Cadillac Repair: Cad.Seville96 air compresser, electronic leak detector, air compresser

Question
When I start the car the computer readout says, low on free-on, compresser won't start. Off and on for 3 yrs. We go to add freon, system won't accept unless compresser is running. Other Mech says so. Could it be the Thermostat or a relay switch. When Mech. does start the compresser and I'm not there to see how, he fills the system, it lasts 3-4 months then compresser won't start, low on freon.  A Mech at work thinks it may be electrical.  We can turn the compresser wheel by hand. Can't really get a good reading on pressure because compresser isn't on.

Answer
Obviously you have a leak in the A/C system which is causing you to lose freon from the system which results in the A/C compressor not engaging. You need to check the complete A/C system for leaks using an electronic leak detector so as to find and fix the leak so you can then remove all the air from the system and install the freon.