A/C Pressure Cycling Switch

A/C System Controls Circuit Diagram:

DESCRIPTION
The A/C pressure cycling switch is an electric switch that toggles "ON" and "OFF" at different pressures. The switch is used to interrupt the A/C request signal if system pressure drops below a certain level. This is to prevent compressor operation if there is not adequate refrigerant charge in the A/C system, and to cycle the system "OFF" if the low side pressure is too low, to prevent evaporator icing.

OPERATION
The pressure cycling switch is installed in low pressure hose and completes the A/C request circuit between the programmer/control head and the PCM. The switch must be closed to conduct the A/C request signal to the PCM. The pressure cycling switch closes at approximately 324 kPa (47 psi) and will remain closed unless the pressure drops below approximately 170 kPa (24.5 psi). The A/C system should have sufficient refrigerant charge pressure to keep the switch closed, even when the system isn't operating. If not, the switch will be open, there will be no request signal at the PCM, and the PCM will not enable the A/C compressor. If during operation, the low side pressure falls below about 170 kPa (24.5 psi), the switch will "Open", interrupting the A/C request signal. The PCM will then disable the compressor clutch, allowing low side pressure to increase and prevent evaporator icing. If the refrigerant charge is only slightly low, the system may cycle "OFF" and "ON" frequently.