What would make the A/C turn on and off. on 2009 GMC Sierra 2500 HD

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It is Fully Charged, The low Pressure Switch has been replaced. but it still turns on and off on its own. The fan does not tirn off it is only the compressure.
(1) Answer
could be overcharged or undercharged. how do you know it is fully charged?? what are the pressure readings??