heated air. This air conditioning system uses a ther-
mal expansion valve between the condenser and the
evaporator coil to meter refrigerant flow to the evap-
orator coil. To maintain minimum evaporator temper-
ature and prevent evaporator freezing, an electronic
cycling clutch switch on the expansion valve cycles
the compressor clutch.
HEATER AND AIR CONDITIONER CONTROL
DESCRIPTION
The heater and A/C system use a combination of
mechanical, electrical, and vacuum controls. These
controls provide the vehicle operator with a number
of setting options to help control the climate and
comfort within the vehicle. Refer to the owner’s man-
ual in the vehicle glove box for more information on
the features, use, and suggested operation of these
controls.
OPERATION
The heater-A/C control panel is located to the right
of the instrument cluster on the instrument panel.
The control panel contains a rotary-type temperature
control knob, a rotary-type mode control switch knob,
and a rotary-type blower motor speed switch knob.
The heater-A/C control panel cannot be repaired. If
faulty or damaged, the entire unit must be replaced.
The illumination lamps are available for service
replacement.
HEATER CORE
DESCRIPTION
The heater core is located in the heater-A/C hous-
ing, under the instrument panel. It is a heat
exchanger made of rows of tubes and fins.
OPERATION
Engine coolant is circulated through heater hoses
to the heater core at all times. As the coolant flows
through the heater core, heat removed from the
engine is transferred to the heater core fins and
tubes. Air directed through the heater core picks up
the heat from the heater core fins. The blend air door
allows control of the heater output air temperature
by controlling how much of the air flowing through
the heater-A/C housing is directed through the
heater core. The blower motor speed controls the vol-
ume of air flowing through the heater-A/C housing.
The heater core cannot be repaired and, if faulty or
damaged, it must be replaced. Refer to Cooling Sys-
tem for more information on the engine cooling sys-
tem, the engine coolant and the heater hoses.
HIGH PRESSURE CUT-OFF SWITCH
DESCRIPTION
The high pressure cut-off switch is located on the
discharge line near the compressor. The switch is
screwed onto a fitting that contains a Schrader-type
valve, which allows the switch to be serviced without
discharging the refrigerant system. The discharge
line fitting is equipped with an O-ring to seal the
switch connection.
OPERATION
The high pressure cut-off switch is connected in
series electrically with the electronic cycling clutch
switch, the low pressure cut-off switch, and the Heat-
er-A/C controls between ground and the Powertrain
Control Module (PCM). The switch contacts open and
close causing the PCM to turn the compressor clutch
on and off. This prevents compressor operation when
the discharge line pressure approaches high levels.
The high pressure cut-off switch contacts will open
when the discharge line pressure rises above about
3100 to 3375 kPa (450 to 490 psi). The switch con-
tacts will close when the discharge line pressure
drops to about 1860 to 2275 kPa (270 to 330 psi).
The high pressure cut-off switch is a factory-cali-
brated unit. The switch cannot be adjusted or
repaired and, if faulty or damaged, it must be
replaced.
HIGH PRESSURE RELIEF VALVE
DESCRIPTION
A high pressure relief valve is located on the com-
pressor cylinder head, which is at the rear of the
compressor. This mechanical valve is designed to
vent refrigerant from the system to protect against
damage to the compressor and other system compo-
nents, caused by condenser air flow restriction or an
overcharge of refrigerant.
OPERATION
The high pressure relief valve vents the system
when a discharge pressure of 3445 to 4135 kPa (500
to 600 psi) or above is reached. The valve closes with
a minimum discharge pressure of 2756 kPa (400 psi)
is reached.
The high pressure relief valve vents only enough
refrigerant to reduce the system pressure, and then
re-seats itself. The majority of the refrigerant is con-
served in the system. If the valve vents refrigerant, it
does not mean the valve is faulty.
The high pressure relief valve is a factory-cali-
brated unit. The valve cannot be adjusted or
repaired, and must not be removed or otherwise dis-
24 - 8 HEATING AND AIR CONDITIONING DN
DESCRIPTION AND OPERATION (Continued)
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