Voltage down

Your alternator could be starting to take a dump. Maybe corroded wire somewhere or a short on a positive wire. I would disconnect batteries, does your volts change. If it goes back to normal, check each battery you should find your problem in the batteries.
If it doesn't return to normal, my guess is alternator. But check your wires first.
I think the low voltage is below 12.4. Not sure on your truck mine is a 99.
My alternator always reads about 14.
Good luck.

 

What is the voltage?

 

14 is normal.

 

Check the batteries, mine just done the same thing and it was a battery pooched. In my experience an alternator doesn't slowly go bad, it either works or it don't.

 

I don't know about where you run, but out here in AZ, 4 years is living on borrowed time for batteries. I would have them load tested and check the wires for corrosion.

 

Belt tension for alternator? Had to have the head taken off mine over winter, for a month after sometimes the voltage would be lower than it should. Was not a consistent thing tho- tightened belts a bit and voila.

 

You can get a digital voltage meter that plugs into the cigarette lighter right? Think there pretty cheap. I'd do that and get a real reading before i did anything.