missed the "tan" part, if my memory doesn't fail me, in the older yorks w1 powers the inducer and after the centrifugal swt closes, the 24v travels through the safties and lands on the Johnson, right? I dont do RTu's as much anymore.
I'm not going to make it easy on you. LOL Look at the heating circuit in your schematic. There is a 3 wire switch whose contacts are welded closed on both sets of contacts. If you follow the schematic, you will see why you have runaway heat. When you replace this you need to order the replacement kit from York, not just the part.
I had one run away over xmass. The customer had a HUGE fish tank. All you could smell was fish. It was so hot
in the bay it melted his hard drives, and cooked his fish. Just some bones bobbing around in the tank. It was the
3 wire limit issue.