Hi folks. Not my first search using the site, but it is my first post...so greetings to all and Sincere Thanks for what appears to be a fantastic forum!

We just purchased a 1998 XJ (2-door, 4.0L, automatic, SE trim level) for our daughter. The alternator light came on after about 10 minutes of driving the second day we had it, tested the voltage while running and it was around 11v at idle. Battery tested 12.2v with key/engine off. So we're jumping to the conclusion it's probably the alternator, based on the searching I've done here. We'll bench test it of course, but upon reviewing available replacement units I see there's 90amp and 120 amp units. Assuming this basic stripped-down model has the 90amp alternator... is there any reason NOT to upgrade to the 120 amp? We'll be adding a few basic accessories as the days wear on (mainly a few lights, as driving at night requires dodging deer and 300lb hogs)...so while we might not *need* the 120 amp alternator, if it's overkill but the same price as the 90amp, why not upgrade?

Asking because we're not very electrically minded, and we've read there's an external voltage regulator built into the PCM... there's no chance the upgraded 120amp alternator would argue with the stock PCM or something like that is there?

Just prefer to ask and be told "you're being stupid" rather than be stupid and not ask...you know what I mean.

Thanks in advance folks.

 

No real reason other than price. If you're adding LED lights, the extra draw shouldn't be that much. There are some aftermarket headlight units that are pretty nice.

Before you pull the alternator, check fuse 5 in the underhood power center and there is a fusible link between the alternator and the power block as well.

 

Also hit Cruisers website for a factory manual and parts diagrams. His site has tons of tips mostly centered on the older Renix Cherokees, but plenty for the newer models as well.
https://cruiser54.com/?page_id=365 (Look for the 1999 XJ)

 

What is the output of the battery terminals while the engine is running?

The stock fuse is within a cable to the PCM. If you do upgrade your alternator you'd need to add a fuse, always round up, if you go to 120amp alternator a 150 or 200amp fuse would work.

 

SINCERE Thanks to you both... very much appreciate all the great info!!

Badblood... the voltage while engine running is 11v at idle. Didn't think to rev it up to see how that affected the output... but will do so in the morning, just to see.

 

11v is too, that would point to the alternator.

I've heard a lot of people upgrading to a dodge Durango alternator.

 

Rumor has it that anything bigger than the stock 117/120 alternator needs to have some grinding done to the mount brackets. I put a larger alt on my old 92 and had to grind a little but I don't remember the amp level I put in.

 

Understood; I'll keep that in mind. I can't weld worth a hoot but I can grind easy enough!

Thanks for the tip!

 

Hey, I recently swapped the alternator on my 98 XJ as well. If you're planning on adding accessories like lights, the 120 amp alternator is definitely worth considering, especially if it's the same price as the 90 amp. I went with the 120 amp myself since I figured it’d give me more leeway in the future if I add more electrical stuff. As long as you’re not going crazy with mods, it won’t put too much strain on the engine. The stock PCM should handle it without issues, so you shouldn’t have any problems with compatibility.

 

Thank you Swisher, that's good info. I appreciate the answer. Definitely not planning on going crazy with the mods... just enough to see off to the sides better at night. Better safe than sorry. Some of the roadkill we see is plenty big enough to total a vehicle I'm afraid.

 

Agreed on this to the OP..... if they are only getting 11v while running.... they already have an alternator issue.

 

If you use say Rockauto... not sponsored but have used them many times in the past for my XJ... don't go cheap... go for the OEM option and you'll thank yourself.
I'm fortunate as my XJ was built as a service XJ and has the higher output alternator that is still original to mine 26 years on.

 

Yep I just repaired my 90a. What I found was pretty simple, dirty brushes. Check resistance across the field terminals, if it’s 1M or so it makes it pretty hard to get any current through the field when the regulator puts 12 v across it.
Half can of brake cleaner and cleaned all the electrical contact points where the brush set screws in, voila! Charging like a champ. To the OP question, I’m still going to add up all my accessories and see why 90 amps would not be enough.
oh, my new brush set came in from eBay today, $10. I’ll keep until needed

 

90 amps is plenty.....AT 5000 rpm. I believe they are rated at max engine rpm, not idle. I just changed mine out with a durolast gold. I will see if I can find the spec sheet that came with it. Add all this up ...blower fan...aux fan...headlights ....w/w....and possibly rear window defroster....Rainy cold night defroster on to keep windows clear stuck in traffic barely moving and your engine is stuck at idle for several minutes at a time. A higher full rated amp load means a higher idle output, I think

 

I think you might be right, I’ll confess the voltage would drop down at idle with my blower motor, fog lamps, and high draw headlights from China that run off relays I put in. Not happening now, though but that’s something I’ll watch. Evaluating. I do recommend the external regulator hack, it’s more assuring to see 14.3 ish than 13.8 ish and it starts much better. lol.