Chrysler Repair: AC Compressor lock up, refrigerant pressure, compressor clutch

Question
QUESTION: I have a 1999 Chryler Cirrus and have replaced the ac compressor twice within the past month (used), why does the compresser keep locking and freezing?

ANSWER: Are you putting in the correct mixture of refrigerant and lubricating oil?

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QUESTION: I have had a mechanic replace it, and he was certified in ac repair

Answer
Hi Pam,
Other than the amount of refrigerant, I don't know. Perhaps the refrigerant pressure cut-off switch is not working. It is designed to disable the compressor clutch if the pressure in the system is either to low OR to high. If that failed the system might be running when it shouldn't and thus causing the lock-up. The mechanic should stand behind his work.