The letter H and L in the car ac hose are the first letters of high pressure and low pressure, H stands for high pressure pipe interface and L stands for low pressure pipe interface. The car ac hose interface is used for the maintenance of air conditioning refrigerant, normally the interface is tightened, mainly to prevent the entry or adhesion of dust.

When there are abnormal conditions such as heat sink blockage, cooling fan does not turn or refrigerant excessive in use of car ac hose, the system pressure will be too high, if not controlled, too high pressure will damage the system components.

The high voltage switch is installed in the high voltage pipeline, generally installed in the liquid storage dryer, in series in the compressor electromagnetic clutch circuit or condenser fan circuit. When the system pressure is too high, the high voltage switch action, cut off the clutch circuit or connect the cooling fan high-speed block circuit, prevent the pressure continues to rise, avoid causing damage to the system.

The low voltage switch is usually installed directly in the high voltage pipeline with threaded joints, in series in the electromagnetic clutch circuit. Its structure is similar to the normally open type high voltage switch. When the refrigerant pressure is normal, the moving contact is connected to the electromagnetic clutch circuit of the compressor. When the refrigerant pressure discharged from the compressor is too low, the low-voltage switch will be disconnected, the electromagnetic clutch circuit will be cut off, and the compressor will stop running to prevent damage to the compressor.