The A/C compressor is a component that circulates and pumps refrigerant throughout the auto A/C system. It takes low side vapor refrigerant and compresses it to become high pressure, high temperature refrigerant that is discharged to the condenser. However, in cases where the A/C condenser can not properly cool that refrigerant and high side pressures begin to rise, the compressor cut off or cut out switch will cause the compressor clutch to disengage. The cut off switch should cause the compressor to disengage before any of the A/C system pressure relief valves cause the refrigerant charge to be released.
Understand that there are several different causes for increasing refrigerant pressures. However, in every case, the increasing pressures are related to the condenser inability to cool the refrigerant. High pressures are not a result of restrictions as so many technicians mistakenly think.
Should high side pressures rise too high, the A/C system has safety valves built in so that pressures are released to atmosphere. Those safety switches or pressure relief switches typically dump the refrigerant charge when pressures rise in the range of 400 to 440 psi. On a properly operating A/C system, the compressor cut out switch should disengage the compressor clutch before pressures rise that high.
When A/C systems are retrofitted to work with R-134a refrigerant, it is common that HPCO or High Pressure Cut Out switches are installed. They should be installed because R-134a refrigerants will typically cause high side operating pressures to be higher than on older R-12 systems. The HPCO switches monitor the high side pressures and disengage the compressor clutch to prevent pressure relief valves from releasing refrigerant.
Also understand that some manufacturers use A/C compressor cut out switches for other purposes. As an example, the WOT or Wide Open Throttle cut off switch disengages the compressor when the driver forces the engine throttle wide open under sever acceleration.
Other A/C compressor cut out switches will disengage the A/C compressor when power steer pressures rise too high. That usually occurs during parking maneuvers. On smaller engines, disengaging the compressor will help prevent the engine from stalling.
In diagnosing A/C system problems where the A/C compressor will not engage, the technician should check any of the cut out switches included in the system. It is possible that they fail and prevent the compressor clutch from engaging under normal operating conditions.