XJ Alternator issue - Weird

[Fredduck]

Here's a weird one I can't figure out, need some help.
My kid has a 93Xj with a 4.0.
When the Jeep sits and the battery drains down a little the jeep will start, but the Alt. won't kick in and the volt meter shows 12V.
When we put the charger on it and start it up the volt meter shows 14V and even after removing the charger the volt meter shows 14v
With the Alt charging.
I've also charged the battery, free of the jeep wiring and then hooked it up, the volt meter shows 12 volts and the Alt won't kick in.
What is going on?
How do I diagnose what's broken?
I'm kind of stumped.

[Vinman]

I had a similar problem on a Chev about 20 years ago and it ended up being a slightly loose fan belt. It was still tight enough that it looked ok the half dozen or so times I checked it but after I tightened it up the problem went away.

[powerguys]

HEY LOTS OF KM,S ON IT IS IT THE ORIGINAL ALT ?

[Fredduck]

HEY LOTS OF KM,S ON IT IS IT THE ORIGINAL ALT ?

Not sure Richy I think he's the 3rd of 4th owner. A replacement from Napa is a stupid price.
Perhaps I should come and see you guys 

[powerguys]

hey fred
yup bring it buy we can fix it up