The components of the compressor are contained in a sealed unit that allows the freon refrigerant to cool the motor. The experts at G&S Mechanical Services say that if the sealants used to protect the motor wear off, the freon will damage the motor and cause it to overheat. An insufficient amount of coolant also leads to overheating. The wiring that delivers power to the motor can also burn out or become disconnected. A burned out compressor will not start, and will need replacing.
Locked up Motor
Air conditioning compressors that sit unused for a long time or run low on oil may lock up. The moving parts in the compressor motor will become tight and hard to move. According to the Inspectapedia website, one of the best ways to determine if this problem is occurring in your air conditioning unit is to test the amount of electricity the compressor is drawing. A tight or locked compressor draws an excessively high current when started up. Some locked compressor motors are salvaged with a special capacitor while others will need replacement.
Squeaks, squeals and rattling noises made by a home air conditioning compressor can indicate serious problems or simply present as one of it's normal operations. Grinding or hammering noises originating in the compressor unit indicate that the motor is beginning to fail, according to the Air Conditioning and Refrigeration Guide. Rattling or light metallic noises are often caused by a loose screw or other piece of metal vibrating in or around the compressor. Hissing or humming when the compressor runs or starts up indicates issues with the hardware in the unit that require replacement quickly.
Shutting on and off
A compressor may cycle on and off rapidly while the rest of the air conditioning unit works properly. No cool air will is provided while the compressor does this because it does not stay on long enough to produce a temperature drop. The experts at the Air Conditioning and Refrigeration Guide say that this is caused by the unit having difficulty starting and drawing too much power into the unit. The unit trips its own internal overload trigger and starts the process over again. A variety of underlying issues cause this problem, including a locked-up motor.