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Hello, while the system is still running in cooling mode, change the thermostat setting higher, one degree at a time, and tell me if the cooling ever shuts off before you get 85F.
That is a test of the differential controls inside the thermostat programming.
Thanks, ***** ***** case you have a 2 degree F 'dead band' programmed into your thermostat... that is the most energy efficient, and keeps the outside unit from short cycling.
If you set the thermostat for 78F and it keeps cooling to 76F then that is how it is programmed to work.
If you want it to shut off at 77F when you have it set to 78F you will have to follow the thermostat set up procedures on the inside of the button cover on most thermostats and set the differential to 1F ... that will however run your ac electrical costs up by maybe 10% or so.
Experiment with this until you can see what is doing. Stay in touch.
some thermostats use the term Differential or Diff, instead of dead band
Good work on the testing, If that is workable for you please locate the 'rate your expert' link, its how I earn a living. (rating will not increase your charges.)