98 XJ charges for a bit, then doesn't unless I reset computer

I'm looking for some input before I dig into this as it's 30 degrees out and windy, so I'd like to make the most of my time before my fingers stop working.

98 XJ 4.0, 261K. Replaced coil and CPS two weeks ago due to no-spark condition. No other recent work.

When I first start my Jeep up, it shows apx. 12V for a couple minutes, then drops to 9V (with the 'check gauges' light of course) and stays there. However, if I flick the ignition key off and right back on again to reset the computers, most times it will show 12V for a couple minutes and then jumps up to 14V and stays there until it's shut off. Then the same thing happens next time I go to start it. Sometimes it doesn't go to 14V but will drop back to 9V, but more often than not it will go up to 14V.

Any thoughts? I'd normally condemn the alternator with these symptoms but since it will go up to 14V and run properly just by resetting the computer I'm leaning towards voltage regulator which I thought was in the ECU..

Thanks in advance!

ok, dumb question - how do you reset the computer?

Flick the key off and then back on again. Quickly, so the motor doesn't die. Used to have to do that in my last Bonneville before I replaced the transmission to reset the computer when the torque converter wouldn't work properly.

cool, thanks

Ignore my first post. Here's a better description, with a more direct question.

When I first start my Jeep up, the volt gauge goes to 12V and sits there. For two minutes or so, and then it drops to 9 and sits there. If I shut it off, and start it up again, it sits for two minutes or so and then drops to 9 and sits there again.

Alternator, or voltage regulator?

I believe I am getting the same issue on my 98. The alternator seems to charge sometimes, but most of the time it isnt doing anything. I have searched for a week and I cant seem to figure it out. I had my alternator and battery tested at autozone and they tell me it is good. I am going to replace the alternator tomorrow due to running out of ideas.

I have checked all connectors and wires. They seem to be fine. I replaced the grounds, and ground terminal on the battery but nothing. It is intermittent so Im also thinking its the voltage regulator but it is in the ECU so I will try the alt first. I will let you know what happens tomorrow. Best of luck

I would get one of those digital battery meters that plugs into your cig lighter. They are less than ten bucks I think at pep boys or wherever... I have one, and I use it any time I suspect a problem. It is more accurate than an analog gauge, and tells you without a doubt exactly how many volts you have, and whether you are in a "charging" state or not. It would confirm or rule out an inaccurate reading, or a bad gauge in your cluster.

Im not sure about the other guy but I have tested with a digital multi-meter on the battery terminals to verify my guage reading. This being said the last time I monitored the battery, I noticed some discrepancies. Here is the list of events:

- Started the jeep and had about 11.9 volts on the battery before and after the start.

- This stayed for about 5 minutes and all of the sudden the voltage jumped up to 13.9V

- I monitored this voltage on the battery for another 5 minutes and it continued to charge, even with all power accesories running(fans, lights, stereo...)

- I took the jeep out for a drive and after a few miles the instrument panel voltmeter dropped to 9V and the check guages light came on.

- Once I was in the driveway I checked the voltage on the battery it was still 13.9!!!???

- Okay, what the heck is going on? I cut the truck off and started back up, back to 12V and running off the battery. I could not get the alternator to start charging again.

I am lost so I will replace the alternator tomorrow!

13.9 is normal... I would get the alt tested. the best about the gauge I mentioned is that it continually monitors and displays voltage and charging status even while you drive around and run lights and accessories.

UPDATE: 98 XJ charges for a bit, then doesn't unless I reset computer

Hey Jayski, I replaced my alternator Tuesday night and the truck has been charging properly so far. Ive driven about 150 miles and no problems yet. I really hope this keeps up. If the alternator fixes it, I dont understand why the other one tested fine but worked sparatically. Anyway, good luck!

Replacing the alternator took care of my problems as well. Thanks!

Question for jeep experts: Could that weird "battery temp sensor" make this happen if it malfunctioned? I've never heard of these before I started reading up on XJ's.