Auto A/C Terms - Glossary
Clutch Cycling Switch

One of the most common auto A/C system designs is the cycling clutch configuration. Utilizing a cycling clutch means that the A/C system can provide proper cooling performance while not having to keep the compressor engaged all the time the A/C system is working. Instead of using valves to control the flow of refrigerant throughout the A/C system, the cycling clutch system turns the compressor on and off as required. Turning the compressor on and off is referred to as cycling and that is controlled by the cycling switch.

Currently, automotive A/C systems use two different types of clutch cycling switches. They are the temperature cycling switch and the pressure cycling switch. Although they both perform the same function, the pressure cycling switch is a more accurate and reliable.

Temperature cycling switches use a temperature sensing bulb and capillary tube. That sensing bulb is typically attached to the evaporator outlet tube and covered in insulation or it is press directly into the evaporator fins. In either case, the proper operation of the switch relies on good contact with the surface in order to read the proper temperature. Depending on the actual switch settings, the switch will cause the compressor clutch to engage as the sensing temperature increases and disengage when the temperature becomes too cold.

The pressure cycling switches are typically mounted on the accumulator or the low side of the A/C system very close to the outlet of the evaporator. The pressure switch is reading the actual low side pressure of the A/C system refrigerant. Conventional switches will cause the compressor to cycle off when the low side is pulled down to about 22 psi. With the compressor off, the low side pressures start to rise. When those pressures reach about 45 psi, the compressor clutch is engaged again.