Hi guys i have a 00 Xj auto with the 4.0. i got a brand new optima yellow top for about 3 months and since 3-4 days im getting weird voltage. sometime s i start the truck and the voltage wont go higher than 12 .. and will then drop to 0.Sometime it goes up to 14 and then drop to 0 again. Even if i roll with the voltage a 0 i still can do 20-25 km and the truck wont die. If i stop the truck at 0 voltage and then restart it it start like brand new and the voltmeter goes to 12 again. I Unplugged my negative battery pole while the truck was running and the voltmeter stayed on 14 so my alternator seems ok. whats up with my XJ ?? thanks guys and sorry for the english im from Québec,Canada , im trying my best haha !

 

My first guess would be a faulty ground, or more, making intermittent contact.

 

I hope you didn't kill your Jeep's computer doing that... Good news is your alternator obviously works, to a certain degree. But removing the battery while running is very very very bad.

It sounds like you have a bad connection somewhere. Normally with just a bad ground you see the volt meter jump up to 19.

First, test with a voltmeter or multimeter. Absolutely verify the voltage at the bettery terminals, or on the power distribution centre, is at 14V while running and 12V when shut off. Refreshing your charging system connectors is always a good thing and may actually fix your problem. One by one remove the power cables, clean the ends until shiny, spray some electrical contact cleaner, and reassemble with a bit of electrician's grease like Gardner Bender OX-GARD (sold at Home Depot, to prevent corrosion between aluminum and copper conductors). Get the battery terminals, alternator terminals, starter, Power Distribution Centre, and grounds. If this doesn't take care of it I'd look at the connection on the instrument cluster itself.

Also your English is better than some native English speakers on this site... Lol

 

Alright , ill take a look at this ! thanks a lot !

 

Having a voltage of zero on that dash, i highly doubt a bad ground. I suspect an issue with the voltage guage circuit.

 

Agree with Salad wholeheartedly.

Removing a battery connection while the engine is running is an "old school" technique which did work.....25 years ago when there were no solid state components. I used that technique a long, long time ago when I had a 61 Dodge and a 66 Chevy. But using this technique on later model vehicles can fry your alternator, your computer OR both. I have repaired a number of vehicles that have had fried alternators and computers when they tried this. Sometimes you get lucky and get by. But sometimes you don't. So do you feel lucky? Here is a link to an article on why this is a bad idea....

http://www.troubleshooters.com/dont_...ct_battery.htm

I agree that first step here is to freshen all grounds and connections on the charging system. Simple to more complex.....always. Grounds can be the cause of many electrical gremlins!

 

Swapped the alternator for a new one , everything works great , thanks for the help guys !