If your refrigerator fails to maintain its interior temperature, or if the freezer compartment becomes encrusted with insulating ice, one or both of the control thermostats has failed. The good news is that replacements are cheap, and it’s fairly easy to access and diagnose an unserviceable thermostat.
As mentioned, there are two refrigerator thermostats – one to control interior temperature and another to control the defrost cycle. The control thermostat usually contains a sensor tube filled with a temperature sensitive liquid such a refrigerant. This liquid expands or contracts with changing temperatures and activates an adjustable bellows mechanism. The bellows is connected to a control knob that the owner uses to set the desired running temperature. When the interior temperature moves out of its set range, a rod attached to the expanding bellows turns the micro switch feeding power to the refrigerant compressor, on or off as required.
The defrost thermostat contains two strips of dissimilar metals, each with different thermal characteristics; these strips are laminated together to interact with each other. This causes the varying expansion and contraction properties of the metals to curl or straighten the strip with changing temperatures. This then opens or closes electrical contacts attached to the end of the strip. The thermostat is clipped to the top of the evaporator coil to monitor freezer compartment temperature; when the temperature drops below a set point showing ice build-up, the bi-metal strip curls up and separates the switch contacts, cutting power to the compressor and at the same time, activating the defrost heater. When the defrost heater has done its job by melting all the insulating ice from the evaporator coils, the temperature rises. This causes the bi-metal strip to straighten and re-establish electrical contact, thereby turning the defrost heater off and restarting the compressor.
Control thermostat disassembly
Unplug the refrigerator power cord from its electrical outlet. Open the door and unclip the thermostat control housing cover plate. Remove the screw securing the control housing to the top of the refrigerator and lower the housing. Remove the thermostat control knob by pulling it straight out. Release the questionable thermostat and temperature control tube from the housing by pulling it out of its retaining slot; be careful not to kink the control tube by bending it. Take note of the electrical connection orientation and pull the spade connectors off the terminals to disconnect the wires. Either install a new thermostat or reinstall the old thermostat if it proves to be sound, by reversing these procedures.
Defrost thermostat disassembly
Unplug the refrigerator power cord. With a side-by-side double-door refrigerator, remove any shelves and trays obstructing the freezer compartment rear panel. Alternatively, remove all obstructing shelves and trays from a standard refrigerator. Remove the light lens cover by lifting it up and out, then unscrew the light bulb. Remove the screws securing the rear panel. Unclip the grounding wire attached to the panel if applicable, and lift the panel up and out to expose the evaporator coil. Unclip the questionable thermostat from the top of the evaporator coil. Make a note of the wire orientation and disconnect the thermostat from the electrical harness by pulling the connectors straight out. Either install a new thermostat or reinstall the old thermostat if it proves to be sound by reversing the disassembly procedures.
Perform continuity tests on both components by first immersing the defrost thermostat in a glass of ice cold water for two or three minutes. This cools the unit down and straightens its internal bi-metal strip, thereby establishing contact between the switch terminals. If you’re using an analogue Multimeter, set the control to the lowest ohms of resistance setting and then pinch the two probes together. Adjust the knurled wheel on the side of the meter until the needle reads zero. If you’re using a digital Multimeter, set it to the lowest ohms of resistance setting; a digital Multimeter zero's itself automatically. Push one of the probes into one of the wire connectors. Monitor the Multimeter reading and press the end of the other probe into the second wire terminal. If the readout shows zero ohms of resistance, the thermostat has continuity and it is serviceable. If the needle does not move or if there is no significant change on the digital display, the terminals are not making contact showing no continuity. In this case, the thermostat is faulty and must be replaced. Repeat this test with the control thermostat. However, instead of immersing the entire thermostat, place the bulb on the end of the control tube in the bottom of the glass of iced water for two or three minutes. This will cause the liquid in the tube to contract and the switch to establish continuity. Once again, if there is no significant movement of the needle or digital readout, the thermostat is faulty and must be replaced.