TXV Thermostatic Expansion Valve
A TXV or thermostatic expansion valve is a metering device used in automotive air conditioning. For the most part there are two types of metering devices used in today's vehicles. The TXV and the FOT or fixed orifice tube. The major difference between the two devices is the TXV offers a variable and changing rate that meters the system refrigerant according to demands. The FOT has a fixed rate and changes only in relation to refrigerant volume and system pressures created by the cycling compressor.
The expansion valve is located at the inlet port of the evaporator. It consists of several parts including a spring to keep the valve in the closed position restricting the flow of refrigerant through the system. At the top of the expansion valve is a dome filled with a similar refrigerant as the rest of the system. The capillary tube and bulb are also filled with this refrigerant and placed against the outlet port of the evaporator. This entire chamber is sealed off from the rest of the system by a flexible diaphragm.
The gas inside the sensing bulb placed against the outlet port of the evaporator expands as the temperature increases causing the diaphragm to move a pintle. This action allows refrigerant to enter the evaporator. As the evaporator cools the gas inside the bulb contracts forcing the pintle back to the closed position blocking refrigerant from entering the system.
This expanding and contracting gas moves the diaphragm and pintle regulating the refrigerant as it enters the evaporator core. The temperature of the surface of the evaporator core is in direct correlation to the air leaving the air conditioners vents. While troubleshooting a stuck open TXV, note that it will flood and freeze the evaporator possibly damaging the compressor with liquid. A stuck closed TXV will leave an evaporator starved of refrigerant and the compressor its necessary lubricating oil.
