14 volts at the battery and alternator and 12 volts on the gage

Most gauges in modern trucks are electronic and feed data from some kind of module. So if your gauge is reading low and you are measuring good voltage everywhere, chances are the module that reads data for that gauge is getting faulty power itself.

 

With 14vdc at the battery, and 12vdc at the fuse panel.. Check for a high resistance connection between the two....

If you are able to test the feed wire (Disconnected at fuse panel), the measurement with no load should be no more than .5vdc lower than battery....

... Philip

 

Add you some extra braided strap ground cables from engine to chassis, and from chassis to cab, you can also go from battery ground to frame chassis rail. Won't hurt nothing, just make sure they're heavy enough to carry the load in case your system is hurting for a heavy voltage ground. In other words you don't want to run a # 16 wire and watch the smoke show.

 

Normal charge rate is 13.9 volts, when your rate starts consistently dropping below 13 volts your going to have problems.

 

Gauges are sometimes inaccurate. Is 12V what the gauge reads or what your VM reads at the gauge terminals?

I don't know much about these newfangled computer trucks. Is the gauge traditionally wired or does it come from the ECM?

 

Did you complete the test I prescribed?

.... Philip

 

Looking at a 387 schematic, you go from battery positive to a series of mega fuses for each load center. See what you got from battery positive to the mega fuses, and then see what you got from mega fuses to cab fuse load center etc. Are all load center voltages lower or just the cab panel one? Schematic also shows an optional battery disconnect switch, which would supply voltage to mega fuses, too. Another thing about the ground situation, are you reading your volt meter with it directly grounded at the bateries?