TM 9-4120-411-14
4.9.3 Overall Control Logic.
4.9.3.1 Normal Operation. When the FDECU is put into a mode of operation at the control panel, the pc board logic
circuits will control the operation of all system components necessary for the desired mode. Various time delays are
programmed into the logic circuits to cause simple make-before-break contactor functions and inrush current limiting. The
length of time for these delays ranges from between one and three seconds to three minutes for the compressor recycle
time delay. When observing the operation of the pc board light emitting diodes (LED's), keep the time delays in mind and
wait before assuming a fault has occurred. Note that a control logic time-out circuit may prevent the unit from operating
immediately. If this occurs, do not turn the unit off, it should start automatically within 5 minutes. Table 4-2 lists the two
modes of operation, vent and climate control. The table breaks down the logical control of these modes and shows the
LED's that indicate the control functions. The status LED's give an indication of the logic process the pc board is using to
control the system components for any given mode. The logic sequence is based on the selected mode and input signals
from the thermistor temperature sensors used in the system. In general, the vent mode can be used under any condition.
When using the control panel integral to the unit (THIS PANEL) the inside blower will remain on anytime the unit is in the
climate control mode. This is necessary for the temperature sensor, integral to the volute assembly, to sense the shelter
air. When using the remote control box (REMOTE BOX) the inside blower will cycle with the cooling or heating demand
because the temperature sensor, integral to the remote control panel, is inside the shelter and can sense the air at any
time. The climate control mode follows certain temperature setting points as follows; cooling will activate when the shelter
temperature is 3 F above the thermostat set point. If the outside temperature drops to a point that causes the outside coil
temperature to go below 80 F, the outside fan will stop. This is necessary to maintain proper refrigerant system
pressures under low ambient conditions. At outside ambient temperatures above 17 F, the mechanical refrigeration
system provides primary (first stage) heating and electric resistance heaters provide supplemental (second stage)
heating. First stage heating activates when the shelter temperature is 2 F below the thermostat set point and second
stage heat will activate if the shelter temperature drops to 5F below the thermostat set point. Ice will form on the outside
coil whenever the mechanical refrigeration system is used for heat. The system must melt the ice that forms on the coil
using the defrost mode of operation. Defrost mode will activate whenever the outside air temperature is below 51 F, and
the outside coil temperature is 10 F below the outside air temperature. Defrost mode is essentially cooling mode without
the outside fan. Supplemental electric resistance heat is used when in defrost mode to offset the cooling effect of the
mechanical refrigeration system. If the outside ambient temperature drops below 17 F, the mechanical refrigeration
system will be disabled and electric resistance heat will provide both primary (first stage) and supplemental (second
stage) heating. When the control logic energizes the reversing valve, for cooling or defrosting, it will remain energized
even after the unit cycles off. It will remain energized until there is a change in the operating mode that requires it to be
deenergized. The defrost status LED may blink slowly during normal operation when the unit is put into climate control
mode and is cooling the shelter air. This is an indication that the logic circuit is monitoring the outside coil and air
temperatures to determine if there is a need to defrost the inside coil. This LED may continue to blink indefinitely or stop
blinking 20 minutes after it starts depending on the temperature conditions. The following cross reference list is provided
to explain the abbreviations used on the pc board:
OL
Overload
HTCO
High Temperature Cutout
BLOW
Blower
LPCO
Low Pressure Cutout
COMP
Compressor
CIT
RH
Resistance Heat
DEF
Defrost
COV
Cover
REM
Remote
HPCO
High Pressure Cutout
R.V.
Reversing Valve
4-21