Yesterday, I went to start my XJ. It turned over with no problems, but did not start. When turning the key for a second time, nothing happened. No clicking or anything. I was able to jump start it, but it would die when idling. Basically, I had to keep my foot on the gas to get it home. My battery gauge was also pegged out at 19+ after the jump start. I put the battery on a charger and the most I could get out of if was clicking. Still no start. Does this sound like a battery, alternator, or something else?

 

Start with having the battery tested. It is probably bad. I would also get the alternator tested to make sure that is not what killed the battery. If these both test good you could have a bad CKPS, crank position sensor.

 

I've experienced something very similar. Originally had a cheap Costco battery. It died. Replaced the battery. It will run a little rough for a bit after you replace the battery. Drive it around for about 30miles. It'll work its self out. At least that's what I did and it worked.
Some tech told me some mumbo jumbo about how disconnecting the battery that it resets mapping or some jargen. Regards if it's accurate info or not. What ever I did. Worked. No issues since.

 

IF YOUR RIG STARTS RIGHT AWAY WITH A JUMP START , IT MIGHT ONLY BE THE BATTERY TERMINALS

 

Aluminum terminals dont work as good as the coper ones

 

I relpaced my battery yesterday. Of course, I changed the terminals when I did it, as they only cost about 3$ a piece. When I snipped the cables from the old terminals and then stripped the casings off the wires, they were heavily corroded on the inside. I cleaned the heck out of them, put the new terminals on, and it fired right up. The terminal heads might not have been the whole issue, but I'm sure they weren't helping.

Good-luck.

 

I ended up taking the battery in to Advance Auto Parts to get tested. It was fine, so I bought an alternator. When I put the battery back in, I decided to give it a shot before even messing with the alternator. It fired right up. It must have been a bad connection (I do have aluminum terminals). I returned the alternator and haven't been having any problems. Thanks, guys.