GE PSH23PGR Series Technical Service Manual page 23

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
two conditions are met: the evaporator thermistor
reaches 35°F and the fresh food thermistor is
2½F° 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 bi-metal safety
thermostat 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.
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