I replaced my alternator last year and never had any issues. Let week the voltage jumped to 18v on the my dash gauge. I look at my monitor and it’s not going above 14.5v. Any suggestions on where the issue may be?

 

Any time electrics act up I go to the grounds. Check every one you can find and grind it and clean/lube it. Free and easy and maybe the fix for your erratic gauge....

 

Adding redundant grounds everywhere won't hurt a thing either...I like using tin-plated copper strap when I can find it......Ben

 

What monitor are you referring to?

 

Well, my first action would be to verify with an independent device (multimeter)

Your monitor will show the measurements from what's communicated by the bus.
Your gauge cluster will show you it's interpretation of the data on the bus.

Now there are 2 possibilities:
a: Something is messing up the Voltage
b: your Voltage is OK, just some indicator messing up.

This leads to two completely different paths of diagnosis and fixing, (Even though the mention of grounds is valid, I'd go and measure reality first.)

 

It’s an Edge monitor that plugs into the OBDII

 

Its acting as a mini scan tool in this case, then - reporting what the PCM sees. Check the wiring diagrams and see how the voltage gauge in the dash is fed. As Alpine Ram says, measure reality at the batteries, too. If a multimeter on the batteries reads in the 14s and below (depending on temperature, time charged, etc) and the PCM is showing the same - then I would suspect an issue with the gauge or the feed to the gauge.

Do you have access to the wiring diagrams?