PREVOST LE MIRAGE XLII BUS SHELLS Maintenance Manual Download

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Section 22: HEATING AND AIR CONDITIONING

PA1564

4. Carefully lift the bonnet assembly off (upper

part of the valve) so that plunger will not fall
out. The diaphragm can now be lifted out.

NOTE

The above procedure must be followed before
brazing solder-type bodies into the line.

CAUTION

Be careful not to damage the machined faces
while the valve is apart.

9.7.4 Valve

Reassembly

1. Place the diaphragm in the body with the

pilot port extension up.

2. Hold the plunger with the synthetic seat

against the pilot port.

3. Make sure the bonnet O-rings are in place.

Lower the bonnet assembly over the
plunger, making sure that the locating
sleeve in the bonnet enters the mating hole
in the body.

4. Insert the four socket head screws and

tighten evenly.

5. Replace the coil as stated previously.

6. Add a small quantity of refrigerant R-134a to

the low side of the system. Check for leaks.
Return the system to normal operation.

FIGURE 34: REFRIGERANT SOLENOID VALVE

22044

9.8 EXPANSION

VALVE

9.8.1 Cabin HVAC Unit

The expansion valve for the cabin HVAC unit is
a thermo-sensitive valve with a remote control
bulb head attached to the evaporator outlet line
and is accessible by the cabin air filters access
door (Fig. 15). The valve regulates the flow of
refrigerant liquid into the evaporator coils and is
controlled by the suction gas temperature
leaving the evaporator. The bulb head senses
the refrigerant gas temperature as it leaves the
evaporator. High temperature will cause
expansion and pressure on the power head and
spring. Such action causes the assembly valve
to open, allowing a flow of refrigerant liquid into
the evaporator.
The remote bulb and power assembly is a
closed system. The pressure within the remote
bulb and power assembly corresponds to the
saturation pressure of the refrigerant
temperature leaving the evaporator and moves
the valve pin in the opening direction. Opposed
to this force, on the under side of the diaphragm
and acting in the closing direction, is the force
exerted by the superheat spring. As the
temperature of the refrigerant gas at the
evaporator outlet increases above the saturation
temperature corresponding to the evaporator
pressure, it becomes superheated. The
pressure thus generated in the remote bulb and
power assembly surpasses the combined
pressures of the evaporator pressure and the
superheat spring, causing the valve pin to move
in the opening direction. Conversely, as the
temperature of the

refrigerant

gas leaving the

evaporator decreases, the pressure in the
remote bulb and power assembly also
decreases and the combined evaporator and
spring pressures cause the valve pin to move in
the closing position.
As the operating superheat is raised, the
evaporator capacity decreases, since more of
the evaporator surface is required to produce
the superheat necessary to open the valve. It is
obvious, then, that it is most important to adjust
the operating superheat correctly and that a
minimum change in superheat to move the valve
pin to full open position, is of vital importance
because it provides savings in both initial
evaporator cost of operation. Accurate and
sensitive control of the refrigerant liquid flowing
to the evaporator is necessary to provide
maximum evaporator capacity under load
conditions. The spring is adjusted to give 12 to

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