Electronic Control System
Under Counter (UC-24) Series
3-17
#7003933 - Revision B - November, 2007
Monitor and Control Refrigerator Fan-Assisted, Off-Cycle Defrost (UC-24B, UC-24R, UC-24RO)
Temperature signals from the compartment and evaporator thermistor’s are observed by the microprocessor. During
off cycle defrost, if a refrigerator compartment temperature reaches high offset (calling for cooling) before evaporator
temperature rises to 38°F (3°C), no power will be supplied to the compressor. But, the evaporator fan will be
switched ON. Once the evaporator temperature reaches 38°F (3°C), normal cooling functions begin. (See Figure 3-
23).
NOTES:
• If refrigerator compartment thermistor is faulty, compressor operation defaults to twenty (20) minutes ON, forty (40)
minutes OFF cycling; EE appears in right of LCD; SERVICE will flash and Error Code 05 will be logged.
• If evaporator thermistor is faulty, the compressor will not energize until zone air temperature exceeds high offset
by 5°F (3°C); SERVICE flashes and Error Code 06 is logged.
• When in Sabbath Mode, the refrigerator compartment thermistor still controls compressor operation, except there
is a random fifteen (15) to twenty-five (25) second delay before the compressor is energized.
Figure 3-23. UC-24B Signal Trace - Refrigerator Fan Assisted Off-Cycle Defrost
1. Compartment thermistor at high
off-set temp. calling for cooling.
4. Evap Thermistor
below 38°F (3°C).
3. Evap fan energized.
2. Door is closed, so no power to lights.
This is monitored via the orange wire.
5. Compressor and condens-
er fan stay OFF until evap
rises to 38°F (3°C).
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