TM 9-4120-398-14
i. ELECTRICAL CONTROL SYSTEM. Figure 1-5 is reference for the following discussion of the air conditioner
electrical control system.
(1) OPERATING MODES. Switch S1 allows selection of two operating modes: ventilate and automatic. In the
ventilate mode, air is circulated without cooling or heating; in the automatic mode, the thermostat controls the temperature of
the output air.
(2) VENTILATE MODE. With S 1 in the VENTILATE position, only the evaporator fan motor operates The
startup and operating sequence is as follows:
(a) Control relay K4 is energized though closed contacts K5-X1 (line 2).
(b) K4-X1 contacts close, locking in K4 and energizing evaporator fan starter K1 (line 3). The evaporator
fan begins operation.
(c) K4-X2 contacts close, energizing control relay KS. K5 locks in through contracts K5-X2. K5-X1
contacts open.
(d) If the evaporator fan overheats, its thermal cutout (TCO) opens, breaking the circuit to K1 (line 3) and
de-energizing the fan motor.
(e) K4 is also de-energized. K4-X1 holding contacts open. K4-X2 contacts open. However, KS remains
energized through its holding contacts K5-X2 (line 1).
(f) With K5 energized, contacts K5-X1 remain open. Therefore, return to normal temperature (evaporator
fan TCO closes) cannot restore operation of the fan (see 1 above). The fan is restarted by turning S1 to the OFF-RESET
position, which de-energizes K5 and allows K5-X1 to reclose.
(3) AUTOMATIC MODE COOLING. With S1 in the AUTO mode, operation of the air conditioner is controlled
by the thermostat S2 however, all evaporator fan controls bypass the thermostat, thereby allowing the evaporator fan to run
continuously. Operation in the AUTO mode with S2 in the high temperature stage (cooling) is as follows:
(a) The evaporator fan starts up and operates as described in paragraph 1-10d.
(b) Closed contacts K4-X3 provide a circuit to thermostat S2 to turn on condenser fan starter K2 (line 6).
(c) Closed contacts K3-X5 provide a circuit to time delay relay K6 (line 7). K6 contacts close after a
one-minute delay.
(d) Contacts K6-X1 close, providing a circuit to turn on compressor motor starter K3 (line S).
(e) Contacts K3-X6 close, locking in the compressor starter. Contacts K3-X5 open, taking the time delay
relay out of the circuit. Contacts K5-X1 open.
(f) In cycling to open, thermostat S2 breaks the circuit through K3-X6 to compressor motor starter K3 and
condenser fan starter K2. In cycling to close, the thermostat initiates the action described in steps (b) through (e).
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