So I have a 2000 xj and my volt meter dropped to zero when I was out wheelin and then my truck died as soon as I got to the exit luckily haha so I just assumed bad alt after the battery tested good and I replaced it with a brand new one and it still won't charge... Everything I've found by searching has said bad alt or connections but the alt tests good and all my battery cables are brand new and I've checked my grounds and they seem to be good. Any ideas?

Bad fusible link? Check the continuity of the main charging wire? Engine grounds?

Max

I'd replace the alternator under warranty, so as to not chase your tail. It's really not that unusual for a new alternator to be bad out of the box. Not saying that is the case, but need to rule it out.

And as for parts store alternator testers; not so good of a report card. If the alternator tests bad, then it truly is bad. But if the alternator tests good, it might not be accurate and the alternator can still be bad; these are called "escapes" (fails that are not caught). Quite common, especially with alternators.

In the event that yet a different alternator still doesn't charge, the problem is likely in the voltage regulator, which is part of the computer on your 2000 XJ. They don't fail often, but like anything else, they can fail. And when they do, a replacement computer is the fix.

Good luck and keep us updated!

Just replaced the alternator in the wife's Buick. The old one checked good at the auto store, but it would charge for a minute & then fail. So much for auto store testing.....
I suspect that it was the internal regulator messing with me....

The problem with testing an alternator in the parts store, is that the tester that most of them use (the bench tester) simulates a perfect world scenario...........It has the correct voltage inputs to excite the voltage regulator (if the alternator has one) and puts a proper load on the alternator as it spins it at simulated engine speed......................
And at that exact moment, when it tests good, it is in-fact a good alternator..........
The problem with this method, is that the alternator is usually tested cold..........either directly off the shelf, or the customer's alternator has usually cooled sufficiently by the time it is brought into the store.......


We all know, the underhood environment is not a "Perfect world" scenario...


Often, when the alternator is run on the engine, and things heat up considerably, things start to expand/shift inside the alternator, and sometimes a short, or a faulty internal connection is created....................


In addition to this, certain alternators with an internal regulator (Renix era Cherokees with Delco CS alternators) require approx. 12 volt input to excite the regulator...............They will not self-excite with a discharged battery.
'91-up 4.0 engines use an alternator that is externally regulated by the PCM.


In some instances, a person's alternator has failed, causing the battery to be drained-dead. Person then installs a new alternator, jump-starts the vehicle and immediately max-loads the alternator causing it to fail.........
Contrary to popular belief, an alternator will not charge a dead battery by driving the car a few miles..........


Another popular myth, is when someone suspects their alternator is bad, and thinks "I'll disconnect the battery with the vehicle running to see if it still runs the car"...........This is a HUGE NO-NO!
What happens next is that the instant the battery is disconnected, the alternator sends a nice voltage spike through the electrical system, and blows the fusible link.(or worse, the PCM)...........Now the vehicle dies, and the person is now under the assumption that the alternator is bad.........................Goes and gets a new alternator, installs it, and vehicle will not charge..............Person now thinks new alternator is bad also......


ZJMAX already suggested to check the fusible link.............(alternator should have battery voltage at the "BATT" post on the back of the housing with the vehicle off) Sometimes the field wires of various cars have fusible links in them as well.............


After replacing a failed alternator, ALWAYS fully charge or replace the dead battery before trying to run the vehicle...............


Also, if it is at all possible, bring the complete vehicle to the parts store or auto shop to test the charging system with the vehicle running, instead of just bringing in the alternator off the vehicle...

Alright thanks guys, I think the first thing I'll try is fully charging the battery cause I tried charging it with an old charger I have but I guess it didn't work cause when I took the truck to test the alternator again today the battery was reading only 7% charge so hopefully the problem's as simple as that haha

Is there any definite way to test the computer/voltage regulator?

This would be my first check... once I had it charged and running again, I would check the voltage at the source wire for the stator ignition. If you have a good alternator and the source wire is giving voltage, you should have charging voltage at the connector directly on the alternator. If you have the signal at the source wire and not charging, then its a bad alternator. It happens even on new ones.

If you are getting a good source voltage and charging volts at the connector on the alternator, but only battery voltage at the cable on the battery, you should check into the fusible link. I would also like to state that I have my doubts that it would be the fusible link, but it could be. They normaly blow when something catastrophic has occured.

I would also say that a bad ground to the engine block can cause poore or no charging. Double check the ground wire on the drivers side that connects to the block and the intake. One is on the fire wall. Heck, check all the grounds to be clean and tight.

Best wishes.

Well I just went through all the grounds again and cleaned em all up but even with a fully charged battery, it wasn't charging. I took it to orielly again to get it tested and their tester came up with bad voltage regulator but the regulators in the ecm... So does that mean bad alternator?

And I just checked the voltage coming straight out of the alternator and it's at 30 volts. Is that what it's supposed to be?

Well it actually seems like it was the fusible link which I thought I checked but I guess not seeing as this happened when I checked it today haha