System Description
HEATING & A/C SYSTEMThe all-new heating and air conditioning system for DN provides more heating and cooling capacity than in previous years. Depending on the model, either a Dual-Zone Air Conditioning System or a Three-Zone Air Conditioning System is currently available in these vehicles.
Air Distribution - Front
The front compartment HVAC unit has four fully adjustable instrument panel outlets. Side-window demister outlets in the instrument panel eliminate the need for door ducts and door-to-instrument panel seals. A single central mounted outlet delivers air for defrosting the windshield. Two outlets, one on each side of the center console underneath the instrument panel, provide airflow to the driver floor area and to the front seat passenger floor area. While two additional outlets, one on each side of the center console near the rear of the front seats, distribute both heated and cooled air across the floor to the intermediate seat passengers. Ducts in the center of the vehicle underneath the carpet deliver air from the front compartment HVAC unit to the two intermediate passenger outlets.
Door Actuator Control
A simplified control system for operation of the mode, recirculation, and temperature control actuators provides positive positioning without the complexity of feedback from position sensors. The A/C - Heater Control Module knows the number of operating actuator revolutions required for full door travel as well as the number of actuator commutator pulses per revolution. Using these parameters, the A/C - Heater Control Module runs the actuator for the number of commutator pulses that correspond to the desired door position. To maintain accuracy, the system recalibrates itself periodically at known zero and full travel conditions.
The dual sliding Blend switches have multiple detents to control the full range of temperature that the system can produce in any mode. On Dual-Zone systems, the upper slide pot controls the driver-side blend door and the lower slide pot controls the passenger-side blend door. On Three-Zone systems, the upper slide pot controls the driver-side blend door, while the lower slide pot controls the passenger-side blend door and the rear A/C - heater unit blend door.
The single rotary Mode switch has 12 detents to either direct airflow to the panel outlets, to a mix of floor and panel outlets, to the floor outlets, to a mix of floor and defrost outlets, or to the defrost outlet. Rotating the Mode switch to either of the two MAX positions results in the A/C - Heater Control Module sending a signal to the recirculation actuator to close the fresh-air door. While rotating the Mode switch to any other position results in the A/C - Heater Control Module sending a signal to the recirculation actuator to open the fresh-air door.
When the Blower switch is OFF, the A/C - Heater Control Module closes the fresh-air door to prevent outside air from entering the vehicle. On Three- Zone systems, the Mode switch also controls the rear A/C - heater unit mode door, which directs rear airflow either to the right quarter trim panel outlet or to the overhead outlets.
Front Blower Control
The front blower circuitry employs a new resistor technology to control front blower speed. Called the "credit card" resistor, the device looks much like its name in that it is thin and compact like a credit card. The credit card resistor, like the coil resistor used in previous years, mounts inside the front HVAC housing assembly in the airflow path. However, unlike the coil resistor, the credit card resistor offers much less airflow restriction.
A/C Operation
The A/C - Heater Control Module monitors the temperature of the evaporator to keep it just above freezing for maximum cooling and dehumidification. A thermistor (temperature variable resistor) placed between the evaporator fins provides the signal for the control. The Powertrain Control Module (PCM) cycles the compressor on and off with a signal from the A/C - Heater Control Module.
Heater Operation
To assist the HVAC system in maintaining a consistent and optimum heat output at low vehicle speeds, an electric coolant pump operates under specific conditions to keep a steady flow of hot coolant circulating through the heater circuit. The A/C - Heater Control Modules controls the operation of the electric coolant pump. The pump runs when all of the following conditions are met: When the vehicle's speed is below 27 Km/h (17 mph), the coolant temperature is between 65.5 °C and 110 °C (150 °F and 230 °F), and the blend control is set above 60% reheat. The A/C - Heater Control Module turns the pump OFF if any one of the following conditions occur: The vehicle speed rises above 48.3 Km/h (30 mph). The coolant temperature drops below 65.5 °C (150 °F). The coolant temperature rises above 110 °C (230 °F). The blend control is set below 60% reheat.
A/C-Heater Control (Rear)
The Three-Zone system allows the intermediate seat occupants to regulate the air temperature and the fan speed of the rear unit from the rotary controls on the headliner mounted Rear A/C - Heater Control.
HVAC Diagnostics
The A/C - Heater Control Module is fully addressable with the DRB III. Two of the Control Module's diagnostic capabilities that the DRB III will actuate include the A/C Cool down Test to test A/C system performance and the HVAC Door Recalibration Test to determine actuator travel span. After performing either test, the Control Module will display one or more messages on the DRB III screen. The message will indicate either that the HVAC system passed the test or that there is a fault currently active in the HVAC system. The DRB III can also extract active and stored Diagnostic Trouble Codes (DTCs) from the control module. Active DTCs are faults that currently exit in the HVAC system. Stored DTCs are faults that occurred in the HVAC system since the control module received the last "clear diagnostic info" message.