3126B Stopped Cranking for No Reason!!!

[CoffeeGuy]

I'm nearing my wit's end. 3 weeks ago, I was working on our conversion, a 2000 Blue Bird AARE with the Cat 3126B and MD3060. The work consisted of adding Amazon's generic version of Dynamat sound dampening panels to reduce the noise entering the cabin. No big deal.

I started the engine at 10AM to check my progress, and there was some improvement for sure, so I finished the job at around 1PM and my Cat acted like the batteries were drained. Thinking that maybe I left the key in the accessory position, I put the charger on the battery bank and forgot about it for the day. As I said, that was 3 weeks ago. Since then I have done just about every diagnostic test I could think of. Batteries are good, load tested and passed. I cleaned terminals. I checked every fuse and relay that I know about, all passed. I have an extra starter, so I swapped that in.

Having eliminated all other variables, I found that there is a thick orange wire (maybe 14 gauge?) on the positive side of the starter that is linked to the problem. It disappears into a harness that runs toward the front of the bus, but I haven't been able to follow the harness through the whole route because I have storage boxes and water tanks mounted underneath. I may need to remove them.

So here's where I could use some help. I would like to know what this orange wire does. If it is not connected to the starter (and all other wires are connected) the dashboard indicates normal operation when the key is turned to the on position, except that the grid heater light remains illuminated and turning the key to the run position does not crank the engine. When I connect the wire back up, turning the key to the on position causes a weak signal with the grid heater barely staying lit, the tones that normally sound make a weak buzzing, the relays in the engine compartment struggle to make contact, lights dim, turn signals stop operating, and turning the key to the start position results in no crank.

Checking this wire it is not grounded in the off position, the on position or the start position. It does not have 12v in the off position, the on position, or the start position.

What am I missing? Do you have any clues what might be going on? This is really slowing down the build because I just can't let go of it without solving the issue.