Rick's Free Auto Repair Advice

Heat load test on thermostatic expansion valve system

How to perform a maximum heat load test on a thermostatic expansion valve system

You’ll need a digital readout thermometer and a temperature probe to perform a maximum heat load test. One inexpensive option is the AGPtek Dual Two Channel Digital Thermometer 2 K-Type Thermocouple Sensor ($21 at amazon.com)

test air conditioning system, AC doesn't cool

Digital temperature meter with two thermocouples

and a PYLE Meters PCTL01 Pipe Clamp Temperature Lead ($14 at amazon.com).

temperature sensor

Clamp temperature sensor

With these thermometers you can check the level of refrigeration charge, the operation of the evaporator and condenser, as well as and restrictions in hoses. Hey, if you want to save the cost of a shop conducting these tests, you simply have to own or have access to the right tools, so stop whining. There’s no cheap magic bullet to fix your car’s air conditioning system.


Set up the heat load test

1) Park the vehicle outside in direct sunlight

2) Open all doors and windows

3) Start and run the engine up to full operating temperature

4) Let engine run at idle

5) Set AC to MAX

6) Set the blower on High

Click on the PDF below to open it

test air conditioning system

Click on image to open full size PDF in new window


Start with the condenser temperature test

Clamp your temperature probe on the inlet A and outlet B tubes on the condenser coil. This test determines how much heat is being removed by the condenser. The condenser coil must remove at least 20° and no more than 50°F. Subract the outlet B reading from the inlet A reading.

If the reading is less than 20°F here are the possible causes:

Poor airflow across condenser coil.

Debris clogging the coil fins. Clean the condenser coil.

Improper airflow across condenser. Check for proper condenser fan operation (if electric) or fan clutch operation if mechanical radiator fan is used.

Check for broken or missing fan shrouds or seals

The system is overcharged

If the reading is more than 50°F here are the possible causes:

There’s air in the system. Evacuate and pull vacuum for at least 45-mins.

System is low on charge.

Restriction in condenser coil—sludge, too much oil, mechanical restriction due to pinched tube

Then conduct an evaporator temperature test

Clamp your temperature probe onto the suction line before the TMX valve at D. Measure the temperature of the suction line and compare it to the air temperature coming out of the center duct at C.

The temperature difference between D and C should be no more than 10°F

If the center duct temperature more than 10° warmer than the temperature of the suction line:


System is low on refrigerant—All refrigerant boils at the bottom of the evaporator and the gas gains superheat

The refrigerant flow is restricted—bad of clogged TMX valve

Heater control valve is leaking or blend air door is open and heating the air after the evaporator coil.

If the center duct temperature is more than 10° COLDER than the suction line here are the possible causes:

System is overcharged

Finally, conduct a duct versus ambient temperature test

Measure the ambient temperature 3-ft. in front of the grille. Do NOT use radio or TV temp readings. Next, measure the air temperature at the center duct.

A properly operating system must be capable of removing at least 30°F of heat from the ambient air under this maximum stress test. If the system passes the maximum stress test, you’ll get better results when driving with the windows closed.
©, 2013 Rick Muscoplat

Posted on by Rick Muscoplat

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