1994 Peterbilt 5EK power loss issues

Have a 1994 Pete with a 3406e 5ek set too 550 non low nox file. Truck was running good then ecm went out. Changed ecm then starting throwing a code for 5 volt issues and only making 15 pounds of boost. Changed engine harness twice and still no change and shows no code for sensors. At a loss any input would be appreciated