The refrigerator cold control or thermostat is an electro-mechanical component that controls the starting/stopping of the compressor and is one of (usually) two adjustable controls that effect the temperature inside the refrigerator. The other is a damper valve to adjust the airflow between the freezer section and refrigerator section. Both these controls are usually knobs with numbers or letters and it is sometimes difficult at first glance to determine which is a mechanical valve and which is an electrical switch. The cold control has an off position that turns off power to compressor. Both controls usually have a default or mid-range position marked on their numbered scales. In a properly operating system setting these two controls to midrange should result in acceptable temperatures in the freezer and refrigerator section. If it does not there is likely a problem.
Cold Control (a pressure actuated switch)
The cold control has two wires. A hot lead that is usually energized as soon as the refrigerator is connected to an outlet and a switched lead that is hot when the pressure against a bellows pushes the contacts to the closed or made position. The compressor then starts and runs until the colder temperature reduces the pressure and the bellows opens the contacts. The pressure is derived from refrigerant inside a sealed capillary tube and bulb assembly attached to cold control body. If the cap tube is damaged and the refrigerant escapes, the cold control will no longer make to provide power to compressor. In this case, the compressor may be judged bad when the problem is a defective control. If the cold control sticks in the closed position and the compressor runs constantly, the evaporator may eventually ice over and look like a defrost problem.
Note that the temperature control (cold control) receives power from line side of cord. If the control is made power is fed to the defrost control. The defrost control sends voltage to the compressor start circuit, evaporator fan, and condenser fan. When the defrost timer calls for defrost the compressor and fans stop and power is sent to the defrost heater circuit. Obviously an open cold control will keep the compressor and fans from running. The easiest way to test is to unplug refrigerator, remove the cold control so that the two wires can be disconnected and take a reading across the terminals with a volt-ohm meter. If the refrigerator is above set point and the cold control reads infinity the control is bad. A control with shorted contacts will cause the refrigerator to run continuously except when the defrost timer switches to a defrost cycle.