Heating and Air Conditioning: Description and Operation
AIR DELIVERYThe air delivery description and operation is divided into six areas:
- HVAC Control Components
- Air Speed
- Auxiliary Air Speed
- Air Delivery
- Auxiliary Air Delivery
- Recirculation Operation
HVAC Control Components
HVAC Control Module
The HVAC control module is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 3 and battery positive voltage circuits provide power to the control module. The control module supports the features as in table.
Mode Actuator
The mode actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit uses a 0-12 volt linear-ramped signal to command the actuator movement. The 0 and 12 volt control values represent the opposite limits of the actuator range of motion. The values in between 0 and 12 volts correspond to the positions between the limits.
When the HVAC control module sets a commanded, or targeted, value, the control signal is set to a value between 0-12 volts. The actuator shaft rotates until the commanded position is reached. The module will maintain the control value until a new commanded value is needed.
Recirculation Actuator
The recirculation actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit is either grounded or opened during normal operation. If the control circuit is grounded, then the shaft rotates in the opposite direction than it would if the circuit was opened. The actuator shaft rotates until the limit is reached. If the module needs to reverse the direction of rotation, then the control circuit's state is reversed. The open circuits are held at ignition 3 voltage from the actuator. The module will maintain the control circuit's state until a new position is needed.
Air Speed
The HVAC control assembly applies voltage to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:
- Low speed-4 resistors
- Medium 1 speed-3 resistors
- Medium 2 speed-2 resistors
- Medium 3 speed-1 resistors
When the operator requests High speed, the HVAC control assembly applies voltage to the blower motor relay through the high blower motor control circuit.
The voltage energizes the blower motor relay, connecting the blower motor to battery positive voltage.
OFF mode
When the OFF position is selected, the HVAC control module applies voltage to the fan off input within the HVAC control module through the off blower motor control circuit. When the HVAC control module receives this input, any A/C request will be cancelled and the A/C request LED will turn off.
Recirculation is adjustable in OFF mode.
Auxiliary Air Speed
The auxiliary HVAC system provides heating and cooling to the rear of the vehicle. There are two types of systems, a Rear Video/Audio/HVAC (RVAC) system and a base auxiliary HVAC system. The only difference between the RVAC and the base model are the switches and internal wiring. The RVAC module has toggle type switches and the base model has rotary type switches. The system is controlled by the HVAC control module, auxiliary blower motor switch and auxiliary HVAC control/RVAC module. The auxiliary HVAC system can be operated whenever the ignition switch is in the ON position. Auxiliary A/C can only be operated when the HVAC control module is in A/C mode.
Power is provided to the auxiliary blower motor from the ignition 3 voltage circuit from the fuse block. Power and ground are provided to the HVAC control module by the ignition 3 voltage and the ground circuits.
Auxiliary Low Blower Speed
When the Low auxiliary blower speed is selected on the auxiliary blower motor switch, voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage through the auxiliary blower motor relay and through the auxiliary medium 1/2 blower motor control circuit. Voltage is divided between 2 series resistors in the auxiliary blower motor resistor assembly to achieve the desired blower speed. Ground is provided by the auxiliary blower motor low speed control, auxiliary blower motor switch and ground circuit.
When the Low auxiliary blower speed is selected on the auxiliary HVAC control module, the auxiliary blower motor switch must be in the R position. Voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage through the auxiliary blower motor relay and through the auxiliary medium 1/2 blower motor control circuit. Voltage is divided between 2 series resistors in the auxiliary blower motor resistor assembly to achieve the desired blower speed.
Ground is provided by the auxiliary blower motor low speed control circuit, the auxiliary HVAC control module, the auxiliary blower motor switch supply voltage circuit and back into the auxiliary HVAC control module. When the ignition switch is in the ON position, an internal relay inside the auxiliary HVAC control module closes. The relay receives power from the ignition 3 voltage circuit. The ground circuit continues through the closed relay contacts, through the auxiliary blower motor control circuit and auxiliary blower motor switch to the ground circuit. Auxiliary HVAC systems with the premium controls are electronically controlled, there is no internal relay.
Auxiliary Medium Blower Speed
When the Medium auxiliary blower speed is selected on the auxiliary blower motor switch, voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage through the auxiliary blower motor relay and through the auxiliary medium 1/2 blower motor control circuit. Voltage is reduced by an in-line resistor in the auxiliary blower motor resistor assembly to achieve the desired blower speed. Ground is provided by the auxiliary blower motor medium speed control, auxiliary blower motor switch and ground circuit.
When the Medium auxiliary blower speed is selected on the auxiliary HVAC control module, the auxiliary blower motor switch must be in the R position.
Voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage through the auxiliary blower motor relay and through the auxiliary medium 1/2 blower motor control circuit. Voltage is reduced by an in-line resistor in the auxiliary blower motor resistor assembly to achieve the desired blower speed.
Ground is provided by the auxiliary blower motor medium speed control circuit, the auxiliary HVAC control module, the auxiliary blower motor switch supply voltage circuit and back into the auxiliary HVAC control module. When the ignition switch is in the ON position, an internal relay inside the auxiliary HVAC control module closes. The relay receives power from the ignition 3 voltage circuit. The ground circuit continues through the closed relay contacts, through the auxiliary blower motor control circuit and auxiliary blower motor switch to the ground circuit. Auxiliary HVAC systems with the premium controls are electronically controlled, there is no internal relay.
Auxiliary High Blower Speed
When the High auxiliary blower speed is selected on the auxiliary blower motor switch, voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage to the auxiliary blower motor relay. Power is also being delivered to the load side of the auxiliary blower motor relay on the ignition 3 voltage circuit. Energizing the relay pulls the internal contact and grounds the blower motor. The load side of the auxiliary blower motor relay is grounded through the auxiliary blower motor supply voltage circuit, the auxiliary blower motor switch and the ground circuit.
When the High auxiliary blower speed is selected on the auxiliary HVAC control module, the auxiliary blower motor switch must be in the R position. Voltage is delivered from the fuse block to the auxiliary blower motor on the ignition 3 voltage circuit. The auxiliary blower motor speed control circuit delivers voltage to the auxiliary blower motor relay. Power is also being delivered to the load side of the auxiliary blower motor relay on the ignition 3 voltage circuit.
Energizing the relay pulls the internal contact and grounds the blower motor. The load side of the auxiliary blower motor relay is grounded through the auxiliary blower motor supply voltage circuit, the auxiliary blower motor switch and the ground circuit.
The load side of the auxiliary blower motor relay is grounded through the auxiliary blower motor supply voltage circuit, the auxiliary HVAC control module, the auxiliary blower motor switch supply voltage circuit and back into the auxiliary HVAC control module. When the ignition switch is in the ON position, an internal relay inside the auxiliary HVAC control module closes. The relay receives power from the ignition 3 voltage circuit. The ground circuit continues through the closed relay contacts, through the auxiliary blower motor control circuit and auxiliary blower motor switch to the ground circuit. Auxiliary HVAC systems with the premium controls are electronically controlled, there is no internal relay.
Air Delivery
The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. When the mode door is moved to the mix-blend or defrost position, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. Regardless of the mode setting, a small amount of air will be diverted to the defrost ducts to reduce windshield fogging. After a malfunction occurs to the mode actuator it is driven to the Floor position. On start-up, the HVAC control module will place the mode door in the last selected position.
Auxiliary Air Delivery
The optional auxiliary HVAC system allows the driver to adjust the amount of air directed to the rear of the vehicle using the front mode switch. To direct air to the rear of the vehicle using the floor vents, set the mode switch on the HVAC control module to the FLOOR, DEFROST, or MIX-BLEND modes. To direct air through the headliner vents, set the mode switch on the HVAC control module to the BI-LEVEL or VENT modes.
Recirculation Operation
The HVAC control module controls the air intake through the recirculation actuator. Recirculation is only available when the HVAC control module is in Vent, Bi-level or OFF. The operator must activate the blower for Recirculation operation. When the recirculation actuator sees an open in the control circuit, the actuator will default to the recirculation position.
AIR TEMPERATURE
The air temperature controls are divided into four primary areas:
- HVAC Control Components
- Heating and A/C Operation
- Engine Coolant
- A/C Cycle
HVAC Control Components
HVAC Control Module
The HVAC control module is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 3 and battery positive voltage circuits provide power to the control module. The control module supports the features as in table.
Air Temperature Actuator
The air temperature actuator is a 3 wire bi-directional electric motor. Ignition 3 voltage, ground and control circuits enable the actuator to operate. The control circuit uses a 0-12 volt linear-ramped signal to command the actuator movement. The 0 and 12 volt control values represent the opposite limits of the actuator range of motion. The values in between 0-12 volts correspond to the positions between the limits.
When the HVAC control module sets a commanded, or targeted, value, the control signal is set to a value between 0-12 volts. The actuator shaft rotates until the commanded position is reached. The module will maintain the control value until a new commanded value is needed.
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.
The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The PCM disables the compressor clutch under the following conditions:
- A/C pressure is more than 2979 kPa (432 psi). The clutch will be enabled after the pressure decreases to less than 1620 kPa (235 psi).
- A/C pressure is less than 179 kPa (26 psi). The clutch will be enabled after the pressure increases to more than 207 kPa (30 psi).
The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:
- Recirculation actuator setting
- Difference between inside and desired temperature
- Difference between ambient and desired temperature
- Blower motor speed setting
- Mode setting
- Auxiliary HVAC settings
Heating And A/C Operation
Obtaining proper HVAC maximum cooling operation:
1. Open the windows to allow the escape of hot interior hot air
2. Press the A/C button the A/C LED will illuminate on the A/C button
3. Set the mode switch to outside air
4. Set the fan control to the 5 or high setting
5. Set the temperature knob to the full cold setting by turning it counterclockwise
6. After 90 seconds, push the Recirculation button the Recirc LED on the Recirc button will illuminate
7. Close the windows
This procedure acts as a purge of the hot passenger compartment air while allowing cool air conditioned air to fill the passenger compartment. The outside air button should be pushed once a desired comfort level has been achieved.
The control module makes the following actions when making a climate selection, and an air temperature setting is selected:
- When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.
- When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.
Pressing the A/C switch engages the A/C system and illuminates the A/C switch LED. The HVAC control module sends a 5-volt request signal to the powertrain control module (PCM). The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 155°C (311°F). The thermal switch is a non-serviceable item. The following conditions must be met in order for the PCM to turn on the compressor clutch:
- HVAC control module
- Evaporator Temperature more than 2°C (36°F)
- PCM
- Engine coolant temperature (ECT) is more than 125°C (257°F)
- Engine RPM is less than 5,000 RPM
- A/C Pressure is between 1620-2979 kPa (235-432 psi)
- Ambient temperature more than 4°C (40°F)
- A/C request from the HVAC control module
- Battery voltage between 10.8-16 volts
Once engaged, the compressor clutch will be disengaged for the following conditions:
- Compressor thermal switch is opened
- Throttle position is 100 percent
- Battery voltage below 10 volts
- A/C Pressure is more than 2979 kPa ( 432 psi)
- A/C Pressure is less than 1620 kPa (235 psi)
- Engine coolant temperature (ECT) is more than 125°C (257°F)
- Engine speed is more than 5,500 RPM
- Transmission shift
- PCM detects excessive torque load
- PCM detects insufficient idle quality
- PCM detects a hard launch condition
When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.
Dual Zone Operation
The dual zone controls allows for maximum temperature offset and comfort between the driver and rear passengers. It is possible to select maximum airflow over the evaporator core with one dual zone switch along with maximum airflow over the heater core with the other dual zone switch. Each air temperature actuator is independent from the other and the rear passenger location is not limited in it's range of temperature offset.
Auxiliary Heating and A/C Operations
The purpose of the auxiliary heating and A/C system is to supply heat or cool air and remove humidity from the rear interior of the vehicle. The rear auxiliary HVAC system will operate with the ignition in the ON position. The auxiliary HVAC system allows the back seat passengers to adjust the temperature of the rear flow of air. The A/C function is only available to the auxiliary system when the HVAC control module is in A/C mode. When the front auxiliary HVAC control is not set in the rear mode the rear auxiliary HVAC is disabled. For maximum cooling operation of the HVAC system set the rear fan control to 0 or OFF if no rear passengers are present. The rear passengers comfort is achieved by setting the rear fan control to the number 3 setting. Setting the rear blower speed to the R setting will allow the rear passengers to control rear blower and temperature settings.
Engine Coolant
Engine coolant is the essential element of the heating system. The thermostat controls the normal operating temperature of the engine. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation. Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. The HVAC module distributes heated air to the passenger compartment for consistent passenger comfort.
Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and is recirculated back through the engine cooling system.
A/C Cycle With Auxiliary
The auxiliary A/C system operates from the vehicles primary A/C system. The front A/C system must be on to allow the auxiliary A/C system to function.
Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automobile use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.
The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system.
Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.
The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line. The liquid line flow is split and the liquid refrigerant flows to both the front A/C system and to the liquid line for the auxiliary A/C system.
Ambient air is drawn through the auxiliary HVAC module and passes through the evaporator core. Warm, moist air will cause the liquid refrigerant to boil inside of the evaporator core. The boiling refrigerant absorbs the moisture and heat from the ambient air. The refrigerant exits the evaporator through the suction line and back to the front A/C systems suction line. Refrigerant in the front A/C system suction line flows back to the compressor, in a vapor state, and completes the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.
The conditioned air is distributed through the auxiliary HVAC module for passenger comfort. The heat and moisture removed from the rear passenger compartment will also change form, or condense, and is discharged from the auxiliary HVAC module as water.
A/C Cycle
Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.
A Mitsubishi scroll compressor is used on this model year vehicle. The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.
Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.
The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the TXV.
The TXV is located at the evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the TXV. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs the moisture and heat from the ambient air. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.
The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.