TM 9-4120-411-14
4.9.3.2 System Monitoring and Fault Isolation. When the pc board logic circuits make a "request" of the system
components to satisfy the desired operating control mode, the logic circuits compare the signal going to the appropriate
solid state relays (SSR's) and the signal coming back from the associated system component contactor. This control
circuit loop will determine if an SSR or system component contactor is defective. If the logic circuits determine that a
signal is going out to an SSR but is not returning from the associated contactor, the pc board will go into a minor failure
mode. A minor failure will cause the control panel LED's to flash to indicate a fault but will allow the unit to continue
operating in a limited capacity depending upon the mode of operation at the time of the failure. If the unit power is not
interrupted, causing the pc board to reset the failure mode, the status LED's will operate in their alternate mode of flashing
the binary code for the terminal point number on TP1 where the failure was detected to help isolate a control loop or
component failure. See table 4-3 to convert the binary code to a numeric value. The defrost status LED may blink slowly
during normal operation. 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 is not a fault indication. A fault will override this monitoring
indicator and display the fault if necessary. If the logic circuits determine that a signal is coming from a contactor but there
is no signal going to the associated SSR, the pc board will go into a major failure mode. A major failure will cause the
system to completely shut down and the fail safe light emitting diode (LED) (D16) will come on. If the unit power is not
interrupted, causing the pc board to reset the failure mode, the status LED's will operate in their alternate mode of flashing
the binary code for the terminal point number on TP1 where the failure was detected to help isolate a component failure.
The pc board also monitors the signals being sent from various safety overload or cutout switches in the unit. A safety
related failure can cause the unit to shut down or allow for limited operation depending on the failure. Refer to
abbreviations listed in paragraph 4.9.3.1 for definition of LED's. If a component or the pc board is determined to have
failed, replace it per the appropriate procedures. Note that the pc board components cannot be replaced individually and
the entire board must be replaced if any one component on it fails. The only exception to this is the output control power
fuse J3/F1, located above TP3, that can be replaced by the spare fuse mounted at the top of the pc board.
Table 4-3. LED Condition For Binary Code.
DIGITAL NUMBER EQUIVALENT
LED
0
1
2
3
4
5
6
7
8
9
10 11
12
13
14
15
16
17
18
19
20
21
COOL
(D35)
HEAT
(D34)
RH 1
(D33)
CCH
(D32)
DEF
(D31)
Note: A dot "" on the table indicates the LED is ON.
Additional LED's or combinations not listed represent higher numbers that are not relevant to this equipment.
In maintenance mode, LED (D30) comes on to indicate that the PC board is in potentiometer adjustment mode and is
not part of the binary code indicators.
4-23