TM 5-3805-281-24-1
Theory of Operation
HEATER AND AIR CONDITIONER CIRCUIT THEORY OF OPERATION
selected, the resistor block is bypassed and the blower
Power flows from the A/C Controller and Relays 5 amp
motor operates at maximum speed.
Fuse to the air conditioner controller and energizes the
following relays:
With air conditioning switch turned ON in the air
conditioner controller, power flows to the A/C high and
A/C compressor clutch relay (K25)
low pressure switch (B27). The switch (B27) contains
A/C blower motor (medium low speed) relay (K26)
both a low pressure switch and a high pressure switch.
A/C blower motor (medium speed) relay (K27)
With low pressure switch closed, power flows through
A/C blower motor (high speed) relay (K28)
high pressure switch. With high pressure switch
A/C blower motor and main power (low speed) relay
closed, power flows through the A/C compressor clutch
(K24)
relay (K25) to operate the A/C compressor clutch (Y1).
Power from the blower motor 20 amp fuse is applied to
The low pressure switch opens if the air conditioning
terminal 1 of the A/C compressor clutch relay (K25).
system loses its refrigerant charge. The switch opens
to stop current flow to the compressor, which prevents
Power from the 20 amp fuse also goes through the
compressor engagement.
A/C blower motor and main power (low speed) relay to
operate the A/C and heater blower motor (M6).
The high pressure switch protects the system from
high pressure. If a malfunction or line restriction
The air conditioner controller (A3) operates the blower
causes the high pressure to increase above the setting
motor in four different speeds:
of the switch, will open to stop current flow to the
compressor clutch.
Low speed
Medium low speed
The A/C and heater thermistor (B29) is used to sense
Medium speed
the temperature in the evaporator core. When the
High speed
temperature in the evaporator core raises, the blower
thermistor senses the warmer temperature and the air
When any of the first three speeds is selected, power
conditioner controller applies voltage to the
flows through the dropping resistor block (R11) and
compressor.
then to the blower motor. When the high speed is
9031
05
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TX,9031,UU3139 1913AUG961/1
