Went on a service call. Bad hi limit switch, but that isn't the good part. Countless code violations is the fun part.
We had lost this replacement installation job to a lower bidder. They installed the two newer Tranes. The two tranes, the old carrier, and the water heater are common vented. There is a ventor on the common trunk of the flue going into the chimney that should energize upon a call for heat. Because the new Tranes have ventor motors on them, apparently the installer figure they didn't need the common ventor. They cut the wires, and tucked them neatly up above some ceiling duct work. When any one unit is running, it dumps its combustion gases right out the draft hood of the old carrier into the room.
The three units, plus a water heater in the other corner, all common vented.
Here I hold the cut wires in front of the relays that used to control the common ventor.