The Orifice Tube — Fixed Restriction Systems

The orifice tube is the expansion device used in accumulator-based AC systems. Unlike the TXV which actively controls refrigerant flow, the orifice tube is completely passive — it is a fixed restriction with no moving parts. The same amount of refrigerant flows through it regardless of system conditions. Simpler, cheaper, and less precise than a TXV, but reliable and widely used.

The orifice tube is a small plastic and metal component about the size of a AA battery. It sits inside the liquid line — usually inside the evaporator inlet line near the firewall. On many GM vehicles it is in the liquid line on the passenger side firewall. On Ford products it is often in the liquid line between the condenser and the firewall. It has a screen on each end to catch any debris before it reaches the evaporator. These screens can clog with contamination from a failing compressor, causing a restriction.

Because the orifice tube cannot adjust its flow, the system controls cooling by cycling the compressor clutch on and off. A low-pressure switch senses evaporator pressure — which corresponds to evaporator temperature. When the evaporator gets cold enough, the switch opens and cuts the compressor clutch. When it warms up, the switch closes and the compressor engages again. This cycling is normal and continuous during AC operation. On a properly charged system the cycle rate is moderate — not too fast and not too slow.

A clogged orifice tube creates a restriction in the high side. High side pressure rises abnormally high. Low side pressure drops abnormally low. The line going into the orifice tube feels hot, the line coming out feels cold — you can identify a restriction by feeling where the temperature change is. A missing or incorrectly installed orifice tube allows too much refrigerant to flood the evaporator, liquid refrigerant returns to the compressor, and compressor failure follows.