Voltage down

14 is considered normal, so diagnostically speaking it would pass, but there's a possibility that the alternator is starting to get weak, corroded or the belt is loose or the alternator pulley bearings are dragging, anything that would cause more energy to be expended in order to turn the alternator. The voltage regulator generally breaks to the high side, so rather than seeing decreasing voltage, you would see high voltage and overcharging. What you're talking about is the voltage with the truck running, so the batteries wouldn't be the issue as the voltage you are seeing should be the alternator voltage, but if you'd like to check the alternator directly you can do that by taking a multimeter and setting it to volts in the tens range and putting the leads directly on the alternator positive and negatives. That will give you a reading on the alternator output directly.

 

I'm about to replace mine. Mine shows 12-13. They are CAT batteries. They been under my steps for 8 years.

 

if you have batteries still functioning after 8 years, they are possessed!

 

Wow, you got lucky you didn't get stranded someplace with the truck shut off. My batteries will be celebrating their 4th this fall too, and I plan to get them out of there before winter trouble happens. What I wanted to say though, maybe you should consider another voltmeter, or even as someone said, the quick plug n play lighter one, just as a back up. The anaolog gauges are prone to not being always spot on, and can be hard to notice when there is a problem. Your system shouldn't be running at 15 volts, I think it's reading too high. That's the nice thing about a digital one with a decimal point. I switched out my analog pyro last winter for a blue lit digital one. Love it. Especially now in summer temps when I have to watch it close on upgrade pulls. Anyhow, glad you found the prob. Keep that coal rollin!