Heating and A/C Operation

The purpose of the heating and A/C system is to provide the following:
^ Heated air
^ Cooled air
^ Remove humidity from the interior of the vehicle
^ Reduce windshield fogging

Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:
^ Recirculation actuator setting
^ Difference between inside and desired temperature
^ Difference between ambient and desired temperature
^ Blower motor speed setting
^ Mode setting
^ Auxiliary HVAC settings

The HVAC control module commands or monitors the following actions when an air temperature setting is selected.

WARMEST POSITION - The air temperature actuator door position directs maximum air flow through the heater core.

COLDEST POSITION - The air temperature actuator door position directs maximum air flow around the heater core.

BETWEEN THE WARMEST AND COLDEST POSITION The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:
^ Sunload
^ Ambient temperature
^ Inside temperature
^ Duct temperatures

The A/C system is engaged by selecting any switch on the HVAC control module (IPM) except the "A/C OFF" switch. The A/C switch will illuminate "A/C OFF" when the A/C switch is selected. The control module (IPM) sends a class 2 A/C request message to the Powertrain Control Module (PCM) for A/C compressor clutch operation. The Dash Integration Module (DIM) must communicate with the PCM in order for the A/C clutch to be engaged. The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 125°C (257°F). The following conditions must be met in order for the PCM to turn on the compressor clutch from the IPM request:
^ DIM will allow A/C operation if the following limits are within normal operating range
- A/C line pressure
- A/C refrigerant low temperature
- Engine coolant temperature
- Battery voltage
^ HVAC control module
- Evaporator Temperature more than 4°C (39°F)
- Control module operating range 9 and 16 volts
^ PCM
- Engine Coolant Temperature (ECT) is less than 128°C (262°F)
- Engine RPM is more than 0 rpm
- A/C Pressure is between 2850 kPa (413 psi) and 204 kPa (30 psi).

Once engaged, the compressor clutch will be disengaged for the following conditions:
^ Compressor thermal switch is opened.
^ Throttle position is 100%
^ A/C Pressure is more than 2850 kPa (413 psi)
^ A/C Pressure is less than 204 kPa (30psi).
^ Engine coolant temperature (ECT) is more than 128°C (262°F)

If there is a malfunction in the A/C system, the driver information center will read "SERVICE A/C SYSTEM" to alert the driver.

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Dual Zone Operation
The HVAC control module has temperature settings for the driver and the passenger. If the passengers setting is turned off then the driver's setting controls both driver and passenger temperature actuators. The passenger's setting can not be used without the driver's setting also being ON. The passenger's setting can be turned ON or OFF by pressing the temperature knob on the passenger's side of the HVAC control module (IPM). When the passenger's setting is ON, the passenger temperature can be adjusted independently from the driver's setting and the passenger temperature is displayed on the passenger's side of the control module. A different sunload or duct temperature on one side of the vehicle may cause different discharge air temperatures even when the passenger's setting is not turned ON.

Automatic Operation
In automatic operation, the HVAC control module (IPM) will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

To place the HVAC system in automatic mode, the following is required:
^ The blower motor switch must be in the AUTO position.
^ The air temperature switch must be in any other position other than 60 or 90 degrees.
^ The mode switch must be in the AUTO position.

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected (except in the extreme temperature positions. The HVAC control module (IPM) performs the following functions to maintain the desired air temperature:
^ Inside air temperature sensor
^ Ambient air temperature sensor
^ Upper duct sensors
^ Lower duct sensors
^ Sunload sensor
^ Regulate blower motor speed
^ Position the air temperature actuator
^ Position the mode actuator
^ Position the recirculation actuator
^ Request A/C operation

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation the blower will stay on high and the air temperature actuators will stay in the full cold position.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

Steering Wheel Controls
The HVAC control module receives class 2 messages that the driver has activated a steering wheel control switch from the radio interface. The HVAC steering wheel control button controls driver set temperature increase and driver set temperature decrease.
^ Fan speed increase
^ Fan speed decrease
^ Driver set temperature increase
^ Driver set temperature decrease

The HVAC system interprets the set temperature switch on the steering wheel as if the driver had activated the same switch function on the HVAC control module (IPM).

Engine Coolant
Engine coolant is the key element of the heating system. The engine thermostat controls the normal engine operating coolant temperature. Coolant pumped out of the engine enters the heater core through the inlet heater hose. The air flowing through the HVAC module absorbs the heat of the coolant flowing through the heater core. The coolant then exits the heater core through the heater outlet hose and returns back to the engine block.