voltmeter pegs 19v wont restart after turning it off

I have a 2000 XJ 4.0L with 285,000 miles. I will give as much detail as I can. all this stuff seems to have happened after I did some wiring under the hood. I didnt add anything extra just made all the wiring look cleaner. So last week I wired in an aux fuse box to try and clean up the wiring under the hood. everything went fine. I drove it after a few miles the volt gauge pegged 19vs. I immediatley pulled over and sat for about 5 minutes with the Jeep off Key off. The Jeep restarted no problem. Volt gauge read 13.5-14volts Next morning i go out to drive to work and my Jeep is dead. I jump it and drive to work and New Hampshire for the weekend with no issues. Yesterday going to leave work I got in the Jeep and it was dead again. I jumped it it ran fine. while it was running I found that I wiggled the harness near the battery (the harness with power for the starter and ground from the battery, and several smaller wires) the jeep would sputter and die. So I got home and cleaned the grounds and checked both battery terminals and cleaned them, cleaned the engine grounds. Go out this morning to head to work and the Jeep starts up fine. I get to the end of my street and volt gauge pegs 19v. when I tried to restart it this time it was stone dead. I had to hook up the jumper cables and let it sit for about 5 mins before I could even start it. I have taked 2 volt meters my dad had to see if the battery was good and if it was charging, the battery read 11.5-12volts with both meters. when i started the jeep to check the charging system both meters read 12v. When i check charging at the battery cable at the alternator it reads 12v right out of the alternantor. So I know I need an alternator. but my big questions is why is my volt gauge pegging to 19v randomly? I have done some research online and found it to beeither a PCM issue or (I am hoping) a bad battery I am going to take the battery to an auto parts store to get it tested. Any help is greatly appreciated. Thanks

Last time I heard of the high voltage overcharge issues, it was a ground. The PCM was loosing ground reference and giving the alternator everything it had. Make sure the engine to fire wall strap is intact. Also the Battery to Chassis ground.

When the gauge pegs to 19v, what is the voltage read on a meter?

some time after my wiring upgrade my dash gauge was randomly reading maxxed out causing the check gauges light to come on.

took apart and sanded every connection clean and made sure they were nicely secured and it fixed it..

Thanks for the replys. I havent had a meter with me to check to check the voltage read out when the gauge pegs 19v. though i now have a meter in my Jeep for the next time this happens. I will check the engine ground at the firewall next.

OutkastBoss- which connections are you talking about? the grounds?

started with the grounds but went ahead and did the positive wires as well for good measure..

I'm pretty sure the culprit was a bad quick disconnect at the positive batt terminal. Basically causing the batt to be disconnected..

another time my headlight wiring for my h4 bulbs upgrade had a bad ground and it messed up real good. Ground is just as important as the power

Last time I dealt with a jeep that had an over voltage issue that was intermittent it was the alternator. Sometimes it would work fine then it would just spike to high voltage, like 17v. Last time I had an alt issue on my 2000 XJ the check gauges light came on and it dropped the gauge to 0, when it still had 10 volts. Yours could be going the same thing, but in a high voltage case. E.g, not actually displaying the correct voltage on the gauge.

If you have the underside engine mat out of the way you can unplug the alternator fairly easy to see if that changes anything next time the voltage goes up.