How Frost Free Refrigerators Work

When a refrigerator is not defrosting properly, in other words when the evaporator coil is blocked by frost, the temperature in the freezer and the fresh food section will go up. As the air passes through the evaporator coil, it is cooled down, and the moisture or humidity in the air is left behind on the evaporator coil in the form of frost. If the defrost heater is not coming on, or it's not coming on for enough time, this frost will build up.

It is important to understand the difference between a frost build up on an evaporator and an ice buildup. When the defrost heater comes on and melts the frost off of the evaporator, it changes from a solid (the frost) to a liquid (water) and runs down a drain tube out of the refrigerator into a drain pan located on or around the compressor. If this drain becomes restricted, the water coming off the evaporator will start to back up. Given enough time and enough defrost cycles, that water will start to flood the evaporator and freeze, forming a block of ice under and on the evaporator. Changing all three major components, the defrost heater, the defrost terminator or thermostat, and the defrost timer or electronic control, will not solve this problem. The drain must be cleared of the debris that is blocking the flow of water down to the drain pan. Even if you unplug the refrigerator with both doors open for several days, it will just be a matter of time before the ice builds up again after you plug the refrigerator back in.

The defrost heater that is used to melt the frost from the evaporator is made up of the same type of material as the bake element in your oven, in most cases. This type of defrost heater is almost always physically attached to the evaporator. In other words, at the factory it is formed to the size and shape of the evaporator; and when the evaporator is manufactured, it is designed to accommodate the defrost heater. Another type of defrost heater is a resistance heater sealed inside a glass tube. The glass tube is mounted in close proximity to the evaporator and is used to radiate heat onto the evaporator to melt the frost. Both types of defrost heaters can actually get red hot. In most cases, both types of the defrost heaters operate on 120 volts A/C.

In close proximity to the evaporator, or sometimes attached to the evaporator, is the defrost terminator, also referred to as the defrost thermostat. The purpose of the defrost terminator is to prevent the defrost heater from coming on if it is warm in the freezer section, or to open the circuit to the defrost heater when all the frost is melted. Power is applied to the defrost heater through the defrost timer or electronic control, and then through the defrost thermostat before the circuit is complete to the heater. Therefore, if you had a defrost terminator that was stuck in the open position, you would have a build up of frost on the evaporator. As mentioned in the section titled, How Refrigerators Work, the defrost timer can be located almost anywhere on a refrigerator. It can be located on the base of the refrigerator behind the toe plate, in the fresh food section, and from time to time actually mounted on the back of the refrigerator.

Refrigerators with defrost problems that use a defrost timer are much easier to diagnose. Here's how to do it. Confirm that you have a frost build up and not an ice buildup, then take your ohmmeter and put it on the cord of the refrigerator. If the refrigerator timer and the cold control is set for the compressor to come on, as soon as you put your meter on the cord, you will be reading the resistance of the compressor. Now, you can take a flat bladed screwdriver and rotate the defrost timer until you hear it click. If at that time you see resistance on the meter, you will be reading the defrost heater resistance. That means that the defrost terminator contacts are closed; and the defrost heater has a complete circuit. Most likely this is an indication that the defrost timer was not advancing into defrost. If you can leave the refrigerator unplugged with the doors open, it will defrost itself; and all you need do then is replace the defrost timer.

When you click the defrost timer over to close the contacts to the defrost system, and you do not read the resistance of the heater on your meter, then you have to start taking the evaporator cover off. Be very careful when working around a refrigerator evaporator. Refrigerator evaporators are made of very thin aluminum that can easily be punctured. If you damage the evaporator and the refrigerant leaks out, it won't be necessary to fix the defrost system because you will be buying a new refrigerator. After you have removed the evaporator cover, look for the defrost terminator. It will have one wire in and one wire out, and be either attached to the evaporator or in close proximity to the evaporator. Use a jumper wire to jump out the terminals on the defrost terminator. Look at your ohmmeter and in most cases you will now see resistance. That resistance will be the defrost heater resistance. You can confirm this by unplugging the defrost heater and watching the meter start blinking, indicating infinite resistance.

If on the other hand, after you jump out the defrost terminator and do not read the resistance of the heater, you will need to check the heater for resistance. Remove your meter from the cord of the refrigerator because you will need it to check the defrost heater. Remove both wires from the defrost heater and use your meter to read the resistance of the heater. The resistance of the heater will be relatively low, it will be anywhere from 5 to 30 ohms. Your meter at this point will blink indicating infinite resistance, and that the resistance wire within the heater is open. Because the defrost terminator is relatively inexpensive, and has been cycling on and off since the refrigerator was new, it's not a bad idea to change the defrost terminator whenever you change a defrost heater.