TESTING THE DEFROST LIMIT SWITCH (Defrost Bimetal or Defrost Thermostat)

A defrost limit switch is a bimetallic device that opens and closes the circuit to the evaporator defrost heater during the defrost cycles in frost-free units. In this cycle, the timer disconnects the power supply to the compressor and at the same time connects power to the evaporator defrost heater to melt the ice built up on the evaporator surface (see fig. 62). The current to the evaporator heater passes through this bimetal device, which controls the on/off function of this circuit. Since this device is mounted on the evaporator coil, once sufficient frost builds up to cause it to contract, it closes the circuit to the heater. As the timer takes the unit into the defrost cycle, the heater melts the ice, the coil becomes warmer, causing the device to expand and open the circuit to de-energize the defrost heater.

Because the bimetal switch is clipped to the evaporator in the freezer compartment, the evaporator cover must be removed to gain access to it. To do this, remove all food from the freezer compartment, all the shelves, and, if it has one, the automatic ice maker. Then remove the shelf studs and their screws. Next, remove the screws holding the evaporator cover to the back of the liner and carefully remove the evaporator cover.

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