HVAC System - Manual
AIR DELIVERY DESCRIPTION AND OPERATIONThe air delivery description and operation is divided into 4 areas:
- HVAC Control Components
- Air Speed
- Air Distribution
- Recirculation Operation
HVAC Control Components
HVAC Control Module
The HVAC control module is a 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 shown features.
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.
Blower Motor Control Processor
The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a PWM signal to the control processor in order to command the blower motor speed. The processor supplies 12 volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed.
Air Speed
The blower motor forces air to circulate within the vehicle's interior. The vehicle operator determines the blower motor's speed by placing the blower motor switch in a desired speed position. The blower motor will only operate if the blower motor switch is in any position other than OFF, and the ignition switch is in the RUN position. Once a blower speed is selected, the blower speed remains constant until a new speed is selected.
As the requested blower speed increases, the following conditions occur:
- The HVAC control module increases the amount of time that the blower motor speed control circuit is modulated to ground.
- The voltage and duty cycle, measured between the blower motor speed control circuit and ground, decrease.
As the requested blower speed decreases, the following conditions occur:
- The HVAC control module decreases the amount of time that the blower motor speed control circuit is modulated to ground.
- The voltage and duty cycle, measured between the blower motor speed control circuit and ground, increase.
Air Distribution
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 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. When VENT is pressed, the following will occur:
- The mode actuator will be moved to the panel position.
- The recirculation actuator will be placed in the outside air position.
- The A/C compressor will be commanded off.
After a malfunction occurs to the mode actuator it is driven to the Defrost position On startup, the HVAC control module will place the mode door in the last selected position.
Recirculation Operation
The HVAC control module controls the air intake through the recirculation actuator. Recirculation is not available when the mode is in defrost. When the mode is in defog, Recirculation will only be available for ten minutes. The operator must activate the blower for Recirculation operation. The A/C high-pressure recirculation switch can cause the HVAC system to recirculate air. If the recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, then the recirculation switch LED will flash three times. When the high side pressure reaches 2206-2620 Kpa (320-380 psi), the PCM will place the HVAC system in recirculation mode. The high side pressure is lowered when the inside air cools the refrigerant within the A/C evaporator. When the high-side pressure reaches 1447-1861 Kpa (210-270 psi), the PCM will place the HVAC system out of recirculation mode.
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 Module
The HVAC control module is a 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 shown features.
Air Temperature Actuator
The 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.
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 2968 kPa (430 psi).
- A/C pressure is less than 255 kPa (35 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 A/C system can be engaged by pressing the A/C switch. The A/C system is engaged when the HVAC control module is in any mode except OFF. The A/C switch will illuminate when the A/C switch is pressed to the on position. The control module sends a Class 2 A/C request message to the dash integration module (DIM). In order for the powertrain control module (PCM) to turn on the compressor clutch, the DIM and PCM must communicate to each other over the Class 2 serial data circuits. The following conditions must be met in order for the PCM to turn on the compressor clutch:
- DIM
- Battery voltage between 9-18 volts
- A/C request from the HVAC control module
- A/C pressure is between 255-2968 kPa (35-430 psi)
- Engine coolant temperature (ECT) is less than 121°C (250°F)
- PCM
- Engine coolant temperature (ECT) is less than 121°C (250°F)
- Engine speed is less than 5000 RPM
- A/C Pressure is between 255-2968 kPa (35-430 psi)
Once engaged, the compressor clutch will be disengaged for the following conditions:
- Throttle position is 100%
- A/C Pressure is more than 2968 kPa (430 psi)
- A/C Pressure is less than 255 kPa (35 psi)
- Engine coolant temperature (ECT) is more than 121°C (250°F)
- Engine speed is more than 5000 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 key element of the heating system. The thermostat controls 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 heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort.
The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module air temperature door. 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.