I do have the wiring diagrams and service manual. But there are only two solid copper wires going from the stat to the controller. One is brown and is the ground and the other is yellow and is the signal wire. So I can't change the input into the controller. I did try swapping thermistors to see if it was a bad temp sensor going into the stat. But the front furnace worked with either thermistor and the rear worked with neither. I could swap the output around, but that wouldn't tell me anything new. How do you test the signal wire? Should it send different voltage based on which unit should kick on?
