Aircon thermostat troubles?

Our tech guru has the answer
by Ferman Lao | Aug 8, 2011

Hi, Ferman!

I own a 1995 Mitsubishi Galant and recently had my air-conditioning system cleaned up. I also had the expansion valve and drier replaced. The system turned out to be much colder than I expected.

After a couple of days, however, I noticed that my thermostat control won't trip off. I told the aircon technician about this. He said it's alright and that I don't have to worry about a thing. I'm doubtful though and I need your advice. Does the thermostat control really have functions?

Right now I'm very much satisfied with the cooling system of my ride.

Thank you and God bless Top Gear!


Hi, Vulcan.

A typical automotive air-conditioning system usually consists of a compressor, a condenser, some tubing, a filter/drier, an expansion valve, a cooling coil/evaporator, a thermostat and a couple of fans/blowers for the condenser and cooling coil.

The thermostat is the one that controls the temperature of the air coming out of the aircon vents. It constantly measures the cabin temperature, and turns the compressor on and off as needed in order to maintain the temperature inside the cabin.

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The compressor, as the name states, compresses the refrigerant gas inside as it travels thru the system. After leaving the compressor the refrigerant goes to the condenser. The condenser is similar in function to your engine's radiator. It removes heat from the refrigerant before it makes its way to the filter/drier. Once at the filter/drier the refrigerant is scrubbed of all contaminants and moisture in order for the refrigerant to be as pure as possible when it gets to the expansion valve. It is the expansion valve that regulates the flow of refrigerant to the cooling coil. It slowly releases the high-pressure refrigerant and allows the refrigerant to flow into the cooling coil, which then absorbs the heat from the surrounding areas (within reason) and makes the air passing thru the cooling coil, er, cold.

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It is possible that your thermostat may be set too high or too low. Too high and the aircon compressor won't turn on. Too low and the compressor might not possibly turn off. This may be what is happening to your car. If the compressor doesn't turn off a condition called high pressure condition will occur. A high pressure condition will result in the aircon not being cold.

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I would suggest that you find out what the thermostat of your car is set at and from there you can determine why your compressor doesn't seem to be turning off. 


Ferman Lao
Technical editor

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