Rick's Free Auto Repair Advice

What is an automotive expansion valve

How does a automotive thermal expansion valve work?

Let’s keep this simple; all you’re trying to do in an auto AC system is regulate the flow of high pressure liquid refrigerant though a small opening, so it can flow into the evaporator. You want the liquid to absorb heat from the cabin and cause the liquid to boil and change into a gas. Once the refrigerant has turned into a gas, you want it to stop absorbing heat and immediately leave the evaporator.

If the system is low on charge, the gas will continue to absorb heat and stay in the evaporator too long. That extra heat absorption is called superheat (because the refrigerant has absorbed more heat than is necessary to cause it to change from a liquid to a gas) and it causes all kinds of problems that reduce cooling.

Types of expansion valves

There are several types of expansion valves, but they all work on the same basic principle; the valve remains open to allow refrigerant flow until the evaporator temperature drops to a certain point. Then the valve closes to prevent superheat and evaporator freeze up.

Remote bulb expansion valve

A remote bulb expansion valve uses a small copper tube and a remote sensing bulb. The tube and bulb are filled with a gas that expands and contracts with changes in evaporator temperature. As the remote bulb gas expands and contracts it pushes up or down on the power head which moves the diaphragm, pushing the metering pin into or out of the machined orifice.
thermal expansion valve exploded diagram

capillary tube expansion valve

Typical remote bulb or capillary tube expansion valve

High pressure liquid refrigerant pushes against the seated metering pin in the expansion valve. When the evaporator is warm the gas in the capillary tube expands and pushes down on the power head and diaphragm, which pushes the metering pin down, allowing refrigerant to flow into the evaporator. As the evaporator chills, the sensing bulb cools, causing the gas to contract, allowing the superheat spring to push up on the metering pin and close off the flow of refrigerant.

H block expansion valve

Similar in function, the H-block expansion valve meters the flow of refrigerant by sensing the temperature of the refrigerant as it flow out of the evaporator.

H block expansion valve with callouts
car AC system with expansion valve

See this post for tips on how to diagnose an automotive expansion valve

See this post for symptoms of a bad expansion valve

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Posted on by Rick Muscoplat

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