While driving recently, the "Check Gauges" light on the dashboard came on, and I noticed that the alternator gauge was all the way down, reading 9. When I got home, put a digital voltmeter on the battery terminals, and the reading was only about 11.7 volts (with the engine running), so I figured the alternator had failed. I didn't have time to deal with it, so I parked it, and started using a different vehicle.

I finally got around to checking it out, and after starting the engine, was surprised to see that the alternator gauge was now reading nearly 14 volts. I drove about 5 miles to town, and the gauge stayed at that mark the whole time. I shut it off, went into the store, and when I came back out and restarted it, the "Check Gauges" light came on, and the gauge showed about 9 volts again. I drove home, and the gauge stayed at 9. I again checked the voltage at the battery with the engine running, and got a reading of about 11.7 volts, so it didn't appear that the alternator was charging.

A couple of hours later, I again started the engine, and was surprised to see the alt gauge now reading just a hair below 14. Keeping the engine running, a took another voltage reading at the battery terminals, and got a reading of about 13.8 volts, so apparently the alternator was then charging.

This intermittent business has me baffled. If the problem were a bad alternator, wouldn't it just never charge?

Any input welcome!

A dirty intermittent connection at the alternator or either end of a battery cable can cause symptoms like this.

When the alternator brushes wear out, they can become stuck and the alternator will stop charging. Typical symptoms would be as you noticed, 9 volts indicated on the gauge, and charging that returns to normal when you hit a bump, or stop and restart the engine. As suggested already, cleaning and checking the alternator wire connections is free and easy.

New brushes are probably all that is needed. If you buy a "rebuilt" alternator have it tested before you leave the auto parts store. 5 - 10 % of Chinese "rebuilt" alternators are rebuilt incorrectly and either are faulty before you leave the parts store, or will fail quickly.

Thanks for the posts, guys. You know, now that I think about it, this issue began not long after I replaced the spark plugs.

The plugs hadn't been changed in a while, and it was a bit of a battle even getting the boots of the spark plug wires off of the plugs (finally had to use one of those tools that looks like a pair of needle nosed pliers with a loop to get them loose). It's possible that I pulled something loose during that project. I will definitely check the connections.

If the problem is due to worn brushes, are replacement brushes readily available? I wouldn't mind trying to tackle replacing them.

Again, thanks!

when i had funny charging it was the belt. snug up, its fine.
there is no way i can tell if/when i get it right.
always feels too tight...

Hmmm...hadn’t thought about belt tension, but another thing to check out. Occasionally, during rainy weather, the belt seems to be a bit noisy, having a “chirp,” maybe caused by being a bit loose? I agree, hard to tell the proper tension...had a better feel for proper tension on the individual v-belts that drove alternators, power steering pump, etc. Thanks for the additional suggestion.

XJs do like a tight belt, tighter than most.

My ZJ was having the same exact problem. Tightening the belt fixed it. Take note in your examples that your problem seems to be arising after the engine/belt are warmed up/stretched.

Have you been mudding recently. About every other trip I take into the woods I usually pull the alternator off and take it apart and clean the brushes on it and put a little oil on them. Sandy mud can make the brushes stick and cause the alternator to intermittently work/not work. Clean everything inside it with contact cleaner.

Thanks again for the suggestions. I will definitely start by checking the belt tension, and take it from there. Will let you know what happens.

Thing about the belt tension is that using the "yeah that looks about right" method has been far off in my experience.
I broke down and bought this:

https://www.amazon.com/Gates-91132-B.../dp/B000CRDLZM

my problem is back. dont feel like pulling the alt. lazy...
it goes half way,pause, drop down...

you may have a Nippon Denso alternator, you can get the brushes and holders on Amazon for about $9. I understand (not done it myself, a mechanic fixed it for $10 last time it dropped output) that this model is very simple for brush replacement.

It may of course also be loose connections.