I bought a TZEMT400BB3 thermostat. I need general install assistance. I get how it works, and I understand the what a common wire is. My current setup doesn’t have a common wire, but the Trane needs one. I had a blue wire behind my thermostat that was unused, so I went down to the furnace and connected the unused blue wire to the “B” terminal. It won’t power the thermostat. I’m getting just under 24V between the red wire and the common wire. It will power on when I connect the ground (fan) wire in place of the common, but this obviously won’t work as a solution. I get 28V between the red wire and the fan wire.
So what gives? I’ve attached a picture of my transformer. Note: My system is old. Like 1987 old. Any help is greatly appreciated.