The car a/c switches on and off when it wants. How do I fix this?

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Happens every time a/c is used. Been happening for years but only drive once a week. When the green light on a/c button is on, the a/c is cool but it shuts off on its own. The air still blows when light is off but it is warm air. Is there a switch that would fix this. My wife wants me to add a/c but that isn't it since been doing this for a long time and if was low in when it started then it would have been empty by now.