My 1999 XJ has started having the battery voltage (as viewed on the dash) hovering between 11-14 volts in a seemingly random manner with the AC on. It does not do this with the heat running. My belt are probably tight enough, could be slightly tighter but doesn’t squeak ever. With the AC off, everything behaves perfectly normal. What could it be?

 

when a high load comes on, voltage should drop. I assume it is a bigger drop if the headlights are on also, right?

However if the belt is old, replace, if belt is loose, tighten it, if pulleys dirty, clean them Belt slippage may make the drop worse if alternator output drops too much do to slippage.

Finally make sure the battery/ cable connections are clean and tight as a high resistance path here can cause system wide drops under loads.


good luck

 

(just posted this on your other thread on NAXJA)I'm going to bet the belt is slipping. Don't guess on the tension. Just pickup a Krikit II belt tension gauge for $12 at Napa and set it correctly. Per the FSM, a new belt gets between 180lbs and 200lbs. A used belt gets 140lbs to 160lbs. Used belt is defined as one that has been run for 15 min or longer.

 

Yes, and don't just crank it as tight as you can get it. That's a great way to shorten the life of bearings.

 

Totally agree with this.
My '00 seems to be picky about the belt tension.
So I invested in the Krikit ll myself.
To experiment when I first got it I used the "Yeah that looks about right" method.
Then checked with the gauge. Was approximately 40lbs to loose. LOL.
I use the specs that lawsoncl posted but try to stay towards the lower end of the specs when setting the tension.

 

Well I sure am surprised at these belt responses...

Clean your battery connections with a battery brush, and clean the ground connections. There are many threads and YouTube videos on how to do that.

I also found a neat little tool called the "piloted bonding brush". It makes quick work of cleaning ground points, but you can get by with some sandpaper.

 

UPDATE: the next day, the battery voltage would not read above 12v with the ac on or off. Replaced alternator and issue was solved