Operation and maintenance manual

Section 260-0130
Operators Compartment - Air Conditioning
SM 1405 Rev 2 11-04
2
slow down this radiation of heat. Cooling fins are
located on the condenser tubes and fans are used to
circulate cool air around the condenser tubes. Keep all
leaves, paper, dirt, etc. clear from the condenser and
condenser filter. The cooling fins should be straight to
permit free flow of air. The condenser is sometimes
located ahead of the engine radiator and blockage of
air flow through the radiator also affects the
condenser. Bent fan blades, slipping fan drive,
inoperable condenser fan motors, or any other fault
that lessens the amount of cool air circulated through
the condenser, should be corrected. The oil, dirt, or
antifreeze will act as an insulator that will inhibit the
radiation of heat.
Since the purpose of the condenser is to radiate heat
energy, anything that prevents or inhibits this action
may affect cooling, but the temperature and pressure
of the R-134a raise and lower together. Heat energy
that has not been radiated will remain in the R-134a
and the result will be pressure that is too high. The
condenser, hoses, connections and seals can be
damaged by the high pressure. Pressure sensing
safety switches may be activated by the high pressure
caused by the condenser not radiating enough heat.
Receiver Drier
The high pressure liquid R-134a moves from the
condenser to the receiver drier, where the R-134a is
stored and filtered. Moisture is the major enemy of the air
conditioning system and the desiccant inside the receiver
drier will absorb only a small amount. The container of
desiccant inside the receiver drier may break open and
contaminate the system if any attempt is made to dry the
desiccant, or, if more moisture is inside the system than
the desiccant can absorb.
Every effort should be made to remove all moisture from
the system and install a new receiver drier if its condition
is questionable. Installation of a new receiver drier is
recommended each time any part of the R-134a system
is open to the atmosphere. Bubbles are observed in the
sight glass on top of the receiver drier during the
charging procedure.
A filter screen is located in the receiver drier to stop
solid contaminates from leaving the unit. Blockage of
the filter will result in a drop in pressure that will be
indicated by a drop in temperature. Connections of
the new receiver drier should be securely capped
before installation to prevent the entrance of moisture
(air) while in storage.
Thermostatic Expansion Valve
An expansion valve is installed in the system to lower
the pressure before the R-134a enters the evaporator.
The reduction in pressure is done by passing the
R-134a through a small hole (orifice). The size of the
orifice must be controlled to compensate for changes
in pressure and temperature. The temperature of
R-134a leaving the evaporator is sensed by a
thermostatic sensor that moves the valve seat via a
diaphragm and actuating pin.
Evaporator - Heat/Cool
The evaporator is the low pressure, low temperature
component where liquid R-134a absorbs heat from
surrounding air. The expansion valve bleeds high
pressure R-134a into the low pressure evaporator.
The R-134a expands rapidly in the evaporator and its
temperature is quickly reduced. The R-134a absorbs
heat from the air when the blower fan circulates air
over the evaporator coil fins. The exchange of heat
from the air to the R-134a depends upon the
difference in temperature. During high heat load, such
as usually encountered when the system is first turned
on, the temperature difference is great and the R-134a
will absorb heat quickly. The blower fan can be set at
its highest setting to circulate large quantities of warm
air around the evaporator. After the cab has cooled,
the fan speed should be reduced so that the already
cool air will have a longer time to yield heat to the
R-134a as it passes the evaporator coils. The heater
circuits utilize engine coolant at approximately 82° C
(180° F).
High Pressure and Low Pressure Switches
The pressure switches are electric switches that
monitor air conditioner operation. The high pressure
and low pressure switches are activated at preset
pressures and engage and disengage the compressor
clutch.