ventilation and air conditioning (HVAC) system, the
engine cooling system must be properly maintained.
The use of a bug screen is not recommended. Any
obstructions in front of the radiator or A/C condenser
will reduce the performance of the A/C and engine
cooling systems.

The engine cooling system includes the radiator,

thermostat, radiator hoses and the engine coolant
pump. Refer to 7 - Cooling for more information
before opening or attempting any service to the
engine cooling system.

All vehicles are equipped with a common ventila-

tion housing (Fig. 1) located in the engine compart-
ment which includes:

• Blower motor

• Blower motor resistor block

• Recirculation door and actuator

All vehicles are also equipped with a common

heater housing (Fig. 2) mounted under the instru-
ment panel which includes:

• Heater core

• Evaporator core

• Evaporator temperature sensor

• Expansion valve

• Air outlet temperature sensor

• Mode doors

Fig. 1 Ventilation Housing

1 - NUT (5)
2 - WASHER (5)
3 - VENTILATION HOUSING
4 - HOUSING COVER

HEATING & AIR CONDITIONING

24 - 1

Some vehicles are equipped with an additional

Denso 10S17 A/C compressor mounted to the engine
which is used for the optional rear A/C system and
various add-on A/C systems (Fig. 3).

A heater booster is used to aid in warming the

engine coolant. The heater booster system can be
switched on while the vehicle is being driven to help
the engine reach its normal operating temperature
quickly and will help aid in heating up the air within
the passenger compartment quickly when requested
by the operator (Fig. 4).

OPERATION

Outside air enters the vehicle through the hood

opening at the base of the windshield, and passes
through the ventilation housing located in the engine
compartment into the heater housing located behind
the instrument panel. Air flow velocity is adjusted
with the blower motor speed selector thumbwheel on
the A/C-heater control. The air intake openings must
be kept free of snow, ice, leaves, and other obstruc-
tions for the HVAC system to receive a sufficient vol-
ume of outside air.

The automatic temperature control (ATC) system

controls interior temperature by taking actual values
from the temperature sensors and the CAN bus and
comparing them to the nominal value of the temper-
ature control switch. The electric pulsed heater valve
is then energized depending on the requested quan-
tity of heat and an electrically-operated water pump
gives a nearly constant water flow for exact temper-
ature regulation. If the solenoid is not energized, the
coolant circuit to the heat exchanger is fully open. To
control the temperature the solenoid valve is pulsed
by the ATC in periods of four seconds.

The mode control knob on the A/C-heater control is

used to direct the conditioned air flow to the selected
air outlets. The mode control knob operates the mode
doors by cables connected to the mode doors.

Fig. 2 Heater Housing

1 - SCREW (12)
2 - UPPER HOUSING
3 - A/C EVAPORATOR
4 - EVAPORATOR O-RING SEAL (2)
5 - EVAPORATOR GASKET
6 - VENTILATION HOUSING GASKET
7 - LOWER HOUSING
8 - WIRING HARNESS
9 - BOLT (3)
10 - HEATER CORE
11 - HEATER CORE TUBE ASSEMBLY
12 - HEATER CORE TUBE GASKET
13 - HEATER CORE TUBE O-RING SEAL (2)

Fig. 3 Denso 10S17 A/C Compressor

Fig. 4 Heater Booster

24 - 2

HEATING & AIR CONDITIONING

When the outside air contains smoke, odors, high

humidity, or if rapid cooling is desired, interior air
can by recirculated by selecting the Recirculation
Mode with the mode control knob. The mode control
knob operates the recirculation door through use of a
vacuum actuator. When the Recirculation Mode is
selected, the recirculation door is closed to prevent
outside air from entering the passenger compart-
ment.

To maintain minimum evaporator temperature and

prevent evaporator freezing, an evaporator tempera-
ture sensor is used.

The A/C system is designed for the use of non-CFC,

R-134a refrigerant only and uses an expansion valve
to meter refrigerant flow to the evaporator.

DIAGNOSIS AND TESTING

A / C PERFORMANCE

The A/C system is designed to provide the passen-

ger compartment with low temperature and low
humidity air. The A/C evaporator, located in the
HVAC housing is cooled to temperatures near the
freezing point. As warm damp air passes over the
fins of the A/C evaporator, the air transfers its heat
to the refrigerant in the evaporator coils and the
moisture in the air condenses on the evaporator fins.
During periods of high heat and humidity, an A/C
system will be more effective in the Recirculation
mode (max-A/C). With the system in the Recircula-
tion mode, only air from the passenger compartment
passes through the A/C evaporator. As the passenger
compartment air dehumidifies, the A/C system per-
formance levels rise.

Humidity has an important bearing on the temper-

ature of the air delivered to the interior of the vehi-
cle. It is important to understand the effect that
humidity has on the performance of the A/C system.
When humidity is high, the A/C evaporator has to
perform a double duty. It must lower the air temper-
ature, and it must lower the temperature of the
moisture in the air that condenses on the evaporator
fins. Condensing the moisture in the air transfers
heat energy into the evaporator fins and coils. This
reduces the amount of heat the A/C evaporator can
absorb from the air. High humidity greatly reduces
the ability of the A/C evaporator to lower the temper-
ature of the air.

However, evaporator capacity used to reduce the

amount of moisture in the air is not wasted. Wring-
ing some of the moisture out of the air entering the
vehicle adds to the comfort of the passengers.
Although, an owner may expect too much from their
A/C system on humid days. A performance test is the
best way to determine whether the system is per-
forming up to design standards. This test also pro-
vides valuable clues as to the possible cause of

trouble with the A/C system. The ambient air tem-
perature in the location where the vehicle will be
tested must be a minimum of 21° C (70° F) for this
test.

A / C PERFORMANCE TEST

WARNING: Refer to the applicable warnings and
cautions for this system before performing the fol-
lowing operation (Refer to 24 - HEATING & AIR
CONDITIONING/PLUMBING - WARNINGS) and (Refer
to 24 - HEATING & AIR CONDITIONING/PLUMBING -
CAUTIONS). Failure to follow the warnings and cau-
tions could result in possible personal injury or
death.

NOTE: Very specific instructions and conditions
pertain to this procedure which are significantly dif-
ferent than procedures used in other vehicle appli-
cations. Follow each step in the order they are
presented. Do not skip steps or change conditions
from those stated or results will be adversely
affected and invalid.

NOTE: When connecting the service equipment
coupling to the line fitting, verify that the valve of
the coupling is fully closed. This will reduce the
amount of effort required to make the connection.

(1) Check for diagnostic trouble codes using a

DRBIII

t scan tool. If no DTCs are found in the

engine control module (ECM), go to Step 2. If any
DTCs are found, repair as required, then proceed to
Step 2.

(2) Place the vehicle in the shade and operate the

heating-A/C system under the following conditions.

• Engine at idle at operating temperature

• All doors or windows open

• Transaxle in Neutral

• All A/C duct louvers open

• A/C-heater controls set to fresh air (NOT Recir-

culate), full cool, panel mode, high blower and with
A/C compressor engaged.

NOTE: The A/C compressor clutch is de-energized
under any of the following conditions:

•

Restricted compressor (thermal fuse in the pul-

ley)

•

Low pressure in the system

•

Low evaporator temperature

•

Hard acceleration (WOT)

•

High coolant temperatures

(3) Insert a thermometer in the driver side center

panel air outlet and operate the A/C system until the
thermometer temperature stabilizes.

HEATING & AIR CONDITIONING

24 - 3

(4) With the A/C compressor clutch engaged, duct

temperature should not be less than 2° C (35° F) or
more than 12° C (54° F). The compressor clutch may
cycle, depending upon the ambient temperature and
humidity. If the clutch cycles, use the readings
obtained before the clutch disengaged.

(5) If the A/C compressor clutch has not cycled off

and the duct temperature is less than 2° C (35° F),
check the evaporator temperature sensor and circuit

by performing the ATC Function Test (Refer to 24 -
HEATING & AIR CONDITIONING - DIAGNOSIS
AND TESTING - ATC FUNCTION TEST).

(6) If the air outlet temperature fails to meet the

specifications, refer to the A/C System Diagnosis
chart.

A/C SYSTEM DIAGNOSIS

Condition

Possible Causes

Correction

Rapid A/C compressor clutch
cycling (ten or more cycles
per minute).

Very low refrigerant system
charge.

See Refrigerant System Leaks in this group.
Test the refrigerant system for leaks. Repair,
evacuate and charge the refrigerant system, if
required.

Equal pressures, but the
compressor clutch does not
engage.

1. No refrigerant in the refrig-
erant system.

1. See Refrigerant System Leaks in this
group. Test the refrigerant system for leaks.
Repair, evacuate and charge the refrigerant
system, if required.

2. Faulty fuse.

2. Check the fuses in the Power distribution
block and junction block. Repair the shorted
circuit or component and replace the fuses, if
required. Refer to Group 8.

3. Faulty A/C compressor
clutch coil.

3. See A/C Compressor Clutch Coil in this
group. Test the compressor clutch coil and
replace, if required.

4. Improperly installed or faulty
evaporator temperature sensor.

4. See Evaporator Temperature Sensor in this
group. Test the sensor and replace, if re-
quired.

5. Faulty A/C pressure trans-
ducer.

5. See A/C Pressure Transducer in this
group. Test the sensor and replace, if re-
quired.

6. Faulty engine Control Mod-
ule (ECM).

6. Refer to Group 9 - Engine Electrical Diag-
nostics for testing of the ECM. Test the ECM
and replace, if required.

Normal pressures, but A/C
Performance Test air temper-
atures at center panel outlet
are too high.

1. Excessive refrigerant oil in
system.

1. See Refrigerant Oil Level in this group.
Recover the refrigerant from the refrigerant
system and inspect the refrigerant oil content.
Restore the refrigerant oil to the proper level,
if required.

2. Blend door cable improperly
installed or faulty.

2. See Mode Door Cables in this group. In-
spect the cable for proper operation and re-
place, if required.

3. Blend-air door(s) inoperative
or sealing improperly.

3. See HVAC Housing in this group. Inspect
the blend-air door(s) for proper operation and
sealing. Repair if required.

24 - 4

HEATING & AIR CONDITIONING

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