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TM 10-4110-262-13&P
0003 00
Defrost mode
The unit will go into a DEFROST mode if the evaporator temperature is below freezing and the unit is
running in the COOL mode and the manual defrost key on the controller is pressed. This will cause
power to flow from pin 8 of connector C-9 through the gray wire to pin 1 of C-3 on the PC board to the
defrost relay. This relay is grounded through the defrost klixon (DK) in the evaporator when this
thermostat is below freezing at 36 F ( 5 F). When the defrost relay is energized power flows from the
power relay through the normally open contacts of the defrost relay, through the 26 circuit to the pilot
solenoid (PS). This solenoid allows hot gas from the compressor to flow directly to the evaporator coil
where the hot gas will melt frost off the coil. Power to the condenser fan is also interrupted by the
normally closed defrost relay contacts so that this fan does not run during defrost. Power also flows from
the defrost relay contacts through diode D6 to the defrost coil. This keeps the relay energized after the
manual defrost key is release and until the defrost klixon opens after the evaporator is defrosted and
warms up.
There is also a timer built into the relay PC board that will also initiate a defrost mode if the unit has been
running in the cool mode for 3 hours. This time is adjustable by way of a potentiometer and dipswitches
located on the PC board. When this timer times out it applies power to the defrost relay coil and defrost is
initiated and continues as described above.
This unit has a few relays that are needed to complete the circuits through their normally closed contacts
but are never energized. This is because this PC board is used on other units that are mounted on trucks
where power is supplied by the vehicle battery. These relays are labeled CR1, CR2, and ER.
During defrost the suction pressure regulator limits the load on the compressor. IT maintains the suction
pressure at 65 psig.
Defrost Timer Settings
The Defrost timer counts plugged-in time and not switch-on time. It is only active when the coil
temperature is below 36 F and the defrost termination switch is closed.
The timer has two switches and a red, round selector located on the PC board in the RU electrical
enclosure. The timer is set from factory at 4 hours. The setting may be changed to any value between 1
minute and ten hours. To change the setting, see Work Package 0029 00
Manual Defrost Switches
Manual defrost will only cycle on if unit is indexed to run in the cooling mode.
NOTE
Manual defrost will only cycle on if unit is indexed to run in the cooling
mode.
A Manual Defrost switch is located on unit controller. Pressing this switch initiates the defrost cycle if the
defrost termination switch is closed and the unit is in Cool mode.
0003 00-7

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