I was on my way home the other night in the rain i noticed my voltage gauge was reading about 12 volts instead of the usual 14. I'm thinking the belt is slipping real bad or the alternator is shot , then the needle drops to 9 the check gauges light comes on and the CEL comes on . I get home and put a volt meter on the battery and I'm reading 13.8 volts at idle . I shut it off do the gauge check and every thing works , restart it and the gauge is showing 14 volts . any clue ? The CEL was a low voltage code by the way . It's a 98 xj that i just did a cluster swap on .

 

Your problem probably lies there. Did you clean the contact points? Why was the cluster swapped?

If you're reading 13.8v with a meter, chances are your charging system is ok. Load it up a little with the lights and blower and bring the engine up to the 2k rpm range. You should be reading around a 1/2 volt less than at idle no load.

 

Sounds like belts a little loose gets wet and slips.

 

swapped the cluster so I could have gauges, the original had idiot lights . it hasn't happened since .

 

Mine has read a bit low lately. The lights get dimmer when idling at first, and my digital volt meter that is on my sub cap reads 12.88 at cold idle. The alternator was replaced about a year ago, so I wouldn't suspect that. It has been pretty cold, but it seems to do better when it has ran for a few minutes, reaching 13.5 like it usually does. Could the cold affect it?

 

Try tightening the belt.

 

Let's make sure the belt is loose before going to try tightening.. Likely you would notice if it was slipping.

I would go and get your battery tested, and ensure that all of your battery/ground cables are in good shape, and that the ground points themselves are in good shape. Of course, this is assuming that the alternator is good.

 

My 97 would do the same thing. I kept checking the battery voltage whenever the gauge dropped and my battery light would come on and got the same reading every time. I thought it was just a wire rubbing somewhere causing the gauge to act up. I went a good month with it like that.
Until one day on my way home at 10pm the lights dimmed till they were almost off, and every dash light came on, and of course it was raining... My alternator died.

 

It hasn't done that since this morning started so cold, I was reading 14 all day even with the lights and blower on, so I feel better.

 

Also my belt is at the perfect tension!

 

Never hurts to freshen your grounds. More below.
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Grounds can be the root cause of many electrical gremlins. Refreshing grounds is NEVER a bad idea, and the investment of your time in this procedure is always well worth it!

You can't tell much of anything by looking at ground connections!! You must remove, scrape, clean until shiny the cable/wire ends and whatever they bolt to. Be sure to remove all paint from any ground connections.

Start with the one on the back corner of the head, and where it attaches to the firewall, as it deteriorates over time and is an area that makes it susceptible to damage. Best to replace that woven cable with a #4 or #2 gauge cable. You can attach the one end to the intake manifold if you would like.

Next go over to the engine dipstick tube stud. Remove the nut and clean the wire ends and scrape the block until shiny at the stud. Reattach tightly.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support.

 

+1 freshen grounds and all other alternator/battery connections, checking belt condition is a good idea any day of the week.