I've got a 1997 XJ 4.0L Auto 4X4. Recently and quite randomly I stopped for gas, got back in and it was very hard to start and would not stay idling, it would just die. After getting it home, and it stalling two more times, I did the key trick to pull codes. No codes. Next day it ran fine, idle was a little low but better than the night prior. It's been hit or miss how it runs from day to day now. Just yesterday on the way home from work the voltage gauge pegged to 19, immediately pulled over and did the key trick. Code 46 "Battery voltage sense input above target charging voltage during engine operation." Had the battery checked at Oreillys and they said it was good but said I was having problems with the voltage regulator. I know that's in the PCM, but could it be the alternator? Could it be a battery temperature sensor? Could it be just a fuzzy ground somewhere? Bad PCM? I've gotten conflicting information from the internet and I'd like to be somewhat confident of what is before I spend a dime. Any and all help would be amazing.

Clean your grounds and battery cables - free and easy.

If that doesn't work, I'd take it to your local parts dealer and do a battery and alternator test. Autozone for sure, but likely the rest of them will do it on the car for you. That will help isolate your issue. A bad battery or bad CPS are also possibilities, but due to the voltage weirdness, I'd focus on the alternator test first and foremost.

If you want to be confident of what it is before you spend a dime, get it diagnosed for free by a competent mechanic over the internet.

Probably not the battery temp sensor, which can definitely change voltage in the system, but not usually by a large amount, and typically downward from 13v (it results in a reduction system voltage as temperature under the battery increases).

Many of the items you mentioned could be implicated. But the most common would be poor connections (battery, alternator, grounds). In that if the computer starts seeing low system voltage, it will command the alternator to increase its output voltage. This can happen when bad connections are causing resistance somewhere between the alternator and the computer.

It is rare that the PCM internal voltage regulator is bad. Though not unheard of. (I helped a member on here with a particularly difficult, and similar set of circumstances, and we troubleshot a ton of items, until finally replacing the PCM was the last option, and it did correct the issue). Just definitely not the first item to jump to, and bad mechanics often will. They think bad voltage regulation = bad voltage regulator. Though its just not as simple as that.

I had very similar symptoms, and it was the battery cables that were causing the problem. I even got the 19 volt reading, which turned out to be BS. Scared me though. When it happened, I'd loosen the positive clamp and turn it a bit before tightening it. That would fix it for a while. I finally replaced the cables with the ones from XJ Wonders, and it's been fine ever since.

Also, your '97 is OBD-I? I thought everything post '96 was OBD-II.

Thank you! I'll be honest, I've driven Fords my entire life and now at 31 I have to learn a new system and all it's nuances, it's been very frustrating at times. I plan on cleaning those connections this evening and see what happens. It ran like a champ this morning, no problems. I appreciate your thorough response and accompanying video. Major help!

Come to think of it, I did have this happen once. However I am fuzzy,..it may have been the opposite, voltage dropping out. Anyhow for what is worth, there way a coolant leak spraying on the alternator. Think I ended up replacing it.

The other oddity that makes no sense to me is that when it does peg out the voltage gauge, if I stop and put it in park, after a few seconds the light clears and the gauge returns to 14 until I turn it off. This part stumps me.

Do the lights seem brighter when this happens? Have you checked it with a voltmeter when this happens, to rule out a problem with the gauges?
I can think only of a couple things that would make it act like that. Either a defect in the PCM, or the alternator, or the ungrounded Field wire is shorting to a +12V wire, which would make the alternator put out full power. Sounds like you could roast some electronics if it is putting out that much, incl. sensors and or pcm.

The lights do not seem to surge when it happens and none of the other gauges seem to go nuts either. I haven't checked it when it happens but I did check with a meter sitting in the parking lot this afternoon. 13.69-13.7 at the battery, battery to body, alternator to battery and alternator to body. I'm about to drive around and see it will do it again so I actually confirm its pushing 19 when it freaks out. I cleaned the terminals and cables at the battery and the grounds next to the computer. Guess I'm going to work my way around to the others and see if things get better

Well yeah the first thing would be to make sure it's not just a flaky gauge problem. Maybe jury rig a voltmeter to any 12v source and watch it while you drive around, if that's when it's happening.
If it truly is going to 19 you would see that everywhere, headlights, interior lights, etc.