goes down to 12.5-12.8 then when I run the lights and the cooling fan it goes down to under 12V.
IN the past when the alternator has stopped charging there is a buzzing noise that comes from behind the steering column and I know i need to change the alternator. This has happened twice. Both times I have purchased a refurbished alt from Oreilly here in Santa Monica.
This time however when I took the alternator in to have it checked, Oreilly's alt test machine said it passed. I took the battery in as well and it was holding a charge fine.
Then when I put the old alt and battery back in it started to work again and the gauge inside the car read 15V again. I thought it might have been a loose positive cable because it didn't take much to unscrew it. Not sure how that happened.
I started it up today and its back at 12.8V
Should I just replace the alternator anyway, even if it isn't making that "not charging" noise?
The intermittently working alt is a 96A one from oreilly.
What would you guys recommend I substitute it with?
I'm running a Dorman "Taurus" fan so that definitely pulls a lot of juice.

cheers,

 

The gages can't be trusted because the electrical connector at the back of the dash is a hokey design prone to intermittent and poor connections. Need to put a real voltmeter at the battery and see what is the charging voltage.

If the charging voltage is fine, then you can improve the function of your gages by taking the gage pod out and cleaning the electrical terminals at the connector with a pencil eraser. Be gentle, the copper is VERY THIN.