While replacing the engine, your mechanic at some point had to disconnect A/C lines from the A/C compressor that's mounted on the motor. In doing so, he lost your refrigerant (134a) charge and never replaced it. If everything worked before, and it was reinstalled correctly, it should run by evacuating/ recharging it with about two 14 ounce cans of 134a. Your mechanic should be willing to finish his work correctly for free. That lost refrigerant would cost him only $15. I hate to see women get ripped off. I hope you get treated fairly this time.
They probably did not properly recharge and check the a/c system such as wiring and filling the Freon.
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