Heating and Air Conditioning: Description and Operation
AIR DELIVERY DESCRIPTION AND OPERATIONThe air delivery description and operation is divided into four areas:
- HVAC Control Components
- Air Speed
- Air Delivery
- Recirculation Operation
HVAC Control Components
HVAC Control Module
The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive and ignition 3 voltage circuits provide power to the control assembly. An integrated potentiometer controls air temperature door position. The integrated vacuum system controls the mode door position. The control assembly supports the features as in table.
Air Speed
The blower motor circulates air through the vehicle's interior. The vehicle operator determines the blower motor's speed when the driver places the blower switch in one of the following positions:
- Low (1)
- Medium 1 (2)
- Medium 2 (3)
- High (4)
The blower motor will operate in any switch position, as long as the ignition switch is in the RUN position and the mode switch is in any position other than OFF. When the ignition switch is ON, power from the instrument panel fuse block is delivered to the mode switch first before going to the blower motor switch. The blower motor and mode switches are located within the HVAC control assembly.
Power is provided to the blower motor from either the instrument panel fuse block or underhood fuse block. Depending upon selected blower speeds, voltage is either supplied directly to the blower motor or voltage is varied by a series of in-line resistors. Power and ground are provided to the HVAC control assembly by the ignition 3 voltage circuit, ground circuits and splice pack.
Low Blower Speed
When the Low blower (1) speed is selected, the HVAC control assembly applies voltage to the blower motor resistor assembly through the low blower motor control circuit. Low blower speed voltage will bypass the blower mode switch completely. Voltage is divided between 3 series resistors in the blower motor resistor assembly, to the blower motor relay on the blower motor supply voltage circuit, and the blower motor to achieve the desired blower speed. The blower motor is included in the series circuit by the blower motor supply voltage and ground circuits. The ground circuit provides a ground path for the blower motor.
Medium Blower Speeds
When the Medium 1 blower (2) speed is selected, the HVAC control assembly applies voltage to the blower motor resistor assembly through a portion of the low blower motor control circuit. The blower switch will direct the voltage through the medium 1 blower motor control circuit. Voltage is divided between 2 series resistors in the blower motor resistor assembly, to the blower motor relay on the blower motor supply voltage circuit, and the blower motor to achieve the desired blower speed. The blower motor is included in the series circuit by the blower motor supply voltage and ground circuits. The ground circuit provides a ground path for the blower motor.
When the Medium 2 blower (3) speed is selected, the HVAC control assembly applies voltage to the blower motor resistor assembly through a portion of the low blower motor control circuit. The blower switch will direct the voltage through the medium 2 blower motor control circuit. Voltage is divided by a single resistor in the blower motor resistor assembly, to the blower motor relay on the blower motor supply voltage circuit, and the blower motor to achieve the desired blower speed. The blower motor is included in the series circuit by the blower motor supply voltage and ground circuits. The ground circuit provides a ground path for the blower motor.
High Blower Speed
When the High blower (4) speed is selected, the HVAC control assembly applies voltage to the blower motor relay through a portion of the high blower motor control circuit. The blower switch will direct the voltage through the high blower motor control circuit. When the load side of the relay coil is energized, the blower motor is connected directly to battery voltage through the blower motor supply voltage circuit. The ground circuit provides a ground path for the blower motor.
Off
The blower motor and A/C compressor are turned off. When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the outside air inlet, into the HVAC assembly and out through the defrost outlets. Since the A/C compressor is not running, the incoming air may be warmed but not cooled.
Air Delivery
Mode Switch
Use the MODE switch in order to change the air delivery mode in the vehicle. The air flow direction will sequence through the following modes:
- MAX A/C
- A/C
- BI-LEVEL
- PANEL
- FLOOR
- DEFOG
- FRONT DEFROST
Max A/C
MAX A/C mode is only available on vehicles with the C60 option. When the driver selects the MAX A/C mode, the blower motor and A/C compressor clutch are activated. The recirculation door is activated, bring air from inside the vehicle for maximum cooling. Air is delivered out the instrument panel outlets with a small amount out the floor outlets.
A/C
A/C mode is only available on vehicles with the C60 option. When the driver selects the A/C mode, the blower motor and A/C compressor clutch are activated. Outside air is brought in to be cooled. Air is delivered out the instrument panel outlets with a small amount out the floor outlets.
Panel
When PANEL is selected, air is delivered through the instrument panel outlets with a small amount of air delivered to the floor. Vacuum is applied to the mode actuator through the Brown vacuum line, and to the defrost actuator through the Red vacuum line. The mode actuator will retract, opening the Panel Door. The defroster actuator will retract, closing the Defroster Door and open the Heater Door through mechanical linkage.
Bi-level
When Bi-Level is selected, air is delivered through the instrument panel outlets and to the floor outlets. Vacuum is applied to the mode actuator through the Brown and the Blue vacuum lines, and to the defrost actuator through the Red vacuum line. Applying vacuum to both sides of the mode actuator will hold the Vent Door stationary in the half open position. The defroster actuator will retract, closing the Defroster Door and open the Heater Door through mechanical linkage.
Floor
When Floor is selected, air is delivered to the floor outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Red vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, closing the Defroster Door and open the Heater Door through mechanical linkage.
Defog
When DEFOG is selected, air is delivered to the windshield outlets and the floor outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Red and the Yellow vacuum lines. The mode actuator will retract, closing the Vent Door. Applying vacuum to both sides of the defroster actuator will hold the Defroster Door stationary in the half open position. The Heater Door will also be held stationary in the half open position through mechanical linkage.
Defrost
When Defrost is selected, air is delivered to the windshield outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Yellow vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, opening the Defroster Door and close the Heater Door through mechanical linkage.
Recirculation Operation
Recirculation is only available in MAX A/C. When MAX A/C mode is selected, vacuum is applied to the recirculation actuator. The force of the vacuum overcomes the spring pressure of the spring inside the actuator. This moves the actuator, which is linked to the recirculation door, into recirculation mode. This brings air from inside the vehicle instead of fresh air from the outside.
AIR TEMPERATURE DESCRIPTION AND OPERATION
The air temperature controls are divided into 4 areas:
- HVAC Control Components
- Heating and A/C Operation
- Engine Coolant
- A/C Cycle
HVAC Control Components
HVAC Control Assembly
The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive and ignition 3 voltage circuits provide power to the control assembly. Two integrated potentiometers control air temperature door position and blower motor speed. The integrated vacuum system controls the mode door position. The control assembly supports the following features:
Air Temperature Actuator
The air temperature actuator is a 3-wire bi-directional electric motor. Ignition 3 voltage, ground and control circuit 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. When full cold is selected, a low voltage, near ground, is applied to control circuit. When full hot is selected, a high voltage, 12 volts, is applied to the control circuit.
A/C Pressure Switches
Two pressure sensors protect the A/C system. If the A/C low pressure falls below 145-172 kPa (21-25 psi), then the A/C low pressure switch will open. This will disable A/C compressor operation. This switch will then close at 262-290 kPa (38-42 psi) to allow A/C operation. If the A/C high pressure exceeds 2826 kPa (410 psi), then the A/C high pressure switch will open and not allow for the PCM interpret an A/C request. This switch will close at 2413 kPa (350 psi).
Heating And A/C Operation
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
The control assembly makes the following actions when an air temperature setting is selected:
- When the air temperature switch is placed in the warmest position, the control assembly 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 assembly commands the air temperature door to direct air to bypass the heater core.
The mode switch must be placed in the following positions for the A/C to be engaged by the PCM.
- Max A/C
- A/C
- Bi-Level A/C
- Blend
- Defrost
The following conditions must be met in order for the PCM to turn ON the compressor clutch:
- Engine coolant temperature (ECT) is less than 121°C (250°F)
- Engine RPM is more than 550 RPM
- A/C low pressure switch signal circuit is grounded
- A/C request signal circuit to the PCM has 8.5 volts or more
Once engaged, the compressor clutch will be disengaged for the following conditions:
- Throttle position is 100 percent
- A/C request signal circuit has 0 volts
- A/C low pressure switch signal circuit has voltage
- Engine coolant temperature (ECT) is more than 121°C (250°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.
Engine Coolant
Engine coolant is the essential element of the heating system. The thermostat controls the normal engine operating coolant temperature. 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. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.
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.
The A/C system used on this vehicle is a non-cycling system. Non-cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal to the compressor clutch, if the refrigerant pressure becomes excessive. After the high and the low sides of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor clutch. The A/C system is also 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 continues to rise, the high pressure relief will pop open and release refrigerant from the system.
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. The refrigerant is discharged from the compressor through the discharge hose, and forced through the condenser and then through the balance of the A/C system.
Compressed refrigerant enters the condenser at a high-temperature, high-pressure vapor state. As the refrigerant flows through the condenser, the heat is transferred to the ambient air passing through the condenser. Cooling 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 orifice tube.
The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered, causing the refrigerant to vaporize at the orifice tube. The orifice tube also measures the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the orifice tube 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 to boil inside 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 flows back to the compressor in a vapor state, 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 condenses, and discharges from the HVAC module as water.