I'm sure this will seem like an elementary problem to some, but I'm at a loss. I left my XJ for about a month and a half while attending some training, and upon returning it will not start up. Here are the facts:

- There were no issues with the battery before I left. Cables, connections, etc are free of debris or corrosion.
- I attempted to jump the vehicle first. The voltmeter jumps to 14 volts immediately after starting the jump vehicle. All lights and guages come on almost instantly as well.
- When turning the key in the ignition, there is a whining/whirring noise that I am assuming is the starter motor spinning. However, the flywheel is never engaged. After about a second of attempting to start the vehicle the voltmeter guage dives to almost zero, lights and electronics dim and act strangely, and the motor noise dies off. There are no clicking or thumping noises.
- Checked the fuzes for all of the relevant systems just to be sure, no issues there.

From my pretty limited knowledge it seems like it could be the solenoid, but I'm not sure. Any thoughts?

 

It sounds like the starter is not engaging with the ring gear. Try hitting the starter with a hammer or whatever you choose.
It may free it up.

I would start with that.

 

Have you tried removing the battery and just hooking the cables to the jumper cables? If your battery has a shorted cell it could be allowing the system to have 14v but essentially no Amps present to turn the starter.

 

After reading around a little, I figured I would try a new battery just in case the starter wasn't receiving enough power even during a jump. That fixed my issue but revealed a completely new one.

When I started it up, it ran perfectly for about 1 or 2 seconds, then I got the infamous puff of smoke from the belt assembly followed by a terrible screeching sound. Needless to say I now need to go through the process of diagnosing whether my Alternator, Power Steering, or water pump pulley is seized up because there is no belt movement whatsoever.

Edit: My bet is that something is that the alternator was already going bad before I left the Jeep sit for a while, could also explain why the battery that was in it died despite being fairly new (1 year old).