Control System Logic

Controls and Compressor Operation

All customer requests for the Electronic Manual Temperature Control (EMTC) system come through the HVAC control module.

A/C (Air Conditioning) (if equipped) Request

The HVAC control module uses a dedicated wire to send the A/C request message to the IPC. The IPC sends the A/C request to the PCM through the HS-CAN.

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when:

the refrigerant pressure is not excessively high or low.
the engine coolant temperature is not excessively high.
the ambient air temperature is above approximately -6.6°C (20°F).
a Wide Open Throttle (WOT) condition is not present.
the evaporator discharge air temperature is above approximately 1.5°C (35°F).

Compressor Clutch

When A/C is requested and all conditions are within normal parameters, a ground is provided to the A/C clutch relay coil from the PCM, energizing the A/C clutch relay. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil from the relay.

Sensor Inputs

The Ambient Air Temperature (AAT) sensor and the evaporator temperature sensor are inputs to the IPC. The A/C pressure transducer is an input to the PCM. If the temperatures are below a predetermined value, or the system pressure becomes too high or too low, the PCM does not allow the A/C compressor clutch to engage.

Heating and Ventilation

The heating and ventilation system:

controls the temperature of the air inside the vehicle.
reduces the relative humidity of the air inside the vehicle (during A/C compressor operation, if equipped).
delivers heated or cooled air to maintain the vehicle interior temperature and comfort level.

Air Handling

All airflow from the blower motor passes through the evaporator core (if equipped). Based on the climate control system temperature settings, the temperature door actuator directs airflow through the heater core as needed. The air distribution door actuator controls the airflow to the defrost, footwell or register vents. The air source is from outside air or recirculated passenger compartment air as determined by the air inlet door position. The cabin air filter is located in the heater core and evaporator core housing. For cabin air filter maintenance intervals, Refer to: Maintenance Schedules - Gasoline Engines .

The air distribution door and temperature door actuators use a potentiometer to sense and communicate the door position to the HVAC control module. When in airflow mode or a desired temperature is requested by the HVAC control 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 resistor uses 3 resistance elements that are wired in series on the ground side of the blower motor. The blower motor has 4 speeds:

for low speed, all 3 resistors are used
for medium-low speed, resistors 1 and 2 are used
for medium-high speed, resistor 1 is used
for high speed, the blower motor is provided a path directly to ground and the blower motor resistors are not used

The HVAC control module grounds 1 of the 4 circuits to control the blower motor speed.

MAX A/C (if equipped)

When MAX A/C mode is selected:

the air inlet door closes, preventing outside air and allowing only recirculated air.
the recirculated air indicator is illuminated (recirculated air forced on).
the mode doors direct airflow to the instrument panel registers.
the A/C button is illuminated.
the A/C compressor operates if the outside temperature is above approximately -6.6°C (20°F).
blended air temperature is available.
the blower motor is on.

PANEL

When PANEL mode is selected:

the mode doors direct airflow to the instrument panel registers.
the recirculated air request button can be enabled. If the recirculated air request button is selected (indicator on), the air inlet door closes off outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door admits only outside air into the passenger compartment.
blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on (if equipped).
the blower motor is 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. If the recirculated air request button is selected (indicator on), the air inlet door closes off outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door admits only outside air into the passenger compartment.
blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on (if equipped).
the blower motor is on.

OFF

When OFF is selected:

the recirculated air request button is disabled.
the air inlet door closes, preventing outside air and allowing only recirculated air.
the A/C request button (if equipped) is disabled.
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. If the recirculated air request button is selected (indicator on), the air inlet door closes off outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door admits only outside air into the passenger compartment. When recirculated air is selected, the air inlet mode door remains in recirculated air mode for 5 minutes. After 5 minutes, the recirculated air indicator turns off and the air inlet mode door changes to fresh air mode.
blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on.
the blower motor is 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. If the recirculated air request button is selected (indicator on), the air inlet door closes off outside air from entering the passenger compartment. If the recirculated air request button is not selected (indicator off), the air inlet door admits only outside air into the passenger compartment. When recirculated air is selected, the air inlet mode door remains in recirculated air mode for 5 minutes. After 5 minutes, the recirculated air indicator turns off and the air inlet mode door changes to fresh air mode.
blended air temperature is available. The airflow temperature can only be cooled below the outside air temperature when the A/C is commanded on (if equipped).
the A/C request button (if equipped) indicates the last status and the indicator can be toggled. To reduce fogging, the A/C compressor operates automatically, regardless of indicator status, if the outside temperature is above approximately -6.6°C (20°F).
the blower motor is 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 is disabled. The air inlet door opens, admitting only outside air into the passenger compartment.
the A/C request button (if equipped) indicates the last status and the indicator can be toggled. To reduce fogging, the A/C compressor operates automatically, regardless of indicator status, if the outside temperature is above approximately -6.6°C (20°F).
blended air temperature is available.
the blower motor is on.