SECTION 22: HEATING AND AIR CONDITIONING
56
PA1621 Maintenance Manual All Series | Section Revised November 2017
Locate the purge valve illustrated in Figure 69 &
Figure 74, and open them momentarily until no
air escapes from the lines. If present, open the
manual vent located inside the HVAC unit, on
the driver’s side (Figure 68).
NOTE
In test mode (see paragraph 5.4 TEST MODE
FOR SWITCHES AND SENSORS), with the
parking brake applied and the passenger set
point set to a value higher than 64°F (18°C),
the hot water circulating pump is not set to
OFF as it would normally do when the outside
temperature gets above 50°F (10°C). This
feature is useful when working on the heating
system to remove air pockets trapped in the
system.
6.5 COOLING
The central system is equipped with 4-cylinder
4NFCY Bitzer compressor with an air
conditioning capacity of 7½ tons. The receiver
tank and filter-dryer are mounted inside the
condenser compartment.
The schematics of FIGURE 4 and FIGURE 7
show the central HVAC system’s cooling function
arrangement.
6.5.1 Refrigeration Cycle
Refrigeration may be defined as the transfer of
heat from a place where it is not wanted to a
place where it is unobjectionable. The air
conditioning system used is the "closed" type
using R134a refrigerant.
1. The refrigerant flowing to the compressor is
compressed to high pressure and reaches a
temperature higher than the surrounding air.
It is passed through the air-cooled fins and
tubes of the condenser causing the hot,
high pressure gas to be condensed into a
liquid form.
2. The liquid refrigerant flows to the receiver
tank, then back to the condenser sub-
cooler. It leaves the condenser and passes
through a filter-dryer where moisture, acids
and dirt are removed and then through a
moisture indicator which indicates if any
moisture is present in the system.
3. By its own pressure, the liquid refrigerant
flows through a thermal expansion valve
where the pressure drop causes the
refrigerant to vaporize in a vapor-liquid state
at a low temperature pressure.
4. The cold low pressure refrigerant passes
through the passenger’s and driver’s unit
evaporator coils which absorb heat from the
air passing over the fins and tubes, and
changes into gas. In this form, the
refrigerant is drawn into the compressor to
repeat the air conditioning cycle.
5. The success of the air conditioning system
depends on retaining the conditioned air
within the vehicle. All windows and intake
vents should be closed. An opening of
approximately 8 in
2
(5162 mm
2
) could easily
neutralize the total capacity of the system.
6. Other causes of inadequate cooling are
dirty coils or filter. Dirt acts as insulation and
is also serves as a restriction to the air flow.
7. The refrigeration load is not constant and
varies. It is also affected by outside
temperature, relative humidity, passenger
load, compressor speed, the number of
stops, etc.
The compressor will load depending on
operating conditions.
6.5.2 Refrigerant
The A/C system of this vehicle has been
designed to use R134a refrigerant as a medium.
Regardless of the brand, only R134a must be
used in this system. The chemical name for this
refrigerant is 1,1,1,2-Tetrafluoroethane.
WARNING
Refrigerant in itself is nonflammable, but if it
comes in contact with an open flame, it will
decompose.
Procurement
Refrigerant is shipped and stored in metal
cylinders. It is serviced in 30 and 100 pound
cylinders.
Approximately 24 lbs are needed in the Central
HVAC system plus an additional 2 lbs will be
needed for the overhead compartment A/C
system if equipped.
For vehicles equipped with the Small HVAC
System, approximately 4 lbs of refrigerant are
needed.
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