Heating and Air Conditioning: Description and Operation

INTRODUCTION
For proper operation of the instrument panel controls, refer to the Owner's Manual provided with the vehicle.

All vehicles are equipped with a common heater A/C unit housing assembly

SYSTEM OPERATION
Air Conditioning Components
A/C Pressure Transducer
The switch is located on the discharge line near the compressor for 2.7L engines and near the condenser for the 3.2/3.5L engines. The pressure transducer functions as the refrigerant system pressure sensor. It supports the condenser/radiator fans and compressor functions.

Clutch, Pulley And Coil
They are mounted on the compressor providing a way to drive the compressor. The compressor clutch / coil and manifold are the only serviced parts on the compressor. when the compressor is not in operation, the pulley free wheels on the clutch hub bearing. when the coil is energized the clutch plate is magnetically engaged with the pulley and turns the compressor shaft.

Compressor
The compressor compresses the low pressure refrigerant vapor from the evaporator into a high pressure, high temperature vapor. The Nippondenso lOPAl7 Compressor is used on all models. The system uses polyalkylene glycol synthetic refrigerant oil ND8 PAG.

Condenser
It is located in front of the engine cooling radiator. Its function is to cool the hot high pressure refrigerant gas. This causes it to condense into a high pressure liquid refrigerant.

Evaporator Coil
The coil removes heat and dehumidifies the air before it enters the vehicle. The coil is located in the A/C housing.

Evaporator Temperature Sensor
The evaporator temperature sensor prevents condensate water on the evaporator coil from freezing which can block airflow. The probe is a thermistor inside a metal tube which is wedged between the evaporator fins. The metal tube is tightly held by the evaporator fins so that the thermistor in the tube will detect the temperature of the evaporator. The thermistor will change resistance as the temperature changes. The Body Control Module (BCM) will send a PCI bus message to the Powertrain Control Module (PCM), which will check engine, coolant temperature and refrigerant pressure temperature before turning on the A/C Compressor Clutch. Turning on the A/C Compressor Clutch will allow the system to cool the evaporator. The BCM will send a message to the PCM when the evaporator temperature becomes too cold. The PCM will then turn off the A/C Compressor Clutch. The DRB scan tool can be used to monitor this operation. Refer to diagnostics for evaporator temperature cycle points.

Expansion Valve:
The valve is used to meter refrigerant into the evaporator in accordance with cooling requirements. The valve is located in front of the evaporator coil.

High Pressure Relief Valve
The valve is located at the rear of the compressor The valve is used to prevent excessive high system pressure. The valve vents the system when a pressure of 3445-4135 kPa (500 - 600 psi) and above is reached. This prevents damage to the compressor and other system components. The valve closes with a minimum pressure of 2756 kPa (400 psi).

Filter/Drier
The drier is used to remove any traces of moisture from the refrigerant system. The filter is used to separate any foreign particles from the refrigerant.

Refrigerant Lines
The lines are used to carry the refrigerant between the various system components.

Service Gauge Port
2.7 L: The high pressure gauge port is located on the liquid line. The low pressure gauge port is located on the suction line.

On 3.2/3.5L engines, they are located on the compressor.

A/C Refrigerant Lines
The air conditioning lines used on this vehicle are made from reinforced rubber with a nylon liner on the inner walls. The ends of the A/C lines are made with light weight aluminum fittings.

CAUTION: Never attempt to disconnect an A/C line without reclaiming all refrigerant from the air conditioning system. The system must be empty.

The O-rings are made from a special type of rubber that is not affected by R-134a refrigerant. 0-ring replacement is required whenever lines are removed and installed. Use only 0-rings specified for this vehicle. Failure to use the correct type of 0-ring will cause the connection to leak within a short period of time.

When it is necessary to open the refrigeration system, have everything needed to service the system ready. The system should not be left open any longer than necessary Cap or plug all lines and fittings as soon as they are opened. This will prevent the entrance of dirt and moisture into the system. All new lines and components should be capped or sealed until they are ready to be used.

WARNING:
- AVOID BREATHING NC REFRIGERANT AND LUBRICANT VAPOR OR MIST. EXPOSURE MAY IRRITATE EYES, NOSE AND THROAT USE ONLY APPROVED SERVICE EQUIPMENT MEETING SAE REQUIREMENTS TO DISCHARGE R-134a SYSTEM. IF ACCIDENTAL SYSTEM DISCHARGE OCCURS, VENTILATE WORK AREA BEFORE RESUMING SERVICE.
- R-134a SERVICE EQUIPMENT OR VEHICLE NC SYSTEM SHOULD NOT BE PRESSURE TESTED OR LEAK TESTED WITH COMPRESSED AIR. SOME MIXTURES OF AIR/R-134a HAVE BEEN SHOWN TO BE COMBUSTIBLE AT ELEVATED PRESSURES. THESE MIXTURES ARE POTENTIALLY DANGEROUS AND MAY RESULT IN FIRE OR EXPLOSION CAUSING INJURY OR PROPERTY DAMAGE.

ATC Sensor Motor-Fan Operation:

ATC In-Car Temperature Sensor
The Automatic Temperature Control (ATC In-Car Temperature Sensor returns electrical signals to the Body Control Module (BCM). The In-Car Temperature Sensor is used on ATC equipped vehicles only. The In-Car Temperature Sensor is made up of two parts. One part is the in-car sensor motor fan assembly and the second part is a temperature thermistor. The in-car sensor motor fan assembly attaches to the back of the ATC Control head. This assembly has a small fan and a motor which draws air through the intake on the front of the ATC control. The in-car sensor thermistor is located inside of the ATC Control Head. Air drawn from the passenger compartment by the in-car sensor motor-fan assembly, flows over the thermistor. The in-car sensor thermistor changes resistance with air temperature. The BCM measures this resistance and calculates the temperature of the air drawn into the ATC Control. The ATC system then makes adjustments to maintain the optimum passenger compartment comfort. Refer to the ATC Sensor Motor-Fan Operation table for when the ATC Sensor is operating.

The in-car sensor motor is a separate serviceable part connected to the ATC Control Head. See Diagnosis and Testing for proper check of airflow. The in-Car Sensor Thermistor is part of the ATC Control Head and not a separate serviceable part. One must replace the ATC Control if the ATC self-diagnostics indicates a fault code. See Self-Diagnostics for information on diagnosing wiring and thermistor.

Blower Motor Resistor
Vehicles with manual air conditioning (non-ATC) systems will have a blower motor resistor. The resistor is located on the lower right side of the HVAC unit housing.

Blower Motor Power Module
The blower motor power module is only used in vehicles equipped with Automatic Temperature Control (ATC). it is located on the lower right side of the HVAC unit housing.

The power module receives pulse width modulated signals from the Body Control Module. The power module varies voltage to the blower motor for different blower speeds based on the ATC software. There are 14 speeds and a blower off signal.

Compressor
The compressor used on this vehicle is a Nippondenso 10PA17 R134a. This compressor uses an aluminum swash plate, teflon coated pistons and aluminum cylinder walls. One-way check valves are used to regulate refrigerant flow through the compressor.

CAUTION: A 10PA17 R-12 compressor looks identical to a 10PA17 R134a and will bolt up to this vehicle. It is extremely important that a R-134a compressor is identified prior to using compressor in question. Check tag located on compressor for model number.

Excessive noise that occurs when the air-conditioning is being used may be caused by:

- Loose Bolts
- Mounting Brackets
- Loose Compressor Clutch
- Excessive High Refrigerant Operating Pressure Verify the following before compressor repair is performed:
1. Compressor drive belt condition
2. Proper refrigerant charge
3. Thermal expansion valve (TXV) operating correctly
4. Head pressure is normal

Engine Cooling System Requirements
To maintain the performance level of the heating/ air-conditioning system, the engine cooling system must be prepared.

The use of a bug screen is not recommended. Any obstructions in front of the radiator or condenser can reduce the performance of the A/C and/or engine cooling system.

WARNING: DO NOT OPEN COOLING SYSTEM WHEN HOT AS PERSONAL INJURY OR DAMAGE TO VEHICLE MAY RESULT.

The engine cooling system is designed to develop internal pressure of 97 to 123 kPa (14 to 18 psi). Wait 15 minutes (after the engine is shut off), or until safe temperature and pressure is attained, before opening cooling system. Coolant temperature can be checked using the scan tool (DRB).

The use of aluminum cylinder heads, intake manifolds, heater cores, and radiators, requires special corrosion protection. Use only Mopar� antifreeze (part #4698542) or ethylene-glycol based antifreeze with an organic type inhibitor for best cooling system protection. To obtain the maximum life of the cooling system, mixing distilled water with ethylene-glycol type coolant is recommended.

CAUTION: Do not add additional inhibitors, antirust products, or soluble oil (sold as "water pump lubricants"). These products may not be compatible with the radiator coolant.

Evaporator Temperature Sensor Setpoint:

Evaporator Temperature Sensor
The evaporator temperature sensor is a temperature sensing element located at the coldest point on the face of the evaporator. Output from the probe is sampled by the Body Control Module (BCM). The sensor is used to switch the A/C compressor clutch OFF before evaporator freeze up occurs. The temperature setpoint at which the clutch is turned OFF varies with the outside ambient temperature. The temperature at which the clutch is turned ON is preset to 2° F above the OFF setpoint mentioned above. Refer to Evaporator Temperature Sensor Set-point table and for the correct setpoint.

Expansion Valve
The expansion valve is located on the engine side of the dash panel, near the right shock tower.

The expansion valve can fail in three different positions (open, closed or restricted). Note, that in each case, it is defective and requires replacement.

In an Open Position: this will result in a noisy compressor or no cooling. The cause can be a broken spring, broken ball or excessive moisture in the A/C system. If the spring or ball are found to be defective, replace the expansion valve. If excessive moisture is found in the A/C system, recycle the refrigerant.

In a Closed Position: There will be low suction pressure and no cooling. This may be caused by a failed power dome or excessive moisture in the A/C system. If the power dome on the expansion valve is found to be defective replace the expansion value. If excessive moisture is found recycle the refrigerant.

A Restricted Orifice: There will be low suction pressure and no cooling. This may be caused by debris in the refrigerant system. If debris is believed to be the cause, recycle the refrigerant and replace the expansion valve and filter/drier.

Filter/Drier
The filter/drier is a receiver for reserve refrigerant. It also has a desiccant pack and a filter. This is used to absorb moisture and filter the refrigerant as it passes through the filter/drier.

The filter/drier is located near the dash panel on the passenger side of the vehicle. The A/C refrigerant must be removed from the system before removing the filter/drier. Always use a refrigerant recovery machine.

Replace the filter/drier if an A/C system is left open for an extended period of time.

Handling Tubing and Fittings
Kinks in the refrigerant tubing or sharp bends in the refrigerant hose lines will greatly reduce the capacity of the entire system. High pressures are produced in the system when it is operating. Extreme care must be exercised to make sure that all connections are pressure tight. Dirt and moisture can enter the system when it is opened for repair or replacement of lines or components. The refrigerant oil will absorb moisture readily out of the air. This moisture will convert into acids within a closed system.

CAUTION: The system must be completely empty before opening any fitting or connection in the refrigeration system. Open fittings with caution even after the system has been emptied. If any pressure is noticed as a fitting is loosened, retighten fitting and evacuate the system again.

Inspect all flexible hose lines to make sure they are in good condition and properly routed.

The use of correct wrenches when making connections is very important. Improper wrenches or improper use of wrenches can damage the fittings.

The internal parts of the NC system will remain stable as long as moisture-free refrigerant and refrigerant oil is used. Abnormal amounts of dirt, moisture or air can upset the chemical stability. This may cause operational troubles or even serious damage if present in more than very small quantities.

When opening a refrigeration system, have everything you will need to repair the system ready. This will minimize the amount of time the system must be opened. Cap or plug all lines and fittings as soon as they are opened. This will help prevent the entrance of dirt and moisture. All new lines and components should be capped or sealed until they are ready to be used.

All tools, including the refrigerant dispensing manifold, the manifold gauge set, and test hoses should be kept clean and dry.

HVAC Forced RECIRCULATION
The HVAC system (both Manual A/C and ATC) may force itself into recirculated airflow when extremely hot outside ambient conditions exist. This action will occur at a range of approximately 95° F to 120° F. The Control Head receives a PCI bus message from the BCM with the feedback from the A/C Pressure Transducer. The HVAC system will be forced into the Recirculation mode, reducing the A/C refrigerant pressure under these extreme conditions. The reduced operating pressure offers improved A/C system performance and reduced wear on A/C components.

Side Window Demisters
The side window demisters direct air from the heater assembly to the side windows. This is done when the mode selector is on FLOOR, DEFROST, MIX OR BI-LEVEL setting. The outlets are located on the top outboard corners of the instrument panel.

Sun Sensor
The sun sensor is only used on vehicles equipped with Automatic Temperature Control (ATC). The sensor is mounted on the top of the instrument panel below the instrument panel top cover. This is not a thermistor type sensor but rather a photo diode. For this reason the sun sensor responds to sun light intensity rather than temperature. It is used to aid in determining proper mode door position, temperature door position and blower speed.

The sun sensor is also used to sense day/night conditions for automatic headlight control if so equipped, and has an LED indicator for the vehicle security system.

The sun sensor is not serviceable and must be replaced if found to be defective.

System Airflow
The system draws outside air through the cowl opening at the base of the windshield. Then it goes into the plenum chamber above the Heater A/C unit housing and passes through the evaporator. At this point airflow can be directed either through or around the heater core. This is done by adjusting the blend-air door with the TEMP control on the control head. After the air passes the blend air door, the air flow is then directed from the PANEL, BI-LEVEL (panel and floor), and FLOOR - DEFROST outlets. Air flow velocity can be adjusted with the blower speed selector switch on the control head.

Ambient air intake can be shut off by closing the recirculating air door. This will recirculate the air that is already inside the vehicle. See owners manual for recirculation operation.