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The evaporator coil behind the cover on the back
wall inside the freezer will ice up under normal conditions. Every 8 to 10
hours for around 20 minutes the defrost timer (or in most newer models the electronic
adaptive defrost control) will turn the defrost heater on to
melt the built up ice. There is a defrost thermostat which prevents the heater from
overheating the freezer by breaking the heater circuit when the temp reaches
close to 32 degrees F. The entire cooling system shuts off during the defrost
cycle and starts back when the timer advances through the cycle.
If this ice is not melted it will continue to
build up until the air can’t flow over the coil to circulate the cold air
through the freezer and into the fridge. The temperature change in the fridge
is usually noticed first followed by the freezer.
If the defrost thermostat is bad it can prevent the heater from
coming on OR it won’t turn the heater off when it gets too warm. It is clamped
to the evaporator coil at the top to sense the temp. If it appears to be
misshapen it is bad.
With an ohm meter it should show continuity when
cold and none when warm.
You can also bypass the thermostat to see if the
heater comes on then. If it does then you know the thermostat is bad and needs
replaced.
The defrost heater is
located on the evaporator. It is in a tube which is at the bottom and can also
go up the sides of the evaporator. On some types you can see a burnt spot if
it’s bad. With an ohm meter it should show continuity from end to end when
disconnected from the wiring in the freezer. You can also test the wiring for
voltage when it’s in the defrost mode.
If
you have a defrost
timer you
can test it. It can be located under the fridge behind the kick panel on the
front. Some are in the fridge with the controls at the top. You can turn the defrost timer till it clicks and everything shuts
down. The heater should now come on. If it does, replace the timer because that
means the timer is not running. If it
doesn't, check the heater and defrost thermostat. Turn the timer again till
everything starts back up to end the defrost cycle.
If you have an adaptive defrost control instead of a timer,
replace it if the heater and thermostat test good. It is located in the fridge
with the controls in some models and on the back in others.
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