The A/C compressor is commonly powered by the main drive belt. An electromagnetic clutch is used to engage the pulley with the compressor shaft. If the clutch circuit senses low refrigerant or another problem, the clutch will not engage. The compressor pressurizes and recirculates refrigerant, which is sent to the condenser. A seized compressor may cause a turning belt to squeal. Since liquid cannot be compressed, only vapor enters the compressor.