Pressure Cycling Switch

Control of the refrigeration cycle (ON and OFF operation of the compressor) is done with a pressure cycling switch which senses low-side (suction) pressure as an indicator of evaporator temperature. The switch is mounted on a fitting that is welded to the accumulator. The switch fitting contains a standard needle-type valve so that the refrigerant charge does not have to be recovered from the system when replacing the switch.

With ambient temperature at 16-26°C (60-80°F) and the system turned OFF, the equalized pressures within the charged A/C system are sufficient to close the contacts on the pressure cycling switch. When an air conditioning mode is selected, voltage is applied to the Powertrain Control Module (PCM) through the switch. The PCM then energizes the A/C Compressor Control Relay to energize the compressor clutch coil. As the compressor runs, low-side (suction) pressure decreases and high-side (discharge) pressure increases. When suction pressure drops to approximately 152 kPa (22 psi), the pressure cycling switch opens, de-energizing the compressor clutch. With the compressor stopped, system pressures again begin to equalize. When low-side pressure rises to approximately 296 kPa (43 psi) the switch contacts close, the clutch coil energizes and the compressor begins to run. This cycling process continues and maintains evaporator discharge air temperature at approximately 6°C (33°F).

Because the compressor is cycled ON and OFF, some slight increases and decreases of engine speed/power may be noticed under certain conditions.