TM 9-4120-411-14
4.9.1 PC Board Layout and Component Functions. See figure 4-9. The pc board logic circuits receive power at TP6
(upper right corner) directly from the unit power supply. The status light emitting diodes (LED's) D30 through D35 (lower
center) indicate the operating condition the pc board "thinks" it's in. Unit power, used by the pc board to control system
components, is completely separate from the logic circuits and enters at TP3 (lower right). The control power then passes
through fuse J3/F1 before going to the output switch side of the solid state relays (SSR's) K3 through K9. The SSR's will
switch the control power at TP3 to energize the various system component contactors. The LED's D17 through D23
adjacent to the SSR's provide a visual indication of when the SSR's are energized. When a system component contactor
is energized, it will send a signal back to the pc board at TP1 (upper left). Feedback LED's D3 through D6 and D11
provide a visual indication of what component contactor is energized. Safety cutout switches used in the unit will send a
signal to the pc board at TP1. Safety LED's D1, D2, and D7 through D10 provide a visual indication of what cutout switch
has indicated a fault. The pc board receives temperature signals from the volute or remote control box for shelter air,
outside coil, and outside air. The temperature potentiometers R19 through R24 (center left) can be adjusted to set the
point at which the temperature signals will cause system components to cycle. When the remote control box is being
used, LED D12 will be illuminated. The compressor is protected from short cycling by a timer circuit on the pc board that,
when activated, is indicated by LED D14. The compressor recycle time delay can be terminated for maintenance
troubleshooting purposes by pressing momentary push button switch S1. The dual inline parallel (DIP) switches S3 are
used for maintenance troubleshooting purposes to override the automatic logic in the pc board and operate the control
circuits manually. Binary codes for adjustment and diagnosis of failures in the system are indicated by LED's D16 and
D30 through D35.
4.9.2 (FDECU-2 ONLY) Compressor Crankcase Heater Function. The compressor crankcase heaters are used to
keep the compressor temperature above the surrounding ambient air temperature to prevent liquid refrigerant from
migrating into the compressor when it is not operating. When the FDECU is put into the climate control mode, and the
compressor needs to operate, the compressor warm up LED (DS1 or DS4) on the control panel will come on if the
compressor temperature is not 23 F above the surrounding ambient air temperature. If this LED is on, the compressor
will not operate until this temperature difference is met. Once the compressor starts, the crankcase heaters may cycle on
and off to maintain the temperature difference. When the compressor is running, the heater cycling will not affect
operation, however, if the compressor attempts to start during a warmup cycle, the temperature difference must be met
before the compressor actually starts. (FDECU-2 ONLY) (FOR AIR FORCE ONLY) On some FDECU-2 units, the
compressor crankcase sensor has been bypassed to eliminate the compressor warm up delay. This is accomplished by
installing a jumper or splice in the harness that will connect the crankcase sensor (CCS) and common (COM) terminals of
terminal block (TP4) on circuit board (A2A1).
4-19
