What is the purpose of the cycling switch in a cycling clutch system?

The A/C cycling switch cycles the compressor clutch on and off to maintain evaporator temperatures above freezing.

Where is the clutch cycling switch located?

Thermostatic cycling switch: Some vehicles mount it in the engine compartment, near the evaporator case. Others mount theirs under the dashboard near the evaporator.

Pressure cycling switch: The A/C pressure cycling switch is located in the low-side. Some vehicles mount theirs on the accumulator, others on the lines.

Some vehicles use a temperature-controlled cycling clutch switch to maintain evaporator temperatures. The switch opens or closes the circuit to the compressor clutch, based on evaporator temperature. The switch has a temperature sensing bulb that sits on the evaporator or along the evaporator outlet line.

The bulb is filled with a gas or refrigerant similar to one in the system. If the evaporator core gets too cold, the pressure of the refrigerant in the sensing bulb drops. That allows the contacts in the cycling clutch switch to open, which releases the compressor clutch and stops the compressor.

As soon as the compressor stops turning, the pressures in the system begin to equalize. The evaporator pressure increases and warms up. That warms the sensing bulb enough to let the cycling clutch switch close. The compressor clutch engages, and the cycle begins again.

Pressure controls maintain evaporator temperature. 

Cycling the compressor clutch based on evaporator temperature is just one way to control evaporator temperature. Since refrigerant temperature can be controlled by adjusting the pressure, some manufacturers use a pressure switch on the accumulator to monitor evaporator pressure and cycle the compressor clutch.

Low-side refrigerant pressure sits on one side of a diaphragm, spring pressure on the other. When refrigerant pressure exceeds a predetermined value about 45 PSI (310 kpa) it overcomes the spring pressure, forcing a set of electrical contacts to complete the circuit, and energizing the compressor clutch.

As the compressor runs, low-side pressure drops. When low-side pressures drop below about 23 PSI (159 kpa), the spring tension overcomes refrigerant pressure; the contacts in the switch open, shutting the compressor clutch off. Pressures rise, and the cycle begins again.

Electronic cycling clutch switch

Electronic cycling switch monitors evaporator temperature, and turns the compressor on and off to maintain evaporator temperatures over 32° F (0ºC).

How do I know if my AC cycling switch is bad?

The AC system is not blowing as cold as it used to, that may be a sign that the switch has failed, or is beginning to fail

Your AC system will completely cease to blow cold air without the cycling switch to activate the compressor’s clutch.

The A/C compressor cycles on and off too often and out of pressure specifications. You can hear the clicking noise of the A/C compressor clutch engaging and disengaging.

Summary
What is the purpose of the thermostatic switch in a cycling clutch system?
Article Name
What is the purpose of the thermostatic switch in a cycling clutch system?
Description
A/C compressor clutch, cycling devices.Pressure cycling switch: The A/C pressure cycling switch is located in the low-side. Some vehicles mount theirs on the accumulator, others on the lines.
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Auto A/C Repair LLC.
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