No a/c at operating temperature

I recently purchased an 88 Cherokee, and am loving it except for a problem with the air conditioning. When the car is cold, the a/c works fine (compressor cycles on and off), but when the vehicle reaches operating temp (can't be sure, no temp gauge), the compressor does not cycle on. It does cycle on vary rarely, but mostly it stays off and the air coming out the vent is warm. Any ideas? I have the equipment to check the pressure in the lines, but I'm not sure what it's supposed to be. Thanks in advance!