How to disable a thermostat to prevent it from turning on the AC

After looking closely at the images you've supplied, this is what it appears is going on in the last photo.

From what I know about HVAC systems, I would guess a broader view of the system would look something like this.

• The transformer supplies power to the thermostat through the RED wire.
• The BLUE wire serves as the COOL call from the thermostat.
• The BLUE wire connects to the WHITE wire, which is connected to the coil of the contactor in the condensing unit.
• The GREEN wire is the other side of the contactor coil.
• The GREEN wire connects to the BROWN wire, which is the other side of the transformer (C).

When the thermostat calls for COOL, the BLUE wire is energized. This energizes the WHITE wire leading to the contactor, which pulls the contactor closed. The GREEN wire connects to the BROWN wire, which leads back to the transformer, thus completing the circuit.

Therefore, if you want to use the YELLOW wire as C. You should include it in the twist-on wire connector with the GREEN and BROWN wire.

You'll just want to make sure the transformer is sized properly to supply the additional current draw of the thermostat.

If it were me, I'd rewire it so that the YELLOW wire was the COOL call, and the BLUE wire was C. As that's a more common color coding.

My second stab at answering this:

Since this is not a heat pump, I now see more clearly you have a photo that shows your blue wire connected to the coil of a contactor in the unit that is separate from the contactor controlled by the white wire but also with heavy supply wires coming into it. Judging from the large size of the wires fed into this contactor, I believe this was originally meant to be control for stage 2 electric heat. However, given the small gauge white wire that is connected to the load side now, I cannot say what has been connected in place of stage 2. I can say that unless that small white wire coming off that contactor is separately fused, then it is probably a fire hazard. I also see in the same photo that you have a lot of bare wire exposed on that heavy conductor.