System Operation
Climate Control - EMTC
System Operation
System Diagram
EMTC (Electronic Manual Temperature Control)
The Refrigerant Cycle
During stabilized conditions (A/C (Air Conditioning) system shutdown), the refrigerant pressures are equal throughout the system. When the A/C (Air Conditioning) compressor is in operation, it uses a piston pump to compress the cool vapor, causing it to become high-temperature/high-pressure vapor. The high-temperature/high-pressure vapor is then released into the top of the A/C (Air Conditioning) condenser core.
The A/C (Air Conditioning) condenser, being close to ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubing. The now liquid refrigerant, still at high pressure, exits from the bottom of the A/C (Air Conditioning) condenser and enters the inlet side of the A/C (Air Conditioning) receiver/drier (integral to the condenser). The receiver/drier is designed to remove moisture and contaminants from the refrigerant system.
The outlet of the receiver/drier is connected to the TXV (Thermostatic Expansion Valve). The TXV (Thermostatic Expansion Valve) provides the orifice, which is the restriction in the refrigerant system, and separates the high and low pressure sides of the A/C (Air Conditioning) system. As the liquid refrigerant passes across this restriction, its pressure and boiling point are reduced.
The liquid refrigerant is now at its lowest pressure and temperature. As it passes through the A/C (Air Conditioning) evaporator, it absorbs heat from the airflow passing over the plate/fin sections of the A/C (Air Conditioning) evaporator. This addition of heat causes the refrigerant to boil (convert to gas). The now cooler air can no longer support the same humidity level of the warmer air and this excess moisture condenses on the exterior of the evaporator coils and fins and drains outside the vehicle.
The refrigerant cycle is now repeated with the A/C (Air Conditioning) compressor again increasing the pressure and temperature of the refrigerant.
A thermistor, which monitors the temperature of the air that has passed through the evaporator core, controls A/C (Air Conditioning) clutch cycling. If the temperature of the evaporator core discharge air is low enough to cause the condensed water vapor to freeze, the A/C (Air Conditioning) clutch is disengaged by the PCM (Powertrain Control Module).
The high-side line pressure is also monitored so that A/C (Air Conditioning) compressor operation is interrupted if the system pressure becomes too high or is determined to be too low (low charge condition).
The A/C (Air Conditioning) compressor relief valve opens and vents refrigerant to relieve unusually high system pressure.
Controls and Compressor Cycling
All customer commands for the EMTC (Electronic Manual Temperature Control) climate control system come through the HVAC (Heating, Ventilation, and Air Conditioning) control head.
A/C (Air Conditioning) Request
The HVAC (Heating, Ventilation, and Air Conditioning) module uses a dedicated wire to send the A/C (Air Conditioning) request message to the BCM (Body Control Module). The BCM (Body Control Module) sends the A/C (Air Conditioning) request through the HS-CAN (High Speed Controller Area Network) to the PCM (Powertrain Control Module).
When an A/C (Air Conditioning) request is received by the PCM (Powertrain Control Module), the PCM (Powertrain Control Module) engages the A/C (Air Conditioning) clutch relay when:
- the refrigerant pressure isn't excessively high or low,
- the engine coolant temperature isn't excessively high.
- the ambient air temperature is above 6°C (42.8°F).
- a WOT (Wide Open Throttle) condition is not present.
- the evaporator discharge air temperature is above 7.5°C (45.5°F).
Compressor Clutch
When A/C (Air Conditioning) is requested and A/C (Air Conditioning) line pressures allows, a ground is provided to the A/C (Air Conditioning) clutch relay coil from the PCM (Powertrain Control Module), energizing the A/C (Air Conditioning) clutch relay. When the PCM (Powertrain Control Module) energizes the relay, voltage is supplied to the A/C (Air Conditioning) compressor clutch field coil from the relay.
A/C (Air Conditioning) Compressor Cycling
The evaporator discharge air temperature sensor is an input to the BCM (Body Control Module) and is relayed to the PCM (Powertrain Control Module) over the HS-CAN (High Speed Controller Area Network). The evaporator discharge air temperature sensor prevents icing of the evaporator core by measuring the temperature of the airflow immediately after the evaporator core. An accurate evaporator temperature is critical for compressor engagement. The PCM (Powertrain Control Module) uses the temperature measurement to regulate the on and off time of the A/C (Air Conditioning) compressor to maintain the evaporator temperature within an acceptable temperature range.
The PCM (Powertrain Control Module) monitors the discharge pressure measured by the A/C (Air Conditioning) pressure transducer. The PCM (Powertrain Control Module) interrupts A/C (Air Conditioning) compressor operation in the event the A/C (Air Conditioning) pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM (Powertrain Control Module) does not allow the A/C (Air Conditioning) clutch to engage.
Air Handling
Based on the climate control system temperature settings, the temperature blend door actuator directs airflow through the evaporator and heater cores as needed. The defrost/panel/floor mode door actuator controls the airflow to the defrost, floor or panel register vents. The air source is from outside air or recirculated passenger compartment air as determined by the air inlet mode door position.
The defrost/panel/floor mode door and temperature blend door actuators use a potentiometer to sense and communicate the door position to the HVAC (Heating, Ventilation, and Air Conditioning) module. When in airflow mode or desired driver or passenger temperature is requested by the HVAC (Heating, Ventilation, and Air Conditioning) module, the appropriate door actuator motor is driven to the desired position, using the position sensed by the door actuator potentiometer to accurately position the door actuator.
The blower motor is controlled using a blower motor resistor. The blower motor resister uses 3 resistors that are wired in series on the ground side of the blower motor. The blower motor has 4 speeds:
- for low speed, the all 3 resistors are used
- for medium-low speed, 2 resistors are used
- for medium-high speed, 1 resistor is used
- for high speed, the blower motor is provided a path directly to ground and the blower motor resistor is not used
The HVAC (Heating, Ventilation, and Air Conditioning) module grounds 1 of the 4 circuits to control the blower motor speed.
Control System Logic
The blower motor knob must be set to any speed other than 0 for the system to operate
MAX AC
When MAX A/C is selected:
- the air inlet door closes, preventing outside air and admits only recirculated air.
- the recirculated air indicator is illuminated (recirculated air forced on).
- the A/C button is illuminated.
- the A/C (Air Conditioning) compressor operates if the outside temperature is above approximately 6°C (42.8°F).
PANEL
When PANEL mode is selected:
- the mode doors direct airflow to the instrument panel registers.
- the recirculated air request button can be enabled.
- blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C (Air Conditioning) is commanded on.
PANEL-FLOOR
When PANEL/FLOOR mode is selected:
- the mode doors direct airflow to the floor duct and the instrument panel registers. A small amount of airflow from the side window demisters and defrost duct is present.
- the recirculated air request button can be enabled.
- blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C (Air Conditioning) is commanded on.
OFF
When the system is OFF:
- the air inlet door closes, preventing outside air and allowing only recirculated air.
- the blower motor is off.
FLOOR
When FLOOR mode is selected:
- the mode doors direct airflow to the floor duct. A small amount of airflow from the defroster duct and side window demisters is present.
- the recirculated air request button can be enabled.
- blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C (Air Conditioning) is commanded on.
FLOOR-DEFROST
When FLOOR/DEFROST mode is selected:
- the mode doors direct airflow to the floor duct, the defroster duct and the side window demisters.
- the recirculated air request button can be enabled, but can automatically turn off.
- blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C (Air Conditioning) is commanded on.
DEFROST
When DEFROST is selected:
- the mode doors direct airflow to the defroster duct and side window demisters. A small amount of airflow from the floor duct is present.
- the recirculated air request button can be enabled, but can automatically turn off.
- the A/C (Air Conditioning) is automatically commanded on and will remain on until the system is either turned off or the airflow mode has changed and A/C (Air Conditioning) button is pressed.
- blended air temperature is available.