My 2000 XJ sport as 182k and runs great but on and off for a couple of weeks the volt meter will sometimes drop to 9.

I turn on the engine and it will be at about 11.8 (usually it is around 14). After a mile or two of driving it will plummet to 9 OR it will shoot up to 14 and all will be well. Usually when it drops to 9 I turn off the engine, hit the battery terminals with a hammer, turn it back on, and everything is fine.The problem has become more frequent though and I tried cleaning the terminals very good, but to no avail.

I don't have any indications that something is wrong except for the voltmeter (no failing power or trouble starting). I am wondering if this is bad reading, failing alternator or something else. I have not gotten the batter/alt. check yet (autozone's is broken).

 

It sounds to me like a voltage regulator going out. My experience has been that they tend to just fail without warning but I've also seen it being jumpy on (rare) occasion. This is just my general automotive $0.02, not necessarily Jeep specific

 

On the Jeep the voltage regulator is in the engine computer. Could be a bad alternator, slipping belt, or rotten battery cables. Are you measuring the voltage with a multimeter at the battery itself?

 

If it isn't an incorrect signal in the gauge, then I'd imagine that the battery cables need to be replaced as the most likely issue. They are wearable parts, not quite like spark plugs...

 

put a voltage meter accross your battery terminal, and start the motor. it should read 13 to 14 volts depending on how much your batter needs at the time. when the volt meter in the dash is low do the same check, see if the dash meter is correct or not, it is pretty common for the dash meter to be bad. I think you will find that the voltage actually going to your battery is fine and that you have a screwey Gage in the dash cluster. hope this helps.

 

Thank you all! I was finally able to check the voltage and found it was a bad alternator. I switched them out and so far all seems fine so far. I need to give it a proper test drive though.

P.S. The bottom bolt is a [expletive] to get off! I read somewhere on these forums that tightening the stuck bolt before loosening it will help "break it free." That actually worked! Had to go back and forth for like 20 minutes but it broke free. I did this in a uni parking lot so heating it up/taking off the bracket wasn't an option, and I didn't have a breaker bar. Just leaving this tip here in case anyone else needs it.