Control System Logic

The blower motor knob must be set to any speed other than 0 for the system to operate.

Controls and Compressor Operation

All customer commands for the EMTC climate control system come through the HVAC control head.

A/C Request

When the A/C button is pressed, the HVAC control module uses a dedicated wire to send the A/C request message to the BCM. The BCM sends the A/C request through the HS-CAN to the PCM.

When an A/C request is received by the PCM, the A/C clutch relay is engaged when all of the following conditions are met:

Excessively high or low refrigerant pressure from the A/C pressure transducer is not detected.
Ambient air temperature is above approximately 32.0 °F (°C).
Evaporator temperature is above approximately 33.8 °F (1 °C).
Engine coolant temperature conditions are within normal parameters.
Wide Open Throttle (WOT) condition is not present.
Engine torque conditions are within normal parameters.
Battery state of charge conditions are within normal parameters.

Compressor control and the evaporator temperature are a function of many parameters, not just a straight on/off, to avoid freezing the evaporator. The PCM monitors multiple temperature sensors for correlation including, but not limited to the AAT, CACT, CHT, ECT, IAT, MAF, MAP, and MAPT (as applicable). The PCM runs this logic after an engine off and a calibrated soak period of 6 to 8 hours. This soak period allows the Ambient Air Temperature (AAT) sensor and the other temperature sensors to stabilize and not differ by greater than a calibrated value, typically 18°C (32.4°F). If a temperature sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C clutch to engage. For more information on PCM sensors, refer to Engine Controls - Description & Operation (Except Diesel & Hybrid) .

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C 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 does not allow the A/C 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).
delivers heated or cooled air to maintain the vehicle interior temperature and comfort level.

The heating and ventilation system uses a reheat method to provide conditioned air to the passenger compartment. Temperature blending is controlled by the temperature door, which regulates the amount of air that flows through and around the heater core, where it is then mixed and distributed. All airflow from the blower motor passes through the A/C evaporator core.

Air Handling

Based on the climate control system temperature settings, the temperature door actuator directs airflow through the evaporator and heater cores as needed. The air distribution 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 door position. The air inlet door actuator contains a reversible electric motor with no potentiometer and is either fully open or fully closed.

The air distribution door and temperature door actuators are stepper motors. There is no feedback circuit or potentiometer for these actuators. The pins labeled Door A, B, C and D are for the different phases (coils) of the motor. All the phases for a given motor are fed from a common power lead, the pin labeled Door Power and the HVAC control module controls each phase by turning a low side output on or off for the different phases. The motor turns, or steps, through a series of motions to position the actuator. The HVAC control module tracks the actuator positions by counting steps and periodically calibrates itself so it knows how many steps there are for the full range movement of the actuator. The HVAC control module sets Diagnostic Trouble Codes (DTCs) by monitoring the current flow through a given phase. If it sees too much current it identifies it as a short circuit. If it does not see any current it identifies it as an open circuit.

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.

Off

When the system is OFF:

the air inlet door closes, preventing outside air and allowing only recirculated air.
the blower motor is off.

Max A/C

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 A/C button is illuminated.
the A/C compressor operates if the outside temperature is above approximately 0°C (32°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 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 is commanded on.

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 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 is commanded on.

Defrost

When DEFROST mode 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 is automatically commanded on and remains on until the system is either turned off or the airflow mode has changed and the A/C button is pressed.