With the engine off clean the compressor clutch with carb cleaner. Allow to dry. Test A/C system. My 91 300TE had a slight leak at the front compressor seal which deposited oil on the clutch and caused the the clutch to slip just enough for the rpm sensor to detect and disable the system until the next start cycle. I had to repeat the cleaning every year until I ultimately replaced the compressor before selling the car to a neighbor. Mark