Spark

Automatic HVAC Description and Operation

The air temperature and the air delivery description and operation are divided into eight areas:

HVAC Control

The HVAC control contains all switches, which are required to control the functions of HVAC. The HVAC control is a LIN device that interfaces between the operator and the HVAC system to maintain and control desired air temperature, recirculation and air distribution settings. The HVAC control provides blower, air delivery mode, recirculation and air temperature settings. To change "°C" to "°F" or "°F" to "°C" for the temperature display, perform the following procedure.

Mode Valve Actuator

The mode valve actuator is a 3-wire stepper motor. The HVAC control supplies a 12 V reference voltage to the stepper motor and energises the stepper motor coils with a pulsed ground signal. The stepper motor puts the mode flap into the calculated position in order to reach the selected position. Stepper motors are used for air distribution control. With the appropriate switches at the HVAC control, the desired air distribution flap position can be put in. The selected values are passed to the HVAC control via the LIN-Bus.

Temperature Valve Actuator

The temperature valve actuator is a 3-wire stepper motor. The HVAC control supplies a 12 V reference voltage to the stepper motor and energises the stepper motor coils with a pulsed ground signal. The stepper motor puts the mixed air flap into the calculated position, in order to reach the selected temperature. Stepper motors are used for temperature regulation. With the appropriate switches at the HVAC control, the desired air temperature flap position can be put in. The selected values are passed to the HVAC control via the LIN-Bus.

Air Inlet Valve Actuator

The air inlet valve actuator is a 3-wire stepper motor. The HVAC control supplies a 12 V reference voltage to the stepper motor and energises the stepper motor coils with a pulsed ground signal. The stepper motor puts the recirculation flap into the calculated position in order to reach the desired position. Stepper motors are used for recirculation flap control. With the appropriate switches at the HVAC control, the desired recirculation flap position can be put in. The selected values are passed to the HVAC control via the LIN-Bus.

Blower Motor Control Module

The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control provides a low side pulse width modulation (PWM) signal to the blower motor control module via the blower motor speed control circuit. As the requested blower speed increases, the HVAC control increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control decreases the amount of time that the signal is modulated to ground.

Evaporator Temperature Sensor

The evaporator temperature sensor is a 2-wire negative temperature co-efficient thermistor. The sensor is installed at the evaporator and measures its temperature. If the temperature drops under 4°C (39°F), the compressor will be switched off in order to prevent a frozen evaporator.

A/C Compressor

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. When the A/C switch is pressed, the HVAC control sends an A/C request message to the ECM via CAN-Bus. Therefore the ECM grounds the A/C compressor clutch relay control circuit, which will switch the A/C compressor clutch relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch. The A/C compressor clutch will be activated.

Sun Load Temperature Sensor

The sun load temperature sensor is a 2-wire photo diode. Low reference and signal circuits enable the sensors to operate. As the sunload increases, the sensor signal decreases. The sensor signal varies between 0-5 V. The sun load sensor provides the HVAC control with a measurement of the amount of light shining on the vehicle.

The blower control switch is part of the HVAC control. The selected value of the blower switch position is sent to the HVAC control via LIN-Bus.

The blower motor control module is an interface between HVAC control and blower motor. The blower motor control module regulates supply voltage and ground circuits to blower motor. The HVAC control provides a PWM signal to the blower motor control module in order to command the desired blower motor speed. The blower motor control module supplies battery voltage to the blower motor and uses the blower motor ground as a low side control to adjust the blower motor speed.

The HVAC control module controls the distribution of air by the use of air inlet valve and mode valve actuator. The modes that may be selected are:

The desired air distribution mode can be selected with the air distribution switches at the HVAC control. The HVAC control controls the air distribution actuator so that it drives the flap to the calculated position. Depending on the position of the flap, air is distributed through various ducts leading to the outlets in the dash. Turning the mode flap to the defrost position, the HVAC control will move the air inlet valve actuator to outside air, reducing window misting. When defrost is selected, the blower motor will be activated, regardless of the coolant temperature. The HVAC control enables a high volume of air delivered to the front defrost vents. A/C is available in all modes.

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 windscreen fogging. Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature:

Pressing the A/C switch or AUTO switch enables the HVAC control to request A/C compressor engagement. The HVAC control sends a message to the engine control module (ECM) for A/C compressor engagement. The ECM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil.

The following conditions must be met in order to activate the A/C compressor:

The sensor information is used by the ECM to determine the following:

The air streams into the passenger compartment through the heater core and the evaporator core. The temperature valve actuator drives the mixed air flap to induce the airflow. If the interior temperature should be increased, the mixed air flap is put into the position in which more air streams through the heater core. If the interior temperature should be decreased, the mixed air flap is put into the position in which more air streams through the evaporator core.

The recirculation switch is part of the HVAC control. The selected recirculation switch position is sent to the HVAC control via LIN-Bus. The HVAC control controls the air intake through the air inlet valve actuator. The recirculation switch closes the recirculation flap in order to circulate the air within the vehicle. Through renewed selection of the recirculation switch, the recirculation flap is opened again in order to route outside air into the vehicle.

Recirculation is only available if the defrost mode is not active. When the defrost mode is active, the air inlet valve actuator opens the recirculation flap and outside air is circulated to the windscreen to reduce misting.

In automatic operation, the HVAC control maintains the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuator, mode valve actuator and air inlet valve actuator.

To put the HVAC system in automatic mode, the auto switch must be activated.

Once the desired temperature is reached, the blower motor, mode valve, air inlet valve and temperature valve actuators automatically adjust to maintain the temperature selected. The HVAC control performs the following functions to maintain the desired air temperature:

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 pressurised state. The heater core is located inside the HVAC control module. The ambient air drawn through the HVAC control module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC control module, for passenger comfort. Opening or closing the air temperature flap controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculates back to the engine cooling system.

Refrigerant is the key element in an air conditioning system. R-134a is presently the only Environmental Protection Agency 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 compressor builds pressure on the vapour 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 A/C refrigerant pressure sensor fails 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 vapour 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 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 aluminium 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. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters 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 control 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 heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the A/C compressor, in a vapour state, and completing the A/C cycle of heat removal. At the A/C compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC control 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 control module as water.