1998, 4.0l.

I was driving and noticed check gauges light, voltmeter was pegged low and Alt pulley was spinning. Turned it off and put a meter on batt and reads 12.4v. Started it right up and the light went off, gauge reads 12.5v. With it running it stays at 12.4v with the multimeter at the Batt. Batt is 2 months old, terminals clean. I'm looking at replacing the Alt unless I'm overlooking something.

 

Sounds like a good place to start.

Test power AT the alternator with the Jeep running at 2k rpm. If it's still 12.5 it's time for a new alternator.

Just don't weld plates on the pulley.

 

Guy at the parts store says he stocks a 90amp and that there's a 120amp unit that is special order. Alldata only lists 56005685AA, which crosses to the 90amp. Mine is a limited with power everything. Is the 120amp for fleet vehicles like police, forestry, etc running sirens,lights, & winches?

 

My 98 Sport came stock with a 117amp alt. If you don't already have a build sheet for yours you may want to get one just to verify which alt. came on yours. Here's the link that you can get it for free it normally only take a couple of hours to get it.
http://www.jeep.com/webselfservice/jeep/index.jsp

 

I can't say 100% but I know the forestry vehicles did have a higher amp alternator I believe that would be a direct fit.

However, I can't find a part number directly related it to it. I checked the Grand Cherokee with 120 amp from several parts places and they list the same part number for the optional 98 XJ 120 amp you are talking about. If I'm not mistaken, I think the mounting bracket has to modified a little bit. From what I can remember, just some simple grinding with a die grinder or Dremel tool. Nothing crazy.

If you upgrade, might want to look at upgrading the cables to handle the extra load as well.

Someone else probably chime in that has a bit more information than I do.

 

90amp is in and 13.5v!!!! I still need belt tension for used belt, looking for my cricket now if anybody has value