Auto A/C Terms - Glossary
Cycling Clutch Orifice Tube - CCOT A/C System

General Motors introduced the cycling clutch orifice tube A/C system in the 1977 model year. The unique advantage to having a cycling clutch with a fixed orifice tube was that now, the only moving part on the A/C system would the A/C compressor. The fixed orifice tube, which provides a one size, fixed opening for the flow of refrigerant into the expansion valve had replaced the typical expansion valve or TXV valve that opened and closed to control the refrigerant flow into the evaporator.

The fixed orifice tube means that there is always a flow of refrigerant into the evaporator. In order to control the evaporator pressure and maintain the coldest possible temperature in the evaporator without freezing the condensation, the system was designed to cycle the compressor on and off.

The cycling clutch system meant that the typical normal pressure gauge readings of a properly operating system would have a wider range. Of course, the low side pressures would be at their lowest just before the compressor cycles off while the high side pressures would be at their highest. As soon as the compressor clutch disengages, the high pressures start to drop while the low pressures increase. That happens because the fixed orifice tube is always allowing refrigerant to flow through it, regardless of whether the compressor is on or off. As soon as low pressures began to rise causing a higher temperature in the evaporator, the compressor would cycle back on to pull the pressures and temperature down on the low, once again increasing the pressures on the high side. The cycle continues in a controlled manner that maintains mean or average pressures in the evaporator so that the A/C system can provide maximum cooling performance with the minimum number of moving components.

Understand that the CCOT or fixed orifice tube system is still slightly different than typical cycling clutch systems. In other system designs, expansion valves are still used to control the flow of refrigerant into the evaporator. Those typical expansion valves open and close to monitor refrigerant flow while the compressor cycles on and off to maintain proper evaporator pressures. The orifice tube system utilizes the fixed opening in the orifice tube which maintains a constant flow of refrigerant.