Are you implying that it charges ok at idle, but not at road speeds?

 

Also, are you sure it's ACTUALLY discharging or is it just the gauge?

You probably just have a loose connection somewhere. The instrument cluster on 97+ Jeeps is known for getting moisture in the connection and causing flaky gauge behavior.

 

Is the belt tight enough? perhaps It's slipping

 

Yep, I drove a Land Rover half way across the country with a belt slipping. The vehicle would charge fine at idle and around-town speeds, but not at highway speeds. Especially with lights on (an alt load). It was hard to debug. The grooves on the alt pulley were worn sharp and glazed. I had to replace the pulley, then the belt ...and make sure it was routed properly (it wasn't). That fixed the problem. I'm assuming your 'old' husband replaced the belt with a new one and the new alternator pulley has flat ridges at the top of the grooves (not worn sharp)?? Is he checking all the cables, the ground and double-checking the output with a separate multimeter?

 

I read the e-mail of the original message and was going to crack a joke about the O.P.'s "first husband" and us making some wild assumptions, but I see you corrected it.. LOL

In all seriousness though - I think that the advice (checking the belt tension) by JeepWalker and EMX is a great place to check - it makes logical sense as well as is backed up by real world experiences. (I also hadn't thought of that, so like usual, I learn new and important things every day on this awesome forum! )

The XJ belt needs to be much tighter than one would think, so what is proper tension on many other vehicles is actually well under spec for the XJ.