Youtube videos-- "TERRY CHRON"-- cummins/n-14 expert? thoughts?

Not knowing how it is wired now but from what you posted it was getting power all the time before the wire swap.
Where this wire is does the large batt. cable from batt. go to that lug on the starter?
If you don't know take a volt meter or test light to see if it is hot all the time now.

My guess is over time that large batt. cable gets corrosion inside the cable you can not see. Happens all the time with old muscle cars batt cables if they crank slowly cold and more so when hot. Only way to see this is to cut open the insulation.

Any way because of the corrosion the resistance of the cable goes up and when you crank up the motor all the volts go to the starter and low or none to the EMC. Because you pick up power for the ECM at the batt. it lessens this to happen.
Dave ----

 

My ISX has 2 unswitched wire from the batt, and 1 from the key on position of the ignition switch, the original power harness from the battery {2 , inline fused wires} was run down along with the transmission cooling lines, that position is not protected enough, and wires shorted out, am replacing with another set of wires in split loom higher up , the 3rd wire off my ECM is the ground wire run to the Batt NEG,-, personally I would hate to have my wires mixed up with the starter, things got so hot it melted out the top lug in the starter, the post above mentioning resistance from corrosion , makes sense to me, and yes they're hot all the time