1990 xj with a 4.0. My dash voltage gauge reads only at about 9 volts while actually testing the battery with vehicle off I get 14.1. While trying to start my battery only dips to 12.5 and charges again around 14.1. With all acc on, V dips to 13.7. The problem I'm having is while driving I'm losing power (voltage inside stays at 9). I've checked and cleaned the connections to battery and alternator but still seems like power is being drained as I drive (dash lights dimming, blower running slow and signal lights flashing slowly, etc.)
I have also pulled over and been unable to start the vehicle as it would only crank a couple time after which it seemed like the battery was dead. I waited ten min, tested the battery (12.55V) and tried to start it again. This time the cranks were very slow but it was able to start.
My alternator seems slightly louder than normal, but does appear to be charging at idle. Does this seem to be a battery or alternator problem? How else can I test it?

 

I just had this same issue with my 89 waggy, if you have power seats check the circuit breaker in the fuse box, mine was sticking and very hot to touch, replaced it, now no battery drain and battery gauge reading 14 again.

 

Also make sure all your grounds are pristine.

Apparently the Renix era Jeeps are particularly sensitive to this.

Cruiser54 has a great write-up on this. The link is in his signature.

 

I would perform Tips 1 and 3 first.

Your alternator can put out plenty of Volts, but not enough Amps. If it's noisy, replace it.