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Hi my name is Robert...
Few things on this.... To start go to the AC compressor and make sure the clutch is coming on for the compressor, you will see it come on and off. IF it is not coming ON then it could be low on freon, a shop can evacuate the Freon and see how much is in the system, if it is low then it should be checked for leaks and a vaccum put on the system.
The coolant has to flow through the heater core box if the coolant is not flowing then the AC could be warm, the system should use a H valve or electronic water valve that meters the amount of coolant through the heater core box. Normally that valve is under the front cap area on the passenger side, the heater core box will have hoses going to it and the valve would be on that line, it will also have wires going to it. IF you feel each side of the rubber hose it should feel warm, if one side is noticeably hotter then that valve can be the issue.
I am not sure which valve you are thinking about shutting off, normally that is not required at all and can cause problems if it is going to the heater core box.
Hi I see you have not selected a rating, if you still need more help on this please let me know... Robert