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Charging system puzzle -- '99 XJ

965 views 12 replies 8 participants last post by  BagusJeep 
#1 ·
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!
 
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#3 ·
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.
 
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#4 ·
Davy A

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!
 
#6 ·
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.
 
#9 ·
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.
 
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#11 ·
#13 ·
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.
 
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