Heater And Air Conditioner Control; High Pressure Cut-Off Switch - Jeep cherokee 2000 Service Manual

24 - 6 HEATING AND AIR CONDITIONING
DESCRIPTION AND OPERATION (Continued)
provides an additional food source for fungal spores,
which enter the housing with the fresh air. Excess
debris, as well as objectionable odors created by
decaying plant matter and growing fungi can be dis-
charged into the passenger compartment during
heater-A/C system operation.
OPERATION
The heater and optional air conditioner are blend-
air type systems. In a blend-air system, a blend-air
door controls the amount of unconditioned air (or
cooled air from the evaporator on models with air
conditioning) that is allowed to flow through, or
around, the heater core. A temperature control knob
on the heater-A/C control panel determines the dis-
charge air temperature by controlling an electric
motor, which moves the blend-air door. This allows
an almost immediate control of the output air tem-
perature of the system.
The mode control knob on the heater-only or heat-
er-A/C control panel is used to direct the conditioned
air to the selected system outlets. Both mode control
switches use engine vacuum to control the mode
doors, which are operated by vacuum actuator
motors.
On air conditioned vehicles, the outside air intake
can be shut off by selecting the Recirculation Mode
with the mode control knob. This will operate a vac-
uum actuated recirculating air door that closes off
the outside fresh air intake and recirculates the air
that is already inside the vehicle.
The optional air conditioner for all models is
designed for the use of non-CFC, R-134a refrigerant.
The air conditioning system has an evaporator to cool
and dehumidify the incoming air prior to blending it
with the heated air. This air conditioning system
uses a fixed orifice tube in the liquid line near the
condenser outlet tube to meter refrigerant flow to the
evaporator coil. To maintain minimum evaporator
temperature and prevent evaporator freezing, a fixed
pressure setting switch on the accumulator cycles the
compressor clutch.

HEATER AND AIR CONDITIONER CONTROL

DESCRIPTION
Both the heater-only and heater-A/C systems 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 manual in the vehicle glove box for more
information on the features, use, and suggested oper-
ation of these controls.
OPERATION
The heater-only or heater-A/C control panel is
located to the right of the instrument cluster on the
instrument panel. The control panel contains a rota-
ry-type temperature control knob, a rotary-type mode
control switch knob, and a rotary-type blower motor
speed switch knob.
The heater-only or heater-A/C control panel cannot
be repaired. If faulty or damaged, the entire unit
must be replaced. The illumination lamps are avail-
able 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 Group 7 -
Cooling System for more information on the engine
cooling system, 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 between the compressor and the con-
denser inlet. The switch is screwed onto a fitting that
contains a Schrader-type valve, which allows the
switch to be serviced without discharging the refrig-
erant 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 low pressure cycling
clutch switch 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.
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