Voltage meter showing incorrect reading

Did you check the voltage with a multimeter while the truck is running?

 

When you tested the battery voltage, did you test each battery independently, while disconnected from all other batteries?

A broken plate or two in a couple batteries can throw some truly weird readings.

If any individual battery reads significantly different from the others, either high or low, replace it.

 

If it’s jumping around it’s possibly your frame ground. Otherwise, your voltage regulator may be going bad and it’s going to cook your batteries. Your truck ECM cannot handle 20 volts, you need to check the voltage coming from the alternator under load.

 

Regulator is bad would be my first guess. Just about all the computer stuff is going to get burned out from within, cooked to junk in a jiffy at 20 volts. Might even total that truck.

 

Could be the/a ground or the alternator. Normally when the volt gauge spikes, it is a sign that the alternator is failing. Let it go to far, you can do some serious and expensive damage to the electrical system. Have the alternator inspected and get under the truck, start at the battery box and start inspecting the cables. A 2007 unit, odds are you will need to pull and replace some cable, and clean up a few grounds, unless you keep track of that stuff. I have a feeling you do not, as I didn't either until I got under mine and had to replace 100 feet of that cable..

 

OK I guess troubleshooting 101 is out the door.

 

I'll admit to not being a diesel mechanic, but looking for component failure in the simplest high potential source of failure should be a good place to start, if the truck isn't giving error codes, right?

What would you do first? Real question. Not sarcasm.

 

I posted exactly what I would do, look at the votage at the batteries while it is running, if it is stable, then it could be other issues.