My alternator seems to not be working, the battery level is low, and when I unplug the battery the car turns off. I did this before and the car stayed on. The check engine light was on and flashes code 41 which says "Alternator field not switching properly". Does this mean I need to replace the alternator, or is there a way to fix it? I also replaced the belt a couple of weeks before. Would that change anything?

 

You probably mean when you disconnect the alternator from the battery?
You're battery is drained because it's not being charged by the alternator.

See Link below for the description of this fault, see answer #1.

​​​​​​I can't see your year, the 'newer' xjs have the regulator in the pcm.

Fully recharged your battery ( have it load tested, it might be gone beyond its life) . Then, Check your connections, clean them up and measure the alternator output. It's probably low or none.

That would be my first step.
https://www.fixya.com/cars/t25729329-showing_code_41_generator_field_not

 

The year is 1991. I did not disconnect the alternator from the battery, I disconnected the cable on the negative terminal on the battery. Maybe that is the same thing ¯\_(ツ)_/¯. Since the car died when I unplugged it this meant the alternator was not working. I measured the battery terminals when the car was running, and it was 10 volts instead of 14-15 like it was when I tested it a couple of months ago (it was working fine then).

 

1. A code 41 is almost always a faulty alternator. 99.918763% of the time anyways.

2. A simple first test for your charging system is to use a simple voltmeter connected to both posts of the battery. At a high idle, on DC VOLTS, you should be around 14.4 volts give or take a bit.

3. Never disconnect a battery cable from a running modern day vehicle. (yes, the XJ is in that category!) That little trick was used back in the day by mechanics on ancient vehicles, but is very dangerous to try on an XJ. Transient and spikes of energy can leave you with even bigger fish to fry than an alternator issue by doing this. Damage doesn't always happen if you do this, but your odds surely aren't good. Below is a link that explains what can happen in more detail.

http://www.troubleshooters.com/dont_...ct_battery.htm

 

Your alternator is grounded through the engine. It does not have a ground wire going back to the battery neg terminal as far as I recall. So I am not sure which cable you now actually removed there.

Apart from that, your battery reading 10Volt means it's completely dead. Probably beyond any charging capacity.

​​​​​​I can't see your location in the app, but if in the US , I understand that your auto/ part stores test battery and alt for free.

 

X2. Do NOT disconnect the battery while running. The alternator puts out voltage spikes up to around 30-volts when you do that, which can fry your electronics. I personally watched someone fry a Ford EEC-IV unit doing that "trick" by accident. He was autocrossing and had the mandatory battery shutoff, but didn't think about the alternator should be wired to the battery side of the switch. He tested it, engine stumbled and ran for about 10 seconds until smoke started rolling out of the engine computer.

 

I am in the US. If I want to get the alternator tested, can I just bring the alternator, or do I have to bring the car too?

 

Is it okay if I get a used alternator off ebay? What about a remanufactured one? (this is all they sell at my local auto parts store)

 

Not sure In the US, but you can just bring the alternator in I'd think lol
I'd buy from partstore rather than eBay, because you can easily bring it back if any issues.

Reman should be good enough, for a 30 year old car.

 

I know that I can't get a new Napa one for my '00 but was pretty sure that for other years they could be had.
If I could have bought a new one I would of but there is nothing wrong with the reman I purchased to date.
And I agree with Roler about buying locally. Should any problems arise.
I believe this is a new one for your Jeep.

https://www.napaonline.com/en/p/NNE1...ndex=Universal

Not cheap but new can apparently be had for some years.