Eric, I doubt the weather's warmed up much, but were you able to get the new switch installed and get back to plowing?
Funny you ask as just yesterday it warmed up to 30°F.
I got the new key switch in and observed for the first time that two pins of the five pins on the harness side have been running hot (likely caused by the failing switch). I snuck needle-nose pliers into the connector and gave those two pins a slight tweak in the hopes that the blade connectors would make better contact.
Still no start off of the key.
Checked all of the fuses that they were good.
Still no start off of the key.
Flip-flopped the seventy amp fuses under the hood (wasn't thinking that was it, but why not, it was easy to do).
Still no start off of the key.
Confirmed fuel heater function to work.
Still no start off of the key.
Yanked off the dash cowling to get at the brake switch which is normally open and closed on brake application and temporarily jumped the switch.
Still no start off of the key.
Then I considered the chain of events the day it broke:
I was plowing my easement and the engine stopped. I assumed gelled fuel as the engine sputtered, the ECU light came on, and then the engine slowly died. I was able to start the engine again and it idled. Since I was only 300 yards away from my garage, I picked up the plow and drove through the 20" or more snowdrifts trying to nurse the tractor back to my garage. The engine stalled any number of times in 50 yards and I restarted it and got it closer to my garage until the engine suddenly stopped starting on the key. I went and got a remote starter, got the engine running with that, lifted the loader up all the way, and popped in a loader lock on one of the lift cylinders before the now cooling off engine died again. With more fiddling around I confirmed that I had gelled the fuel that I had purchased on December 3rd (which subsequently I learned that my local gas station did not have the winter blend at the time—a thing that other people are now warning me about from that particular operator).
So right now key on with a remote starter, I'm off to the races. The ECU is behind a paywall so I cannot simply plug in a scan tool and find out what the ECU is not seeing to trigger the starter circuit and I never did test the ground or power sides of the brake switch to look for that potential brake fault mentioned from rubbing wires, but I'm pretty sure that my fault is something pretty stupid.
My working theory, for now, is that snow got compacted on top of my skid plate and knocked a neutral or PTO safety switch off and since it is still cold out, and the undercarriage of my tractor is an icy winter wonderland, I'm going to let puzzling out what is really wrong buck until spring.
Life without a shop.