Karjalahti

Some mornings I go start my jeep and it starts up just fine, but it is only running at 12 volts, the lights are dimmer the wipers move in super slow mo, so i pin it and rap it around to 4-5 k on the fix o meter (rpm gauge). and sure enough it pops right up to the 14 volts and it is fine for the rest of the day.

Should I just go ahead and replace the alternator, or is this something more sinister those electrical gremlins are not fun to find.

any help ?

thakmfb

I would freshen up your grounds and battery connections. Make sure there is no corrosion on the battery terminals at the battery and at the alternator/starter.

montanaman

In addition to the above, try this:

When the motor is running at 12 volts, rap on the back of the alternator with a screwdriver handle or something. If that fixes it, your brushes might be stuck, and not engaging with the slip rings.

If that is the case, it is an easy fix. Just remove the alt, pull off the back cover, remove several parts on top of the brush holder, then pull out the brush holder. Spray it clean inside and out with brake cleaner, and double check that the brushes are moving in and out freely.

Reassemble the alternator, reinstall, and you're back in business. It's really very easy.

DFlintstone

Some guys sometimes will stick a Delco "One Wire" alternator in about anything. They may not kick in until revved.

Karjalahti

battery is new, grounds are new, starter is new.... had some starting issues previous so i think that should be good. The alternator is original 215k miles on it.... would it just be more cost effective to buy a rebuilt or tear it all down, I am wondering if it is the voltage regulator, something i no nothing about, is that inside the alternator.

SeriousOffroad

Nope, it's in the PCM.

montanaman

Your '93 XJ has a Nippondenso alternator with no internal voltage regulator. The computer (ECU) controls the voltage regulation, and basically never goes wrong. If it is an original alternator, you are in luck ... they are very high quality. I recommend you rebuild it yourself with new bearings and brushes. I have a bearings/brushes kit that I can send to you for $28.00 + shipping. Here is a link to a rebuild writeup:

http://www.jeepforum.com/forum/f11/r...1822aa-835511/

But first ... knock on the back of it when it's not charging. And don't rev the motor before you do that ... I'll bet that temporarily fixes the problem. If your alt. has 215K miles, the brushes are probably worn all the way down.

New brushes and bearings solve 99% of the problems with Denso alternators.

Karjalahti

so today i started it and did not rev it, got underneath rapped on it once, bam instant power. but my check engine light was on.... seriously i thought that cel was non functioning, due to the age of mater (the jeeps name). so is there any way to check the codes on these older vehicles.... it went away now but if it does come back on.

montanaman

Yes ... you can get the code easily ... Here is a complete explanation, with instructions and a list of code explanations:

http://www.jeepforum.com/wiki/index....k_Engine_Codes

It will probably be code 41, which means your alternator field circuit isn't energized (open circuit). That's what happens when the brushes aren't contacting the slip rings properly ... thus the need to knock it on the back.