ELECTRICAL CIRCUIT
The air conditioner’s electrical circuit is fed from an
accessory circuit and is fused with a 30-ampere fuse
or circuit breaker.
The blower control is a switch which provides a range
of blower speeds from fast to slow. When the blower
switch is turned on, current is fed to the thermostat.
Once the blower is turned on, fan speeds may be
changed without affecting the thermostat sensing
level.
The thermostat reacts to changing temperatures which
cause electrical contacts to open and close. The
thermostat has a capillary tube extended into the
evaporator coil to sense temperature.
When the contacts are closed, current flows to the
clutch field and energizes the clutch, causing the
crankshaft to turn which starts the refrigeration cycle.
When the temperature of the evaporator coil drops to
a predetermined point, the contacts open and the
clutch disengages.
When the clutch is disengaged, the blower remains at
the set speed. After the evaporator temperature rises
about twelve degrees above the cutout point, the con-
tacts in the thermostat close and the refrigeration cycle
resumes.
THERMOSTAT
An electromagnetic clutch is used on the compressor
to provide a means of constant temperature control of
the cab. The clutch is controlled by a thermostat in the
evaporator which is set initially by the driver to a pre-
determined point. Coil temperature is then maintained
by the cycling action of the clutch.
The thermostat is simply a thermal device which con-
trols an electrical switch. When warm, the switch is
closed; when cold, it is open. Most thermostats have a
positive OFF position as a means to turn the clutch OFF
regardless of temperature.
The bellows type thermostat has a capillary tube con-
nected to it which is filled with refrigerant. The capillary
tube is attached to the bellows inside of the thermostat.
Expansion of the gases inside the capillary tube exerts
pressure on the bellows, which in turn closes the con-
tacts at a predetermined temperature.
COMPRESSOR CLUTCH
An electromagnetic clutch is used in conjunction with
the thermostat to disengage the compressor when it is
not needed, such as when a defrost cycle is indicated
in the evaporator, or when the system is turned off.
The stationary field clutch is the most desirable type
since it has fewer parts to wear out. The field is mounted
to the compressor by mechanical means depending
on the type field and compressor. The rotor is held on
the armature by a bearing and snap rings. The arma-
ture is mounted on the compressor crankshaft.
When no current is fed to the field, there is no magnetic
force applied to the clutch and the rotor is free to rotate
on the armature, which remains stationary on the
crankshaft.
When the thermostat or switch is closed, current is fed
to the field. This sets up a magnetic force between the
field and armature, pulling it into the rotor. When the
armature becomes engaged with the rotor, the com-
plete unit rotates while the field remains stationary. This
causes the compressor crankshaft to turn, starting the
refrigeration cycle.
When the switch or thermostat is opened, current is cut
off. The armature snaps back out and stops while the
rotor continues to turn. Pumping action of the com-
pressor is stopped until current is again applied to the
field. In addition, safety switches in the compressor
clutch electrical circuit control clutch operation, disen-
gaging the clutch if system pressures are abnormal.
SAFETY SWITCHES
Trinary Switch
The Trinary switch performs three distinct functions
to monitor and control refrigerant pressure in the sys-
tem. This switch is installed between the condenser
and expansion valve, usually on the receiver-drier. The
switch functions are:
The low-pressure switch prevents compressor
operation if the refrigerant has been lost or the
ambient temperature is too low. Low ambient
temperature results in very low system pressure.
The mid-range function actuates the engine fan
clutch if installed.
The high-range pressure function disengages
the compressor clutch if system pressure is too
high.
The switch functions will automatically reset when sys-
tem pressure returns to normal.
M9-6 Air Conditioning System M09010 6/97
for HFC 134a Refrigerant
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