I have an older rheem imperial 90 furnace with central a/c. a few years ago I had a problem with it and the furnace circuit board and the thermostat were both replaced. Unfortunately this only solved part of the problem. I could use only the a/c unit or the heat unit but not both. The repairman would come and switch a wire on the furnace circuit board to change the working unit. He did mention something like changing a black wire on the right side of the circuit board on the y1 or y2 terminal to the fan speed. This repairman is no longer available. Can anyone help? Does this procedure sound remotely familiar to anyone? I know nothing about furnaces and I don't see any terminal labeled y1 or y2 on the board. I need to switch from heat to a/c.

I really need to suggest that a proper control be installed, switching wires is not done and is not allowed to be done. The system should run automatically without any need to fiddle with it.

Without specific details, pictures, model numbers ,trying to determine which wire and terminal is which is impossible.