Alternator Failing?

97 XJ 4.0 Auto,

Good clean jeep, rarely wheeled, never put under water.

Its been going on for a bit, start it up gauges sits at 12v

Then it drops to 9v check gauges light comes on. While it was running and gauge was there I put a voltmeter on the battery and had 13.9v across the terminals while it was running.

Occasionally if you shut it off and restart it the gauge will go up to 14v on the dash.

Grounds all look good, any ideas? I don't want to throw parts at it and see what sticks.

Turn everything on head lights on high ac on hight dome lights stereo and any thing else the check voltage. If it drops below 13.9 its the alt. Also just curious his old is the battery?

when the alternator crapped out in my xj it occasionally would drop output. a sharp rap on the case would make it start working again for a spell. fun replacement too, kinda a tight fit and stubborn bolts through the aluminum bracket.

Sounds like the brushes are lifting off of the slip ring(s). A new set of brushes will repair it. Find a local repair shop ans either buy the parts or just have them pop them in. Easy repair.

o-gauge-steamer said:

Sounds like the brushes are lifting off of the slip ring(s). A new set of brushes will repair it. Find a local repair shop ans either buy the parts or just have them pop them in. Easy repair.

Click to expand...

X2. Classic symptoms of bad brushes. An easy repair if (big "if" I'm afraid) the slip rings are not badly worn and gouged. If only scorched and dirty, no problem. You can clean up the slip rings with a gentle and minimally abrasive stuff like crocus cloth, but if they're grooved your new brushes won't last long. Check slip ring condition and proceed accordingly.

Brushes come either as an assembly with holder or alone. If alone you need to solder them into the holder. Pretty easy job, but not quite plug and play as it is if you get the whole assembly.

The tight fit of the alternator is due to a big roll pin in the mount. If you push the pin out a tiny bit the alternator will go right in. When you tighten the pivot bolt the pin will snug up again.

Unless you get stuck with a no start/ dead battery situation, consider the well documented "intermitant poor contact on the gauge cluster" condition.

The small little screws on the back of the alternator are the field terminals, and say how much voltage is being produced by the alternator. If that little part cracks or the screws get loose, then the computer and gauge will show sporadic low readings. I had to replace the little component part on mine when it started doing that. Most people just replace the whole alternator. I had another one laying around that didn't work anymore (seized bearing) and was able to scavenge the part.

I haven't popped in here in a while but was since my 2000 is doing the intermittent gauge drop and very please to see this thread. Thanks guys -- Great info! I will probably end up having mine rebuilt locally after the holidays anyway.