1987 peterbilt 379 voltage surge

I think that block on the inside of the frame rail is just a circuit breaker, at least that's what the call it on the wiring diagram.

 

I had that problem. At that block there is a thick wire that goes to the cab that was corroded and was almost brocken at the connection. I took it of and splice it and put it back on. Fixed it.

 

I would try going over a bad bump with a known good volt meter hooked up to rule out the gauge before replacing any parts

 

I would inspect the battery cables from batteries to starter for any chaffing and loose connection at the starter or batteries.The early years of 379 used metal straps w/rubber lining to suspend battery cables to the starter .They had a tendency to wear through cables and short out.Also check the ground cables from the starter to the frame and engine

 

You will also want to check the cab ground ,frame ground and the battery ground, any corrosion at these points can cause the volt meter to read low, also the cab wiring is on a common ground in the cab, check that too, a sealbeam type headlight makes a good tester for this type of stuff, hi beam draws about 10 amp, lo 5, together 15. A corroded connection will often light a test lamp, but not carry much load.