Expansion valve / orifice tube
Definition
The expansion valve is the metering device between the high-pressure liquid line and the evaporator: it sprays liquid refrigerant through a small orifice so that it drops in pressure and boils inside the evaporator, absorbing heat from the cabin air. A thermostatic expansion valve (TXV, usually a block or H-valve bolted to the evaporator pipes) senses the evaporator outlet temperature and pressure and adjusts the flow so the evaporator is fully used without flooding liquid back to the compressor; the simpler orifice tube is a fixed restriction used with an accumulator on the suction side. Valves fail when the sensing bulb or diaphragm loses its charge (valve shuts, no cooling, low low-side and low high-side pressure), when debris or ice blocks the orifice, or when moisture freezes inside it causing cooling that works then stops after a few minutes. Symptoms are poor or intermittent cooling, frost on the valve or liquid line, a hissing noise behind the dash and abnormal pressure readings on the gauges that do not respond to charge changes. Diagnose with the pressure pattern and temperature of the inlet and outlet pipes, replace the valve with new O-rings when the system has been contaminated, and always fit a new drier. No fault codes except evaporator temperature sensor codes (B1xxx) when the evaporator freezes or never gets cold; related terms are receiver drier, evaporator and compressor.