Control System Logic

The FCIM provides the interface for the vehicle occupants to control the climate control system.

When the FCIM receives the climate control selections from pressing the buttons on its own controls (interface), the APIM or the IPC (remote start). The IPC sends the information to the BCM over the HS-CAN. The BCM sends the requests to the HVAC control module over the MS-CAN.

A/C Request

When the A/C is requested, the HVAC control module sends the message over the Medium Speed Controller Area Network (MS-CAN) to the Body Control Module (BCM). The BCM sends the A/C request to the PCM through the High Speed Controller Area Network (HS-CAN).

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

the refrigerant pressure is not excessively high or low.
the engine coolant temperature is not excessively high.
the ambient air temperature is above approximately 0°C (32°F).
a Wide Open Throttle (WOT) condition is not present.
the evaporator temperature is above approximately 1°C (33.8°F).
the PCM does not detect temperatures from the Mass Airflow/Intake Air Temperature sensor that does not correlate with other temperature sensor readings at ignition ON. (Most Mass Airflow (MAF) sensors have an integrated Intake Air Temperature (IAT) sensor).

The PCM monitors multiple temperature sensors correlation (AAT, CHT, ECT IAT, MAF, MAP (as equipped)). The PCM runs this logic after an engine off and a calibrated soak period, typically 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 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 - INTRODUCTION (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.

The FCIM provides the interface for the vehicle occupants to control the climate control system.

A/C Request

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

the refrigerant pressure is not excessively high or low.
the engine coolant temperature is not excessively high.
the ambient air temperature is above approximately 0°C (32°F).
a Wide Open Throttle (WOT) condition is not present.
the evaporator temperature is above approximately 1°C (33.8°F).
the PCM does not detect temperatures from the Mass Airflow/Intake Air Temperature sensor that does not correlate with other temperature sensor readings at ignition ON. (Most Mass Airflow (MAF) sensors have an integrated Intake Air Temperature (IAT) sensor).

Air Handling

All of the door actuators contain a reversible electric motor and a potentiometer. The potentiometer circuit consists of a 5-volt reference signal connected to one end of a variable resistor, and a signal ground connected to the other. A signal circuit is connected to a contact wiper, which is driven along the variable resistor by the actuator shaft. The signal to the HVAC control module from the contact wiper indicates the position of the actuator door. The HVAC control module powers the actuator motors to move the doors to the desired positions. The desired door positions are calculated by the HVAC control module based on the set temperature, in-vehicle temperature and ambient air temperature.

The HVAC control module controls the blower speed by sending a PWM signal to the blower motor speed control.

All airflow from the blower motor passes through the evaporator core. Based on the climate control system temperature settings, the temperature door actuator directs airflow through the heater core as needed. The defrost/panel/floor mode 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.

Max A/C

When MAX A/C is selected:

the air inlet door actuator closes off outside air and admits only recirculated air.
the recirc button is disabled and the indicator is illuminated.
the defrost/panel/floor door directs airflow to the instrument panel A/C registers.
blended air temperature is available.
the A/C request button is illuminated and disabled.
the A/C compressor operates if the outside temperature is above approximately 0°C (32°F).
the blower motor is on.

PANEL

When PANEL is selected:

the recirc request button is enabled. If the recirc request button is selected (indicator on), the air inlet door actuator closes off outside air from entering the passenger compartment. If the recirc request button is not selected (indicator off), the air inlet door actuator admits only outside air into the passenger compartment.
the defrost/panel/floor door directs airflow to the instrument panel A/C registers.
blended air temperature is available.
the A/C request button is enabled. The A/C compressor operates and the indicator illuminates if the A/C request button is selected and the outside temperature is above approximately 0°C (32°F).
the blower motor is on.

PANEL/FLOOR

When PANEL/FLOOR is selected:

the recirc request button is enabled. If the recirc request button is selected (indicator on), the air inlet door actuator closes off outside air from entering the passenger compartment. If the recirc request button is not selected (indicator off), the air inlet door actuator admits only outside air into the passenger compartment.
the defrost/panel/floor door directs airflow to the floor duct and the instrument panel A/C registers. A small amount of airflow from the side window demisters and defrost duct is present.
blended air temperature is available.
the A/C request button is enabled. The A/C compressor operates and the indicator illuminates if the A/C request button is selected and the outside temperature is above approximately 0°C (32°F).
the blower motor is on.

OFF

When OFF is selected:

the recirc request button is disabled.
the air inlet door actuator closes off outside air and admits only recirculated air.
the A/C request button is disabled.
the blower motor is off.

FLOOR

When FLOOR is selected:

the recirc request button is enabled. If the recirc request button is selected (indicator on), the air inlet door actuator closes off outside air from entering the passenger compartment. If the recirc request button is not selected (indicator off), the air inlet door actuator admits only outside air into the passenger compartment.
the defrost/panel/floor door directs airflow to the floor duct. A small amount of airflow from the defroster duct and side window demisters is present.
blended air temperature is available.
the A/C request button is enabled. The A/C compressor operates and the indicator illuminates if the A/C request button is selected and the outside temperature is above approximately 0°C (32°F).
the blower motor is on.

FLOOR/DEFROST

When the FLOOR/DEFROST is selected:

the recirc request button is enabled. If the recirc request button is selected (indicator on), the air inlet door actuator closes off outside air from entering the passenger compartment. If the recirc request button is not selected (indicator off), the air inlet door actuator admits only outside air into the passenger compartment.
the defrost/panel/floor door directs airflow to the floor duct, the defroster duct and the side window demisters.
blended air temperature is available.
the A/C request button and indicator can be toggled. To reduce fogging, the A/C compressor operates automatically, regardless of indicator status, if the outside temperature is above approximately 0°C (32°F).
the blower motor is on.

DEFROST

When DEFROST is selected:

the air inlet door actuator opens, admitting only outside air into the passenger compartment.
the defrost/panel/floor door directs airflow to the defroster duct and side window demisters. A small amount of airflow from the floor duct is present.
blended air temperature is available.
the A/C request button and indicator can be toggled. To reduce fogging, the A/C compressor operates automatically, regardless of indicator status, if the outside temperature is above approximately 0°C (32°F).
the blower motor is on.