Thermostat failure will have three common reasons:

1. Contactors are “melted” together, causing the compressor to run continuously.
2. Contactors that are carbonized prevent the compressor to start at all.
3. The thermostat sensor, springs or bellows have failed, causing either the compressor to run continuously or not at all.

Either instance will require the replacement of the thermostat.

Possible Solution

To test the thermostat you will need to check for warm and cold switching.

Not all thermostats will be marked or the same, however, this example will apply to a commercial thermostat.

1. You will need a multimeter and a glass of ice water.
2. Locate the thermostat and remove the sensing bulb from its position. You will need to test the thermostat by warming up and cooling the sensing bulb.
3. Take a photo of the wiring connections to the thermostat and remove all the connectors.

Warm Testing

When a thermostat is heated up its job is to turn the compressor off.

Connect your multimeter to the connectors. Note P1, 2 and 1 from the drawing.

P1 is the power

2 is the compressor

1 is the defrost unit where applicable. If there is no defrost system this pin is not used.

Important to remember, that some thermostats are “off” in the fully turned position. If you hear a “click”, however, it should not be mistaken for the “clicking” sound of the actual thermostat.

After connecting the multimeter and setting it to Ohm or Continuity, hold the sending bulb to warm it up a little. Turn the thermostat until you hear a click or you see a reading on the multimeter.

Next is to dip the sensing bulb in a glass of water to cool it down. Turn the thermostat on in either direction (depending on the thermostat type), you should either see no reading meaning the thermostat is now off. or you will hear another click.

The aim of this is to test the thermostat to turn on when heated and to turn off when cooled.

If you do not get a reading at all with either test, there is a good chance the thermostat is faulty and will need to be replaced.