Esteemed Jag colleagues, my '94 has begun exhibiting a low charge (11 volts) upon first starting while at idle. When moving off (higher engine revs) the voltmeter will increase to ~15 volts and remain there whilst cruising at 40-50 mph. After which, when sitting at a stop, volts will decrease to 12-13-ish. Moving off, the volts will return to ~15. Is this indicative of a failing alternator?
Could be a loose/slipping belt, which is what I would check first. The fact that it comes up at higher RPM leads me here. If it stayed at the lower voltage throughout revs, I would suspect the load dump module, which can be fixed by you if you are handy with a soldering pen.
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Mine has behaved like this for years, slipping belt when cold almost always the issue. I really should get off my duff and fix it LOL …maybe tomorrow, when I do the oil change I’ll have a look at it …BTW my GTX 20-50 has suddenly doubled in price 
When my car started behaving exactly the same as yours it turned out it was the alternator carbon brushes worn down to virtually nothing, but it could also be a sign that the battery is faulty and not accepting the charge. I adjusted the belt first but it made no difference, the last thing you need is for the belt to be TOO tight and putting a strain on the water pump and alternator bearings.
When the car is idling after it has warmed up, try switching on lots of accessories, fan blowers, heated screen etc and watch the voltmeter, mine struggled to maintain 12V when normally it would be around the 15V mark.
I ordered a rebuild kit for mine and did it myself, it’s not too difficult if you are handy with a soldering iron.
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I used to get my GTX 20-50 from the local Asda supermarket, and it was really cheap, about £22 for 2 x 4 litre plastic cans, but for the last 3 or 4 years it’s been impossible to source it from anywhere regardless of price. The Castrol 5W 40 or 10W 40 is available so I’ve been using that but I would always prefer the 20-50 if I could find some.
Asda was a great source for engine oil when I lived in the UK 15 years ago. I think I still have a can of Castrol and also the really cheap ASDA branded 20w50.
I also ordered several times from https://www.opieoils.co.uk/
They have almost any oil you can imagine and the prices were quite competitive.
Cheers,
Harald
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Casso, I’m thinking perhaps you are on the right track. I haven’t searched yet - where did you source a rebuild kit? And are the various makes alike enough the kit is “universal”? If new brushes would restore operation I wonder if I might find such at a local hardware store? Hmmm…
Just pull out the old ones, measure them and go here. They can be filed/cut to size, just like sharpening a pencil!
or even here
Hi Mike, I found mine on E.bay. The kit I ordered was specific for my alternator type. The company selling them had kits for lots of alternators in common use and also had an excellent tutorial video available with step by step instructions through a link they provided on Facebook. Although the video demo used a different alternator to mine the principle was basically the same. The kit contained the brushes, both bearings, and the stator plate with all the diodes and rectifier already soldered in place so it made the rebuild quite straightforward. The kit was around £20, far cheaper than the £120 I was quoted for a 'service /exchange ’ unit from my local motor factors.
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Here in the USA I find the best price for Castrol GTX 20-50 in 5 quart jugs has been the Walmart Superstore.
Mike, I found this after a quick google search.
Thanks to all that have weighed in on this question. Given the right weather and MF (Motivation Factor) I’ll dig into this and see what I can figure out.
Measure your brushes and find replacements. Mine are for a vacuum cleaner and cost under £5 delivered, and I soldered them to the old connector. You can probably do a similar repair.
If they are available cheap just buy them of course.
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Update, with more to come. I finally emerged from my senior citizen’s fog just long enough to go ‘hey, I know I’ve replaced the alternator before [DIY]…you don’t suppose…’. Sure enough, in August 2008 - nearly 15 years ago - I purchased from a local O’Reilly Auto store (headquartered right here in Springfield, Missouri, US of A) their top-of-the-line reman’d unit with a lifetime warranty. I dug up the receipt and stopped in a store Wed. evening and explained the symptoms. They didn’t even bother to test the alternator in situ, saying it definitely sounded like bad diodes (?). They rang up a new one and said my money will be refunded when the old one is returned. Now the fun could begin.
The last time I replaced it I did it all from beneath and I remember it being a PITA. This time I thought I would try removing the air pump and start from the top side. Long story short, probably not a good move as I ended up with a double PITA. Five hours later, after much fiddling about and several rounds of blessing everything in sight, the new alternator is in place. I considered just chucking the air pump but oh well what the hell…so everything is back in place. Started the old girl and let it idle while I put things up. After starting around 11.5 volts it was purring along around 14.5 volts when I shut it down. So we shall see how she does in real-life driving, and what kind of horse-traders the good folks of O’Reillys prove to be.
Drove the car to the office today, charge-o-meter showing ~13 volts at idle and arond 14.5 volts at higher RPMs. After driving to the office (~9 miles, 20 min.) and on the return trip home the c-o-m was showing ~13.5v at idle and a skosch below 15v at higher RPM, so about back to the old girl’s normal. I stopped in at the O’Reilly store from whence the replacement alternator came and turned in the old one and received a full refund so kudos to the fine O’Relly folks for keeping their word.
BTW, this was what I call penultimate Monday, as I am retiring April 15, so only one more Monday (4/10) to go!
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Good to know O’Reilly honors their commitments. We have them around here and they are my go to for generic parts and supplies.
And, congrats on your upcoming retirement! I hope you enjoy yours as much as I have mine. Been 5+ years for me.
Congrats on the retirement, but now you have nothing to do on April 16th!
PS. i’m having the same problem and have a replacement in a box waiting for the job. Luckily I ditched the pump when I did my engine swap.
Thanks for the good wishes and you are spot on, I’m wondering what I will do with myself come April 16. I bet the wife will be able to think of something! 
So I have noticed after driving the car more that the voltage is still dropping down at idle. I need to do more research but I recall that the earlier XJ40s used a load dump module (?) that could act up and cause symptoms similar to those I am experiencing. Do the '94 models utilize a load dump module, or someting similar? I seem to recall reading the later XJ40s incorporated that functionality into the alternator.
Just a WAG but how tight is the belt?
I believe the belt is fine. Now that I’ve driven the car a while the behavior is nearly back to normal. I can’t explain it…