GE Profile PSB42LGRBV, Technical Service Manual

Page: 40 / 72

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Defrost Cycles
The refrigerator incorporates two different
methods of defrost. Once the compressor has
accumulated 2 hours of compressor run time, and
certain conditions are satisfi ed, the fresh food
evaporator goes through a "natural defrost" cycle.
In addition, at a specifi c time interval, both the
fresh food and freezer evaporators go through a
"heated defrost" cycle.
Natural Defrost (Fresh Food Only)
Once the fresh food temperature is satisfi ed,
natural defrost occurs on the fresh food
evaporator, if the following conditions have been
met:
• The compressor has accumulated 2 hours of
run time.
• The ambient room temperature is above 66°F
(19°C).
• The freezer temperature is satisfi ed.
If the above conditions have been met, when
the fresh food temperature is satisfi ed, and the
compressor cycles off, the 3-way valve will open
to both the fresh food and freezer evaporators.
This allows the refrigerant within the system to
equalize back through the evaporators. This
warm thermal mass (refrigerant) fl owing through
the evaporators helps to remove any frost
buildup on the fresh food evaporator (it has very
little impact on the freezer evaporator due to
the freezer temperature). In addition, when the
fresh food evaporator thermistor is below 23°F
(-5°C), the fresh food evaporator fan will run to
help move air across the evaporator. Once the
thermistor reaches 28°F (-2°C), the fan will cycle
off.
Note: If the compressor is still cooling the freezer
section when the fresh food cycles off, or the
ambient room temperature is below 66°F (19°C),
natural defrost will not occur.
Heated Defrost
In normal cooling operation (normal food load,
door openings, etc.), the power control board
(PCB) will energize the fresh food defrost heater
every 5 hours of compressor running time and
the freezer defrost heater every 10 hours of
compressor running time.
For example, after approximately 10 accumulated
hours of compressor running time, the freezer
evaporator will have gone through one defrost
cycle, and the fresh food evaporator will have
gone through two defrost cycles.
The fresh food heater cycles off when the fresh
food evaporator thermistor reaches 63°F (17°C).
The freezer heater cycles off when the freezer
evaporator thermistor reaches 68°F (20°C). The
compressor and fans do not operate when either
section is defrosting, regardless of the cabinet
temperature. There is a 10-minute dwell period
after the heater cycles off.
Note: Upon initial power-up, the fresh food and
freezer go into heated defrost simultaneously,
after 4 hours of accumulated compressor run
time.
During unusual cooling situations where there are
no door openings (such as vacations), the time
between heated defrost cycles may increase.
If the ambient room temperature is above 82°F
(28°C) and there are no door openings, the fresh
food defrost occurs every 8 hours, and the freezer
defrost occurs every 16 hours. If the ambient
room temperature is below 82°F (28°C), defrost
occurs at the normal intervals – fresh food defrost
every 5 hours and freezer defrost every 10 hours
of accumulated compressor run time.
Thermistor Error Defrost Operation
If either evaporator thermistor fails (open or
shorted), the power control board (PCB) defaults
to a timed defrost cycle based on cabinet
temperature.
If the freezer evaporator thermistor fails (open
or shorted) and the freezer temperature is
below 27°F (-3°C), the freezer defrost heater
will be energized for 20 minutes. If the freezer
temperature is above 27°F (-3°C), the heater will
not come on.
If the fresh food evaporator thermistor fails (open
or shorted) and the fresh food temperature is
below 68°F (20°C), the fresh food defrost heater
will be energized for 10 minutes. If the fresh food
temperature is above 68°F (20°C), the heater will
not come on.
During thermistor error operation, the control
defaults to fresh food defrost every 5 hours and
freezer defrost every 10 hours of accumulated
compressor run time.