Rick's Free Auto Repair Advice

Test car thermostatic expansion valve

Test car thermostatic expansion valve

Determine which type of expansion valve you have

There are two types of car expansion valves: capillary tube and block style expansion valves.

Both types meter liquid refrigerant into the evaporator based on the temperature of the exiting refrigerant gas. A capillary tube expansion valve uses a remote sensing bulb that’s firmly attached to the evaporator outlet. The gas in the sensing bulb and capillary tube expands or contracts based on the temperature of the exiting refrigerant. The gas opens or closes the port on the expansion valve.

Capillary tube expansion valve

Capillary tube expansion valve

H-block style expansion valve

H-block style expansion valve

capillary tube expansion valve











car expansion valve

A block style expansion valve uses a temperature sensing disc rather than a remote sensing bulb. A block expansion valve has both the liquid and vapor lines running through it. The sensing disc detect changes in exiting gas temperature and moves a tapered needle into or out of a metering port.

How expansion valves fail

• The sensing bulb/capillary tubes breaks and loses it gas charge
• The sensing disc loses its gas charge
• The tapered metering rod wears or binds in the orifice or sticks closed
• The liquid orifice gets clogged with sealer, debris or ice.

clogged expansion valve

Clogged expansion valve

How to test an H-block style expansion valve

First, determine if the H-block expansion valve is accessible. It’s either near the firewall or under the dash. You’ll need a manifold gauge set and a full can of dust off spray.

Preparing for the expansion valve test

Connect your gauges to the car AC high and low service ports.

Start the engine and turn on the AC to the MAX (recirculate) position and the coldest setting, along with the highest blower speed.

Test car expansion valve

Test car expansion valve

Run the engine at normal speed until the gauges stabilize—needles move in a predicable pattern.

Record the high and low pressure readings.

Spray the sensing disc

Invert the can of dust off and aim the straw at the sensing disc. Depress the trigger and spray the liquid refrigerant onto the sensing disc. Watch the high pressure gauge. It should drop by about 50-psi. Stop spraying and watch the high pressure gauge. As the frost melts from the sensing disc, the pressure to rise to the first reading you recorded.

If the high pressure dropped by about 50-psi and then returned to the starting pressure you recorded once the frost melted, that confirms the expansion valve is not plugged and the sensing disc is working properly.

How to test a capillary tube expansion valve

Disconnect the capillary tube from the evaporator coil. With gauges attached, warm the capillary tube with your hands. Record the pressure readings. Then spray the capillary tube with an inverted can of dust off spray to chill the capillary tube. The pressure should drop with heat and rise with cold.

Signs of a bad expansion valve

• The high side pressure only drops by about 10-psi. That’s an indication of a faulty sensing disc, sticking metering pin/metering ball or a clog.
• Low side pressure is very low or negative (in the vacuum area)
• High side pressure is high

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat

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