General Information; Heater And Air Conditioner; Heater And Air Conditioner Control - Jeep Wrangler 1999 Electronic Service Manual

24 - 2 HEATING AND AIR CONDITIONING

GENERAL INFORMATION

HEATER AND AIR CONDITIONER

All vehicles are equipped with a common heater-
A/C housing assembly (Fig. 1). The system combines
air conditioning, heating, and ventilating capabilities
in a single unit housing mounted under the instru-
ment panel. On heater-only systems, the evaporator
coil and recirculating air door are omitted from the
housing.
Fig. 1 Common Blend-Air Heater-Air Conditioner
System - Typical
Outside fresh air enters the vehicle through the
cowl top opening at the base of the windshield, and
passes through a plenum chamber to the heater-A/C
system blower housing. Air flow velocity can then be
adjusted with the blower motor speed selector switch
on the heater-A/C control panel. The air intake open-
ings must be kept free of snow, ice, leaves, and other
obstructions for the heater-A/C system to receive a
sufficient volume of outside air.
It is also important to keep the air intake openings
clear of debris because leaf particles and other debris
that is small enough to pass through the cowl ple-
num screen can accumulate within the heater-A/C
housing. The closed, warm, damp and dark environ-
ment created within the heater-A/C housing is ideal
for the growth of certain molds, mildews and other
fungi. Any accumulation of decaying plant matter
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.
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 actuating an electric
motor, which operates 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

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 suggested operation and use of
these controls.
The heater-only or heater-A/C control panel is
located in the instrument panel center bezel below
the radio and above the accessory switch bezel and
ash receiver. The control panel contains a rotary-type
temperature control, a rotary-type mode control
switch, and a rotary-type blower motor speed switch.
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, the blower
motor switch, and the control knobs are available for
service replacement.
TJ