GE Profile PSF26PGWBB Technical Service Manual page 36

The system is designed to run the fan until the fresh
food evaporator thermistor reaches 35°F.
Once this temperature is reached, the fan continues
to run for an additional 5 minutes. Under normal
conditions, the defrost time takes approximately
30 minutes. The maximum time the fan runs in low
speed is 60 minutes. If the evaporator thermistor
has not reached 35°F after 60 minutes, the control
switches to extended defrost #1.
Fresh Food Extended Defrost #1
Occasionally there may be excessive frost on the
evaporator coil. This can be attributed to numerous
door openings, extremely high humidity, poor
door gasket seal, etc. If the normal defrost fan
time exceeds 60 minutes, the fan switches to high
speed. The control continues to monitor the fresh
food evaporator thermistor for 35°F. When 35°F is
reached, the fan runs for an additional 5 minutes at
high speed. The fan can run up to an additional 30
minutes at high speed, trying to reach 35°F. After 90
total minutes of fan time (60 at low speed and 30 at
high speed), if the fresh food evaporator is still below
35°F, the control will switch to extended defrost #2.
Fresh Food Extended Defrost #2
If the fresh food fan has been operating for 90
minutes, the main control searches for a fresh food
thermistor temperature above 52°F. If the fresh food
thermistor is above 52°F, the main control board
assumes there is a problem reading the fresh food
evaporator thermistor, ends the defrost cycle, and
returns to normal cooling.
If the fresh food thermistor is less than 52°F, the
main control board assumes there is refrigerant
leaking through the 3-way valve, keeping the fresh
food evaporator cold during the defrost process
while the freezer evaporator is cooling. The main
control board shuts off the compressor and runs
the fresh food fan at high speed until 2 conditions
are met: the evaporator thermistor reaches 35°F
and the fresh food thermistor is 2½°F above the
set point. As in the other cycles, the fan continues
to run for an additional 5 minutes after the 35°F
evaporator temperature is reached.
Fresh Food Forced Defrost
If the main control board senses the fresh food
section has been cooling for 45 minutes, it
immediately stops the refrigerant fl ow through
the fresh food evaporator. The main control board
changes the position of the 3-way valve if cooling is
still required in the freezer, or turns the compressor
off if the freezer is satisfi ed. The fresh food fan
operates on high speed until the evaporator
reaches 35°F, plus an additional 5 minutes after the
evaporator reaches 35°F.
To prevent the refrigerator from going into forced
defrost when fi rst installed or after a power failure,
the control will disregard the 45 minute time limit if
the freezer temperature is above 20°F.
Note: If the fresh food evaporator thermistor is
either open or shorted and the 35°F cutoff cannot
be determined, the main control board defaults to a
fi xed defrost cycle of 1 hour at high-speed fan.
Freezer Defrost Cycle
The freezer evaporator utilizes a defrost heater
to remove frost from the coil. The control board
determines the length of time the heater is
energized. It does this by monitoring the freezer
evaporator thermistor. Once the temperature of
the thermistor reaches 75°F, the control cycles the
defrost heater off. A bimetal safety thermostat
(L140-30) provides a backup in the event the
evaporator thermistor fails. The safety thermostat
prevents the temperature from exceeding 140°F.
Note:
• During the freezer defrost cycle, the compressor
does not operate, even if the fresh food section
is above the set point.
• When initiating a freezer defrost cycle, there is
a 6-second delay from the time the main board
shuts off the power to the compressor and when
the defrost heater is energized.
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