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©2019 Tecumseh Compressor Company LLC. All rights reserved. TPC Part #71511 | Rev A 05/13/2019www.tecumseh.com
GENERAL SEQUENCE OF OPERATION
Refrigeration Cycle
1. Power is supplied to the timer at terminals “1” and “N.”
2. The fan delay and the defrost termination thermostat is closed in the fan delay position and open in the defrost
termination position. The unit cooler fans run continuously on medium temperature applications.
3. The defrost heaters are off.
4. The room thermostat closes when the temperature rises above the desired setting.
5. The liquid line solenoid is energized and opens, which allows liquid refrigerant to flow through the unit cooler.
6. The low-pressure control closes when the suction pressure rises above the cut in setting of the control.
7. The compressor contactor closes. The compressor and condenser fan start simultaneously.
8. The room temperature gradually decreases to the desired temperature.
9. Once the desired temperature is reached, the thermostat opens and the liquid line solenoid closes,
stopping refrigerant flow through the evaporator.
10. Suction pressure decreases, and the compressor contactor opens when the pressure drops below
the cutout setting on the low-pressure control. The compressor and condenser fan stop running.
11. This cycle is repeated as many times as necessary to satisfy the room thermostat.
12. Frost starts to form on the evaporator coil and continues to form until the defrost cycle is initiated.
Defrost Cycle
1. The defrost cycle starts automatically by the timer at predetermined times. Typical settings are two to four
defrost cycles per day for freezers. For heavier frost loads additional settings may be required.
2. Switch ‘2” to ‘4’ opens in the timer which breaks the circuit to the room thermostat, liquid line solenoid, and
evaporator fan motors, allowing the compressor to pump down and shut off. Simultaneously switch “1” to “3”
closes in the timer allowing current to flow to one side of the defrost heater contactor. When the compressor
shuts off, an auxiliary contact will send power to the contactor holding coil; thus, energizing the defrost heaters.
3. The heaters raise the temperature of the coil to 32°F causing the frost to melt off the coil.
4. When the coil warms to 45°F to 55°F, the defrost termination thermostat closes, which allows current to the
switching solenoid in the timer allowing the refrigeration cycle to begin again.
5. The evaporator heaters are off. If the termination thermostat fails to close, the fail-safe set on the
timer will terminate defrost.
6. The low-pressure control closes, and the compressor will start.
7. When the coil temperature reaches 23°F to 30°F, the fan delay closes. This allows the current to flow
to the fan motors. The fan motors start running.
8. The system will now operate in the refrigeration cycle until another defrost period is initiated by the timer.
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