1989 F 250 Air Conditioning
 

My 1989 F 250 air conditioning is not working; the pump seems to go on for a short time then off then in a short time it goes back on like it is cycling on off on off. Everything else seems fine. The air coming out is lightly cool, I know something is working as it is over a hundred out side and there is no way I would get anything but hot air if it was not working at all. What I would like to know is what could be wrong? Is this just a case of needing recharged or more?? Thank you for your help.