2013 peterbilt 386 with mx13 455hp. Low voltage to ecm.

i was told by mechanic that my ecm is getting 4.7 volts. He has checked all grounds and batteries. Alternator and starter cables. Fuses and relays. Im thinking the problem might be in the wiring harness. I cant find anything online that would show me the electrical pathways from batteries to ecm. There are three power supply wires to the ecm ?

 

Ty so much.. i will study this and see if i can make heads or tails of it.. lol problem is the truck is at mechanics shop a coupl of hours away and i work so many hours finding time to check out wat u sent me will be hard to do. Thanks again

 

Thanks for input.. ill be sure and check that area