Wag90

Hey guys.
I own a 1990 Jeep Wagoneer Limited (4.0l, inline 6, Renix), with 170K miles on the clock. It’s my DD, unmodified, and it runs great. About a month ago, my alternator died and I had it rebuild and it works fine now. My battery gauge on the dash was always accurate, but couple of days ago, it started reading high (close to or in red line), like it was overcharging, and after an hour of two of driving, it would come down a bit (with ac blowing, lights and radio on). I checked with my multimeter and it reads 14V when the Jeep is running (cca 13,5V with the ac and the lights on). I also have a dashcam, and it reads the same as my multimeter (13,5 - 14V). My battery is new (12,5V when the car is not running). So, everything is as it should be, and I’m wondering why my dash battery gauge is reading higher than it should. All of my grounds are refreshed, and I added new ones a few years ago (per Cruiser’s tips).
What could cause this issue, could it have something with the alt being rebuilt?
Also, with the engine running, does the dash battery gauge show an alternator output or the battery?
Thanx everyone!

bluejeep2001

With the battery connected to the alternator, they are the same, the alternator charges the battery to its output but with alt off the battery instantly drops to its internal voltage. If the alternator put out 0 volts you would still read whatever the battery charge is

Wag90

Thank you bluejeep2001. I found a lot of info online that says that on these old vehicles with the ignition on (engine not running), the battery gauge shows the battery voltage, and with the engine running, the gauge shows the alternator voltage (which is higher than the battery voltage in order for it to charge the battery). I'm not sure if this is the case with the XJs. Can someone verify if this is correct? Cheers!

Phydeaux33

That is correct, although the dash gauge is not completely correct, when I turn key on, not start the gauge reads a little over the 9 line ( even though a multi-meter reads 12.8 at the battery) and when running just under the 14 line (multi-meter reads 14.2). It is correct enough that I can see if there is a problem.

Wag90

Thank you Phydeaux33. That's exactly the same how the gauges on my Jeep operated prior to this issue. Not always 100% correct, but correct enough to spot the problem. It's weird seeing it go to the red line now. So, it is possible that my dash gauge shows 16-18V from the alt, but my digital voltmeter shows 14V when connected to the battery (with engine running)?