Basic Principles

CLIMATE CONTROL SYSTEM

WARNING:
- Carbon monoxide is colorless, odorless and dangerous. If it is necessary to operate the engine with vehicle in a closed area such as a garage, always use an exhaust collector to vent the exhaust gases outside the closed area.
- R-134a is classified as a safe refrigerant, but misuse can make it dangerous. The following precautions must be observed.
- Always wear safety goggles when repairing an air conditioning system.
- Avoid contact with liquid refrigerant R-134a. R-134a vaporizes at approximately -25°C (-13°F) under atmospheric pressure and it will freeze skin tissue.
- Never allow refrigerant R-134a gas to escape in quantity in an occupied space. R-134a is non-toxic, but it will displace the oxygen needed to support life.
- Never use a torch in an atmosphere containing R-134a gas. R-134a is non-toxic at all normal conditions, but when it is exposed to high temperatures, such as a torch flame, it decomposes. During decomposition it releases irritating and toxic gases (as described in the MSDS sheet from the manufacturer). Decomposition products are hydrofluoric acid, carbon dioxide and water.
- Do not allow any portion of the charged air conditioning system to become too hot. The pressure in an air conditioning system rises as the temperature rises and temperatures of approximately 85°C (185°F) can be dangerous.
- Allow the engine to cool sufficiently prior to carrying out maintenance or serious burns and injury can occur.

CAUTION:
- To avoid damaging the vehicle or A/C components, the following precautions must be observed.
- The A/C refrigerant of all vehicles must be identified and analyzed prior to refrigerant charging. Failure to do so can contaminate the shop bulk refrigerant and other vehicles.
- Do not add R-12 refrigerant to an A/C system that requires the use of R-134a refrigerant. These two types of refrigerant must never be mixed. Doing so can damage the A/C system.
- Charge the A/C system with the engine running only at the low-pressure side to prevent refrigerant slugging from damaging the A/C compressor.
- Use only R-134a refrigerant. Due to environmental concerns, when the air conditioning system is drained, the refrigerant must be collected using refrigerant recovery/recycling equipment. Federal law REQUIRES that R-134a be recovered into appropriate recovery equipment and the process be conducted by qualified technicians who have been certified by an approved organization, such as MACS, ASI, etc. Use of a recovery machine dedicated to R-134a is necessary to reduce the possibility of oil and refrigerant incompatibility concerns. Refer to the instructions provided by the equipment manufacturer when removing refrigerant from or charging the air conditioning system.
- Refrigerant R-134a must not be mixed with air for leak testing or used with air for any other purpose above atmospheric pressure. R-134a is combustible when mixed with high concentrations of air and higher pressures.
- A number of manufacturers are producing refrigerant products that are described as direct substitutes for Refrigerant R-134a. The use of any unauthorized substitute refrigerant can severely damage the A/C components. If repair is required, use only new or recycled Refrigerant R-134a.

- To avoid contamination of the A/C system:
- Never open or loosen a connection before recovering the refrigerant.
- When loosening a connection, if any residual pressure is evident, allow it to leak out before opening the fitting.
- Evacuate a system that has been opened to install a new component or one that has discharged through leakage before charging.
- Seal open fittings with a cap or plug immediately after disconnecting a component from the system.
- Clean the outside of the fittings thoroughly before disconnecting a component from the system.
- Do not remove the sealing caps from a new component until ready to install.
- Refrigerant oil will absorb moisture from the atmosphere if left uncapped. Do not open an oil container until ready to use and install the cap immediately after using. Store the oil in a clean, moisture-free container.
- Install a new O-ring seal before connecting an open fitting. Coat the fitting and O-ring seal with mineral oil before connecting.
- When installing a refrigerant line, avoid sharp bends. Position the line away from the exhaust or any sharp edges that can chafe the line.
- Tighten threaded fittings only to specifications. The steel and aluminum fittings used in the refrigerant system will not tolerate overtightening.
- When disconnecting a fitting, use a wrench on both halves of the fitting to prevent twisting of the refrigerant lines or tubes.
- Do not open a refrigerant system or uncap a new component unless it is as close as possible to room temperature. This will prevent condensation from forming inside a component that is cooler than the surrounding air.

The manual climate control system heats or cools the vehicle depending on the function selector switch position and the temperature selected.
- The function selector switch position determines heating or cooling, air distribution and enables blower motor operation.
- The temperature control setting determines the air temperature.
- The blower motor switch varies the blower motor speed.

The dual automatic temperature control (DATC) system maintains the selected vehicle interior temperature by heating and/or cooling the air.
- During A/C operation the system also reduces the relative humidity of the air.
- The driver may override the automatic mode of operation.
- The driver side and passenger side temperature settings can be individually controlled.

PRINCIPLES OF OPERATION

There are four main principles involved with the basic theory of operation:
- heat transfer
- latent heat of vaporization
- relative humidity
- effects of pressure

Heat Transfer
If two substances of different temperature are placed near each other, the heat in the warmer substance will transfer to the colder substance.

Latent Heat of Vaporization
When a liquid boils (converts to gas) it absorbs heat without raising the temperature of the resulting gas. When the gas condenses (converts back to a liquid), it gives off heat without lowering the temperature of the resulting liquid.

Relative Humidity
The amount of moisture (water vapor content) that the air can hold is directly related to the air temperature. The more heat there is in the air, the more moisture the air can hold. The lower the moisture content in the air, the more comfortable you feel. Removing the moisture from the air lowers its relative humidity and improves personal comfort.

Effects of Pressure on Boiling or Condensation
As the pressure is increased on a liquid, the temperature at which the liquid boils (converts to gas) also increases. Conversely, when the pressure on a liquid is reduced, its boiling point is also reduced. When in the gas state, an increase in pressure causes an increase in temperature, while a decrease in pressure will decrease the temperature of the gas.

Compressor Anti-Slugging Strategy
Liquid refrigerant may accumulate in the A/C compressor under certain conditions. To alleviate damage to the A/C compressor, compressor anti-slugging strategy (CASS) is utilized.

CASS is initiated only under specific conditions:
- the ignition is off for more than 8 hours
- the ambient temperature is above -4°C (25°F)
- battery voltage is above 8.5 volts during engine cranking

When these conditions are present, the powertrain control module (PCM) will activate the A/C control relay prior to cranking of the engine. The A/C control relay engages the A/C compressor for approximately 4-15 A/C compressor revolutions or a maximum of 2 seconds (depending upon vehicle application), allowing the liquid refrigerant to be pushed from the A/C compressor. CASS is initiated by the PCM regardless of the function selector switch position or the DATC system settings.

Clutch Cycling Orifice Tube Type Refrigerant System, Part 1:

Clutch Cycling Orifice Tube Type Refrigerant System, Part 2:

The Refrigerant Cycle
During stabilized conditions (air conditioning system shutdown), the refrigerant is in a vaporized state and pressures are equal throughout the system. When the A/C compressor is in operation it increases pressure on the refrigerant vapor, raising its temperature. The high-pressure and high-temperature vapor is then released into the top of the condenser core.

The condenser core, being close to ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubing. The now liquid refrigerant, still at high pressure, exits from the bottom of the condenser core and enters the inlet side of the evaporator core orifice.

The evaporator core orifice is the restriction in the refrigerant system that creates the low pressure drop in the evaporator core and separates the high and low pressure sides of the A/C system. As the liquid refrigerant leaves this restriction, its pressure and boiling point are reduced.

The liquid refrigerant is now at its lowest pressure and temperature. As it passes through the evaporator core, it absorbs heat from the passenger compartment airflow passing over the plate/fin sections of the evaporator core. This addition of heat causes the refrigerant to boil (convert to gas). The now cooler passenger compartment air can no longer support the same humidity level of the warmer air and this excess moisture condenses on the exterior of the evaporator coils and fins and drains outside the vehicle.

The suction accumulator is designed to remove moisture from the refrigerant and to prevent any liquid refrigerant that may not have been vaporized in the evaporator core from reaching the A/C compressor. The A/C compressor is designed to pump refrigerant vapor only, as liquid refrigerant will not compress and can damage the A/C compressor.

The refrigerant cycle is now repeated with the A/C compressor again increasing the pressure and temperature of the refrigerant.

The A/C cycling switch interrupts compressor operation before the external temperature of the evaporator core gets low enough to cause the condensed water vapor (excess humidity) to turn to ice. It does this by monitoring low side line pressure. It is known that a refrigerant pressure of approximately 210 kPa (30 psi) will yield an operating temperature of 0°C (32°F). The A/C cycling switch controls system operation in an effort to maintain this temperature.

The high side line pressure is also monitored so that A/C compressor operation can be interrupted if system pressure becomes too high. When the A/C compressor discharge pressure rises, the A/C high pressure cutoff switch contacts open, disengaging the A/C compressor. When the pressure drops, the contacts close to allow operation of the A/C compressor.

The A/C pressure relief valve will open and vent refrigerant to relieve unusually high system pressure.

CLIMATE CONTROL SYSTEM OPERATION

Manual Climate Control System
The manual climate control system operation is determined by the settings on the climate control assembly. The climate control assembly includes a serviceable function selector switch and blower motor switch. The temperature control switch, A/C request switch and rear window defrost switch are integral to the climate control assembly and are serviced as an assembly.

The blower motor switch:
- sets the front blower motor speed.
- directs the blower motor path to ground through the blower motor resistor to allow blower motor operation in LO, MED LO and MED HI.
- actuates the high speed blower motor relay, giving the blower motor a direct path to ground which allows the blower motor to operate in HI.

The temperature control switch:
- is a potentiometer that outputs a variable voltage to the temperature blend door actuator to set the position of the temperature blend door.
- initiates closing of the heater control valve. When set to full COOL, a lever on the temperature blend door pivot shaft depresses the pintle of the heater control valve vacuum switch. The heater control valve vacuum switch then applies vacuum to the heater control valve, closing off engine coolant flow to the heater core.
- is serviced only with the climate control assembly.

The function selector switch setting:
- determines where the system airflow is directed.
- enables blower motor operation.
- enables auxiliary blower motor operation, if equipped, by supplying voltage to the auxiliary blower motor relay.
- commands A/C compressor operation in MAX A/C, FLOOR/DEFROST and DEFROST.

The A/C request switch:
- can command the A/C compressor on when the function selector switch is in PANEL, FLOOR/PANEL and FLOOR when the A/C request switch is pressed.
- is inoperative in MAX A/C, OFF, FLOOR/DEFROST and DEFROST.
- indicator illuminates when the function selector switch is in MAX A/C and cannot be toggled.
- indicator does not illuminate in OFF and cannot be toggled.
- indicator does not illuminate in FLOOR/DEFROST and DEFROST and cannot be toggled. The A/C compressor will operate if the outside air temperature is above 6°C (43°F).
- is serviced only with the climate control assembly.

Dual Automatic Temperature Control System
The dual automatic temperature control (DATC) system operation is determined by the settings on the DATC module. The DATC system automatically maintains a selected temperature for vehicle interior comfort, including capabilities to maintain separate temperature settings for the driver and front passenger. The dual temperature control capabilities are achieved by splitting the air distribution system internally and utilizing two individually operated temperature blend doors. The DATC system is composed of the DATC module, automatic temperature control (ATC) solenoid and manifold, blower motor speed control, four vacuum control motors, two temperature blend door actuators, in-car temperature sensor, ambient air temperature sensor and solar amplifier (sunload sensor) to control the various functions.

When the DATC module is set to AUTO:
- the air distribution direction, heater control valve operation and blower motor speed are automatically controlled based on the temperature(s) selected.

The DATC module manual override settings:
- allow the air distribution direction to be manually selected.
- allow the blower motor speed to be manually selected.
- allow RECIRCULATION mode to be manually selected except in FLOOR/DEFROST and DEFROST. When RECIRCULATION mode is selected, A/C compressor operation is automatically requested (A/C request switch illuminates). The A/C request can be cancelled by pressing the A/C request switch, turning the indicator off.
- allow A/C compressor operation to be manually selected except in FLOOR/DEFROST and DEFROST.

The DATC module also includes:
- an EXT button to monitor ambient (outside) air temperature. By pressing the EXT button the ambient temperature will be displayed on the DATC module display until the EXT button is pressed again.
- a °F °C temperature conversion button to toggle the temperature being displayed from Fahrenheit and Celsius.
- a rear window defrost button.

Dual temperature control can be enabled in AUTO or in all manual override settings. When the DATC module is set to DUAL:
- the DATC system enables varying temperatures to be selected by the driver and the front passenger. Dual temperature control is indicated by the presence of two temperature settings on the DATC module display. To return to single temperature control press the DUAL button, turning the DUAL button indicator off and the front passenger temperature display off.
- the air distribution direction and the blower motor speed cannot be varied between the driver and front passenger. Only the temperature settings can be varied.
- the system will favor the driver temperature setting if the driver and front passenger temperature set points vary greatly.

Auxiliary Climate Control System
An auxiliary climate control system is available as optional equipment. The auxiliary climate control system operation is determined by the settings on the front or rear auxiliary climate control assembly. The auxiliary climate control assemblies include a blower motor switch, temperature control switch and air distribution mode switch, none of which can be individually serviced.

On vehicles with manual climate control the auxiliary blower motor operation is enabled when the function selector switch is in any position except OFF. The auxiliary system air can be cooled below ambient temperature only when A/C compressor operation has been requested by the settings of the climate control assembly of the base (front) system. The auxiliary system air can be heated at any time, the heater control valve does not affect the auxiliary climate control system operation.

On vehicles with DATC the auxiliary blower motor is operational regardless of the DATC system setting. The auxiliary system air can be cooled below ambient temperature only when A/C compressor operation has been requested by the settings of the DATC module. The auxiliary system air can be heated at any time, the heater control valve operation does not affect the auxiliary climate control system.

The front auxiliary climate control assembly:
- blower motor switch controls the auxiliary blower motor speed. The blower motor speed can be set to OFF, MED LO, MED HI, or HI. When the front auxiliary blower motor switch is set to REAR all control of the auxiliary climate control system functions are then transferred to the rear auxiliary climate control assembly.
- air distribution mode switch toggles airflow between the overhead vents and the floor vent by outputting signals to the auxiliary airflow mode door actuator which positions the auxiliary airflow mode door in all blower motor switch positions except REAR. Back-lit icons on the front climate control display indicate the source of the auxiliary system airflow.
- temperature control switch is a potentiometer that outputs a variable voltage to the auxiliary temperature blend door actuator to set the position of the auxiliary temperature blend door in all blower motor switch positions except REAR. The temperature of the auxiliary system air may be affected by the manual climate control assembly or DATC module settings (as equipped). When A/C compressor operation has not been initiated (A/C compressor commanded on) by the manual climate control assembly or the DATC module (as equipped), the auxiliary climate control system cannot cool the air below that of the ambient temperature.

The rear auxiliary climate control assembly:
- blower motor switch controls the auxiliary blower motor speed only when the front auxiliary climate control assembly blower motor switch is set to REAR. The blower motor speed can be set to OFF, LO, MED LO, MED HI, or HI.
- air distribution mode switch toggles airflow between the overhead vents and the floor vent by outputting signals to the auxiliary air distribution door actuator which positions the auxiliary air distribution door only when the front auxiliary climate control assembly blower motor switch is set to REAR. Back-lit icons on the rear climate control display indicate the source of the auxiliary system airflow.
- temperature control switch is a potentiometer that outputs a variable voltage to the auxiliary temperature blend door actuator to set the position of the auxiliary temperature blend door only when the front auxiliary climate control assembly blower motor switch is set to REAR. The temperature of the auxiliary system air may be affected by the manual climate control assembly or DATC module settings (as equipped). When A/C compressor operation has not been initiated (A/C compressor commanded on) by the manual climate control assembly or the DATC module (as equipped), the auxiliary climate control system cannot cool the air below that of the ambient temperature.