If the defrost heater stuck on the bi-metal thermostat would kill power to it, so that would cause the problem more so then the heater. Here is a walk through I typed up that may help.
Not sure exactly what the situation is I'm going to give you a detailed overview on how the system works that way you'll be able to isolate the problem. Your machine is a single compressor unit, meaning that even though there are two seperate sides, the freezer and the refridgerator, they do not operate independantly. The sealed system, which is what cools the machine, has two parts, a high side and a low side. The high side is located under the machine and consist of a condensor, for which freon is pressurized back into a liquid and passed throug the compresoor to be released as a gas into the evaperator, the low side. This is where the freon is at a cold state. Ok, now that you have the bare basics of a sealed system, we move forward. The Evaporator is located in the freezer side, behind the freezer wall. As freon goes through the evaporator, a fan, called the evaporator fan, is located above it and circulates air throughout the freezer compartment. Also, an airway connects the freezer to the refridgerator, usually top right of freezer, or top left refridgerator on side by sides and through the freezer floor on top mount models. This means the freezer cools the refridgerator side. ****First check point**** Is the fan running in the freezer? This would cause the freezer not to cool as well but also a rise in the refrigeration side would be noticed. As freon goes throught the evaporator, which is made of aluminum, the temperature levels are well below the freezing point, so frost and ice will form on the coils. Over time, this kind of build up would form a blockage, not letting air be drawn from the bottom of the coils by the fan to circulate the cold air. To prevent this from happening the a defrost heater is installed to thaw the ice away and comes on every 6-8 hours. This is initiated by the defrost timer, or in some machines an adaptive defrost control. The heater will be located at the bottom of the evaporator because hot air rises, getting maximum effect. A third part to the defrost system, the first two being the timer and heater, is a bi-metal,or terminating thermost. Its called this because when the thermostat is cold, the metal inside it constricts making a pathway for electricity to flow through to the defrost heaters so it can get hot, and as the heat rises and the ice melts away, the terminating thermostat will warm up, and the metal will expand causing the path of electricity to be broken, turing the heaters off. This is to prevent any high temperature situation from happening, like melting plastic or even a fire. If any of these parts fail, the machine will nit defrost. ***Checkpoint two***** Do you see any ice build up on the freezer wall. This ice will be completely across the back wall. Most signs of defrost problems are noticed first by a rise in temperature on the refrigerator side first. The above can be assessed with a quick look and listen. If no frost is present and the fan is running you can rule out the above. ****Please not that ice build up on just a small section, for example, upper left or right corner only is not a defrost situation but a sealed system issue, caused from low freon or a restriction in the lines and a EPA certified technician should be called.****** The last thing you should check you'll need to pull the machine away from the wall and remove the lower panel to gain access to the compressor area. What you are checking here is to see if the condensor fan motor is running and that the coils are somewhat clean enough for air to flow across them. The fan draws air from across the compressor and through the coils helping to keep the compressor temperature down and also to evaporate the water that is drain when the machine goes into defrost. Rule of thumb is if the compressor is on the fan should be. If the fan isn't runnning the heat from the compressor will be transferred to the machine and notice first on the freezer more so than the refrigerator. ****checkpoint three**** IS the condensor fan running? Are the coils clean, and can you feel the air from the fan with your hand placed on the opposite side of the coils?
The refrigerator side is usually where your defrost timer, discussed earlier is located. Also on this side is where your temperature control, or controls , are located, These controls are what are called thermostats, and have a direct line to the compressor. This means when the temperature has reached the desired setting the thermostat will kill power to the compressor in the same manner the terminating thermostat does with the defrost heater. Some models have a temperature control thermostat for the freezer and the refrigeratot, but some only for the refrigerator. The most noticeable sign of a thermostat being bad is the food in the refrigerator side will freeze, usually on any setting or the machine won't come on at all, including the fans. Problems are usually noticed on the refrigeratot side first. The last thing to discuss is The freezer and the refrigerator both are rising in temperature, but the fans are running, etc. Normally in cases like this a humming sound followed by click is heard or sometimes just the click. The humming sound is the compressor trying to start but failing and the click is the relay breaking contact. On newer machines, especially ones that fall under the whirlpool umbrella, the compressor relay, also called the start device has failed and needs to be replaced, but also the compressor itself could be bad. Well, thats the basics. I know it's long winded but I wanted to put something that you could print out and use as reference. Good luck.
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