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 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 battery positive and ignition 1 voltage circuits provide power to the module. The control module 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 1 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 air conditioning (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) or powertrain control module (PCM) 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 or PCM 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 air conditioning (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 or PCM will operate the A/C system automatically in FRONT DEFROST and FLOOR 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 module. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST and FLOOR mode. The following conditions must be met in order for the ECM or PCM to turn ON the compressor clutch:
* Body control module (BCM)
- Battery voltage is between 9-16 volts.
- A/C request from the HVAC control module
* ECM or PCM
- Engine coolant temperature (ECT) is less than 125°C (257°F).
- Engine speed is less than 6,100 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).
* Engine coolant temperature (ECT) is more than 125°C (257°F).
* Engine speed is less than 475 RPM.
* Engine speed is 6,100 or more RPM.
* ECM or PCM detects excessive torque load.
* ECM or PCM 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.