Expansion Valve

📖 YOUCANIC Automotive Glossary

The expansion valve (TXV — Thermostatic Expansion Valve, or EXV — Electronic Expansion Valve) is the metering device that controls the flow of high-pressure liquid refrigerant into the evaporator, where it expands into a low-pressure cold gas that absorbs heat from the cabin air. The TXV uses a sensing bulb clamped to the evaporator outlet to monitor the temperature of refrigerant leaving the evaporator (superheat). If the evaporator outlet is too warm (high superheat indicating insufficient refrigerant flow), the valve opens further. If too cold (low superheat indicating flooding), the valve closes. This maintains optimal evaporator efficiency and prevents liquid refrigerant from reaching the compressor, which would cause damage.

Expansion valve failures cause either too much or too little refrigerant entering the evaporator. A stuck-closed valve causes poor cooling (evaporator starved of refrigerant), very low low-side pressure, and potential compressor overheating. A stuck-open valve causes evaporator freeze-up, liquid flood-back to the compressor (slugging), and high low-side pressure. An intermittently sticking valve causes cycling temperature from the vents. Expansion valves often fail due to moisture or debris in the system clogging the small metering orifice — this is why proper system evacuation (removing moisture) and replacing the receiver-drier (filtering debris) are essential during A/C service.