The alternator on the wife's '94 XJ charges quite normal, for about 6 miles, or until the engine warms up. Then it drops off, and the voltage goes down as the electrical draw brings it down.

Looking in my service manual, the DTCs listed don't really solve a problem that seems related to what this system is doing. If it simply just quit/failed, it would be an easy fix.
And I see that the PCM is the voltage regulator, so maybe that thing is the problem ???
Any ideas are welcome

Of course the problem occurred when the temp here is in the low teens, and my pickup is apart in the shop & I can't move it !

 

Take it by a parts store or garage and have a voltage load test done on it.

You may have one or more bad diodes in the alternator. Diagnose before spending money on parts.

 

Not sure if local parts stores can check it. Seems like it wouldn't charge up to 14+ volts at the beginning if it had some bad diodes.
I may just replace it with a new ( not rebuilt ) one. Looks like it comes off & out the bottom....

 

Mine came out from the top on my '92.

 

I was under there today, and it does come out the bottom, so the service manual was correct !
I have a new alternator coming, from Summit racing ( not a rebuilt one )
70 amp unit..$107.95 with free shipping. All the wiring & grounds look fine, so I'll go with the alternator first.

Local store wanted $109.99 plus 15 buck core charge, and of course 6% tax on that. They tax the core also, even if you bring it in tho buy a another one.
The state figures out all kinds of way to skin a few more bucks out of us, here in Michigan.