99 Hyundai Sonata AC problem...please help!

I have a 99 Sonata that has a problem. The air conditioning only works intermittently...meaning the clutch only kicks in sometimes when it's in the mood. There is no problem with the fan. Is there some kind of sensor
that tells the clutch to start working? I cannot get a good reading on the system without the clutch working so I am still unsure if it has a low charge. Is there a low pressure sensor keeping it from starting? When it does work, it is cold therefore hinting to me that it has a good charge.
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