Defender 90 NAS. Instruction - page 88

AIR CONDITIONING

1

DESCRIPTION AND OPERATION

SCHEMATIC LAYOUT OF THE AIR CONDITIONING
SYSTEM

1. Compressor
2. Condenser
3. Receiver/drier
4. Thermostatic expansion valve
5. Evaporator
6. Capillary tube
7. Trinary pressure switch
8. Condenser cooling fan - single only
9. Compressor high pressure relief valve

10. Sight glass - refrigerant
11. Drying agent - receiver/drier
12. Blower motor

A1 Ambient air flow through condenser
A2 Recirclatory air flow through fan and evaporator
A3 Cooled air flow to vehicle interior

F1 High pressure high temperature refrigerant

vapour

F2 High pressure slightly subcooled refrigerant

liquid

F3 High pressure slightly subcooled refrigerant

liquid with moisture, vapour bubbles and foreign
matter removed

F4 Low pressure low temperature mixed liquid and

vapour

F5 Low pressure slightly super heated refrigerant

vapour

82

AIR CONDITIONING

2

DESCRIPTION AND OPERATION

AIR CONDITIONING SYSTEM OPERATION

The air conditioning system provides the means of
supplying cooled and dehumidified, recirculated air to
the interior of the vehicle. The cooling effect is
obtained by blowing air through the matrix of an
evaporator unit which has a variable temperature
control to provide the conditions required inside the
vehicle. The volume of cooled air supplied is
controlled by a variable speed blower.

A sealed system, charged with Refrigerant R134a,
together with a blower unit and evaporator
temperature control combine to achieve the cooled air
condition.

The air conditioning system comprises six major units:

1. An engine-mounted compressor.
2. A condenser mounted in front of the radiator.
3. A receiver/drier unit mounted on RH front

valance.

4. Thermostatic expansion valve mounted in front

of the evaporator.

5. An evaporator and blower unit mounted under

the RH side of the fascia panel.

6. Blower and temperature control switches.

These units are interconnected by hoses and pipes
carrying Refrigerant R134a, the evaporator is
seporate from the vehicle ventilation system.

Refrigeration cycle

1. Compressor

The compressor (1), belt driven from the crankshaft
pulley, is brought into operation by an AC switch on
the fascia which when selected engages an
electro-magnetic clutch to operate the compressor,
which pressurises and circulates the refrigerant
through the system. The electro-mechanical clutch
maintains the correct temperature and pressure by
engaging or disengaging to support the system’s
requirements. The clutch action is normally controlled
by a thermostat located at the evaporator (5). The
compressor is of the swashplate type, having fixed
displacement.

Should the temperature at the evaporator (5) fall low
enough for ice to begin to form on the fins, the
thermostat disengages the clutch and also isolates the
cooling fans relays. When the temperature at the
evaporator rises to the control temperature, the clutch
is re-engaged.

Should the system pressure become excessive or
drop sufficiently to cause damage to the compressor
(1) a trinary pressure switch (7), located in the high
pressure line, signals the relay unit to disengage the
clutch. The compressor also has an emergency high
pressure relief valve (9) fitted.

The condenser cooling fan operates continuously
when air conditioning is selected.

2. Condenser

From the compressor, hot high pressure vaporised
refrigerant (F1) passes to the condenser (2), which is
mounted in front of the engine coolant radiator. Ram
air (A1) passing through the condenser (2),
supplemented by the cooling fan (8) mounted in front
of the condenser, cools the refrigerant vapour
sufficiently to form a high pressure slightly subcooled
liquid (F2).

3. Receiver/drier

This liquid then passes to a receiver/drier (3) which
fulfils two functions. It acts as a reservoir and moisture
extractor (11).

A sight glass (10), in the high pressure line, provides a
method of determining the state of the refrigerant
without breaking into the system.

4. Expansion valve

From the receiver/drier (3) the moisture free high
pressure liquid refrigerant (F3) passes through a
thermostatic expansion valve (4). A severe pressure
drop occurs across the valve and as the refrigerant
enters the evaporator space at a temperature of
approximately -5

°

C, 23

°

F it boils and vaporises.

AIR CONDITIONING

3

DESCRIPTION AND OPERATION

5. Evaporator

As this change of state occurs, a large amount of
latent heat is absorbed. The evaporator is therefore
cooled and as a result heat is extracted from the air
flowing across the evaporator. The air flow is
controlled by the blower fans which can be operated
at various speeds.

To prevent liquid passing through to the compressor,
a capillary tube (6), attached to the outlet pipe of the
evaporator (5) and connected to the thermostatic
expansion valve (4), controls the amount that the
valve opens and closes in relation to the temperature
of the low pressure high temperature refrigerant
vapour (F4) at the outlet. The atomised refrigerant
then passes through the evaporator (5). Fan blown air
(A2) passes through the matrix (A3) of the evaporator
and is cooled by absorption due to the low
temperature refrigerant passing through the
evaporator.

A thermostat is positioned in the airflow leaving the
evaporator to sense the temperature of the exterior
fins. Should ice begin to form, due to a too cold
condition, it will signal to disengage the
electro-mechanical clutch on the compressor (1).

From the evaporator, low pressure, slightly
superheated refrigerant (F5) passes to the
compressor to complete the cycle.

AIR CONDITIONING CONTROL SYSTEM

The air conditioning control system comprises relays,
thermostat, pressure switches, and a control panel.
Together these controls, in conjunction with the
cooling fan, compressor clutch and blower, enable
minimal input to maintain the required environment
inside the vehicle.

Selecting air conditioning provides the added facility of
cooled air available to be mixed with warm air from
the heater system to give the required interior
environmental conditions.

Trinary pressure switch
This switch, located in the high pressure line between
the receiver drier and the expansion valve, monitors
refrigerant pressure and by means of the relay
controls the following system functions:

1. Refrigerant pressure drops below 2.0 bar, 29

lbf/in

2

(due to possible leakage), the

compressor’s electro-mechanical clutch is
dis-engaged.

2. When pressure rises above 2.0 bar, 29 lbf/in

2

the

compressor’s clutch is re-engaged.

3. Refrigerant pressure rises above 32 bar, 455

lbf/in

2

(due to possible blockage), even with

cooling fan operation, the compressor’s
electro-mechanical clutch is dis-engaged.
When the pressure drops below 26 bar, 375
lbf/in

2

the compressor clutch is re-engaged.

Condenser cooling fan
The condenser cooling fan operates automatically
whenever the air conditioning system is switched on.

1. Blower control
The blower can be operated at various speeds by
turning the blower switch to the required position.
When the blower is switched off, the air conditioning
system will not operate.

2. Temperature control
Rotate the temperature control switch clockwise to
increase the cooling effect. Use in conjuction with the
blower control as required.

AIR CONDITIONING

1

FAULT DIAGNOSIS

REFRIGERATION SYSTEM FAULTS

For any refrigeration system to function properly all
components must be in good working order. The unit
cooling cycle and the relationship between air
discharge temperature and ambient temperature and
the pressures at the compressor can help to
determine proper operation of the system.
The length of any cooling cycle is determined by such
factors as ambient temperature and humidity,
thermostat setting, compressor speed and air leakage
into the cooled area, etc. With these factors constant,
any sudden increase in the length of the cooling cycle
would be indicative of abnormal operation of the air
conditioner.
The low and high side pressures at the compressor
will vary with changing ambient temperature, humidity,
in-car temperature and altitude.
The following items should be checked before
operating the system:

1. Compressor drive belt tension.
2. Compressor magnetic clutch operation.
3. Condenser fan operation.
4. Condenser fins, dirt will cause poor cooling and

higher operating temperatures.

System check

The following conditions should be checked after
operating the system for several minutes:

1. All high pressure lines and components should

be hot to the touch.

2. All low pressure lines should be cool to the

touch.

3. Inlet and outlet temperatures at the receiver/drier

should be at the same temperature (warm). Any
very noticeable temperature difference indicates
a blocked receiver/drier.

4. Heavy frost on the inlet to the expansion valve

may indicate a defective valve or moisture in the
system.

5. Evaporation air temperature will vary with

ambient temperature and humidity. As humidity
increases the outlet temperature will be higher.

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