I have a 96 Cherokee XJ with 207K miles. She runs fine but when you put a large draw (fan/heater/headlights) on the battery the factory dash gauge gauge dips down to about 12 volts.
I had the battery and alternator tested and both are good. I replaced the alternator anyway because I found large cracks in the case. I also pulled all the fuses and they are all good. I poked around some and didn't find any obvious wire damage or evidence of arcing.
If you drive it around with the heater and headlights on it kills the battery and it won't start the next day, but if you let it idle for a while with everything electric turned off the battery recharges and it's fine.

Does anyone have any advice on what might cause this?

Thanks!

 

Start with a simple meter reading. Don't trust your dash gauge; a meter will tell you an accurate story. With a multimeter, attach one end to each of the battery terminals. With a healthy battery and charging system, you should see approximately 13.5 to 14.0 volts on that meter.

Also know that the "voltage regulator" for your 4.0 is part of the computer itself. So if you have a charging problem that is not related to the alternator, the computer can be a suspect.

 

I do not know what specfically would cause this, but I have a motorcycle that has an instrument cluster that draws power when the bike is off. I do not ride it that often so the battery sometimes dies. I now leave it on a trickle charger when I am not using it. Maybe while you are solving this problem, you could put your battery on a charger at night so that it will start the next day. A battery charger is a good thing to have anyway. Thank you. JB

 

upgrade the battery cables is the first thing i would try ,mine was doing a very similar thing it now sits at around 12 with all things on and 14 with them off

 

Thanks for the help everbody. I left the Jeep at home for my family to use while I was stationed overseas so I'm trying to catch up on all the work that wasn't done. The first thing I found was that the radiator fan was hard wired to the fuse box for some reason. I pulled the wire and the fan still kicks on and off as normal. Didn't fix anything but it didn't need to be there. I checked the battery cables next. They are the cheap bolt on kind. Not my favorite but they do work when you don't have anything else. I meant to pick up a multimeter but I forgot it. I'll check it out today and see what I can find.

 

Charging is controlled by the computer so after cable and ground cleaning, get that checked.Also thanks for serving and protecting our way of life.

 

I finally got the multimeter I needed. I tested at the battery and I am getting 13.6 volts. I still have some checking to do but I'm working on a few other things at the same time too. I need to try and get to the alternator and check it from there but that will be a little harder without getting my fingers caught up in a pulley I think.