Wiring My Thermostat

I have an old Lennox system with 4 wires between the thermostat and the air handler control board. It is not clear to me in the instructions on how I should connect. I know I need the “C” adapter, but I have no wire connected to the “Y” terminal. On my Thermostat, I have a blue connected to the “Y” terminal. This blue wire heads out to the compressor unit outside and then comes back to the “T” terminal (Common).
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@speadie how you doing

Can we get some help here from the hvac guru himself

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I believe that on these old lennox systems, T is C. Therefore, the blue wire is Y from the thermostat, the red wire is Y from the A/C compressor, and you would hook your C adapter up as follows:

Sorry about the overlapping R and W wires on the diagram, no way to avoid it with this configuration, but the C adapter ribbon cable wires should all connect to their same respective letters other than C, which connects with the white wire coming from the A/C to the T terminal, and Y, which connects directly to the red wire coming from the A/C compressor.