Air Temperature Description and Operation
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 assembly. The control assembly supports the following features:
Air Temperature Actuator
The air temperature actuators are a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5, or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.
The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates, the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.
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 engine control module (ECM) converts the voltage signal to a pressure value.
The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The ECM disables the compressor clutch under the following conditions:
A/C pressure is more than 2,951 kPa (428 psi). The clutch will be enabled after the pressure decreases to less than 2,068 kPa (300 psi). A/C high side pressure is less than 310 kPa (44 psi). The clutch will be enabled after the A/C high side pressure increases to more than 310 kPa (44 psi).
Ambient Air Temperature Sensor
The ambient air temperature sensor is a 2-wire negative temperature co-efficient thermistor.
A 5-volt reference and signal circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases.
The sensor operates within a temperature range of -40°C to +60°C (-40°F to +140°F). The HVAC control module receives a class 2 message from the instrument panel cluster (IPC). The ambient air temperature sensor is mounted in the forward engine area, behind the left front headlamp. This causes an air flow issue due to the sensor's location and can effect the ambient air update display parameter in the DIC of the vehicle. For model year 2008 the ambient air temperature sensor will have a new location closer to the grill airflow. In the present position, it is exposed to limited airflow from the grill before it reaches the radiator. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a working temperature range.
The ambient air temperature sensor algorithm uses 3 pieces of information, outside air temperature data, vehicle speed, and time the engine was turned off.
The ambient air temperature is updated and displayed by the DIC under the following conditions:
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
* Difference between inside and desired temperature
* Difference between ambient and desired temperature
* Blower motor speed setting
* Mode setting
The vehicle operator can activate the A/C system by pressing the A/C switch. The A/C system can operate regardless of the temperature setting.
The ECM will operate the A/C system automatically in FRONT DEFROST mode to help reduce moisture inside the vehicle. The A/C LED will not illuminate unless the driver presses the A/C request switch on the HVAC control assembly. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode. The following conditions must be met in order for the ECM to turn ON the compressor clutch:
* Body control module (BCM)
- Battery voltage is between 9-16 volts.
- A/C request from the HVAC control assembly
* ECM
- Engine coolant temperature (ECT) is less than 123°C (253°F).
- Engine speed is less than 5,300 RPM.
- Engine speed is more than 600 RPM.
- A/C high side pressure is between 2951-310 kPa (428-44 psi).
Once engaged, the compressor clutch will be disengaged for the following conditions:
* Throttle position is 100 percent for 10 seconds.
* A/C high side pressure is more than 2951 kPa (428 psi) and will reengage once the pressure drops below 2068 kPa (300 psi).
* A/C high side pressure is less than 310 kPa (44 psi).
* ECT is more than 123°C (253°F).
* Engine speed is less than 475 RPM.
* Engine speed is more than 6,000 RPM.
* ECM detects excessive torque load.
* ECM detects insufficient idle quality.
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 is 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 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 thermal expansion valve (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 heat from the ambient air and draws moisture onto the evaporator. 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.