Principles of Operation

For additional refrigerant system and component information, refer to Climate Control .

Climate Control System Network Communication 

The controls for the climate control system are in one or more locations depending on vehicle option content:

• FCIM
• IPC if equipped with steering wheel controls
• FDIM (attached to the APIM)

When the FDIM touchscreen or voice commands are used, the APIM sends a function request message over the I-CAN to the FCIM.

When the IPC steering wheel controls are used, the IPC sends a function request message over the I-CAN to the FCIM.

The FCIM receives the climate control selections from pressing the buttons on its own controls (interface), the APIM or IPC and sends the requests to the HVAC module in the following message path:

• The FCIM sends the requests over the I-CAN to the IPC.
• The IPC relays the requests to the BCM over the HS-CAN.
• The BCM sends the requests to the HVAC module over the MS-CAN.

The messaging path is followed in reverse for any status updates that need to be sent from the HVAC module to the FCIM, FDIM (through the APIM) and IPC message center.

Climate Control System Logic 

Blower Motor Speed 

When blower speed is selected, the FCIM sends the desired blower speed to the HVAC module using the message path described above. The HVAC module then commands the blower motor speed control to the desired speed. The HVAC module monitors the blower motor speed control feedback circuit to make sure the blower motor is at the desired speed.

Airflow Mode Selection 

When an airflow mode is selected, the FCIM determines the applicable and allowable airflow direction. The FCIM then sends this desired airflow direction to the HVAC module using the message path described above. The HVAC module determines the actuator position required to deliver the correct airflow direction. While monitoring the defrost/panel/floor feedback circuit, the HVAC module drives the actuator until the feedback circuit indicates the actuator has reached its required position.

Temperature Selection 

When a temperature is selected, the FCIM sends the desired temperature selection to the HVAC module using the message path described above. The HVAC module then determines the temperature blend door desired position. While monitoring the temperature blend door feedback circuit, the HVAC module drives the actuator until the feedback circuit indicates the actuator has reached its desired position.

Air Inlet Selection 

When fresh air or RECIRC mode is selected, the FCIM sends the desired selection to the HVAC module using the message path described above. The HVAC module drives the air inlet mode door actuator until the HVAC module detects the actuator reached its end of travel. A spike in current draw tells the HVAC module the actuator has reached the end of its travel.

A/C Selection 

When A/C is selected, the FCIM sends the selection to the HVAC module using the message path described above. If the ambient air temperature is sufficient, the HVAC module then sends the request to the BCM over the MS-CAN. The BCM then sends the request to the PCM over the HS-CAN.

FET Protection 

A FET is a type of transistor that, when used with module software, monitors and controls current flow on module outputs. The FET protection strategy prevents module damage in the event of excessive current flow.

The HVAC module utilizes a FET protective circuit strategy for its actuator outputs. Output load (current level) is monitored for excessive current (typically short circuits) and shuts down (turns off the voltage or ground provided by the module) when a fault event is detected. A short circuit DTC is stored at the fault event and a cumulative counter is started.

When the demand for the output is no longer present, the module resets the FET circuit protection to allow the circuit to function. The next time the driver requests a circuit to activate that has been shut down by a previous short (FET protection) and the circuit is still shorted, the FET protection shuts off the circuit again and the cumulative counter advances. If excessive circuit load occurs repeatedly, the module shuts down the output until a repair procedure is carried out.

When each tolerance level is reached, DTC U1000:00 should set (U1000:00 may or may not set) along with the short circuit DTC that was stored on the first failure. These DTCs cannot be cleared by a command to clear the DTCs. The module does not allow the DTC to be cleared or the circuit to be restored to normal operation until a successful self-test  proves the fault has been repaired. After the self-test has successfully completed (no on-demand DTCs present), DTC U1000:00 and the associated DTC (the DTC related to the shorted circuit) automatically clears and the circuit function returns.

The module never resets the fault event counter to zero and continues to advance the fault event counter as short circuit fault events occur. If the number of short circuit fault events reach the third level, DTC U3000:49 set along with the associated short circuit DTC. DTC U3000:49 cannot be cleared and a new module must be installed after the initial fault is repaired  .

CASS 

Liquid refrigerant may accumulate in the A/C compressor under certain conditions. To alleviate damage to the A/C compressor, CASS is utilized.

CASS is initiated only under specific conditions:

• The ignition is off for more than 8 hours
• The ambient temperature is above -4 °C (25 °F)
• Battery voltage is above 8.5 volts during engine cranking

When these conditions are present, the PCM activates the A/C control relay prior to cranking of the engine. The A/C control relay engages the A/C compressor for approximately 4-15 A/C compressor revolutions or a maximum of 2 seconds (depending upon vehicle application), allowing the liquid refrigerant to be pushed from the A/C compressor. CASS is initiated by the PCM regardless of the HVAC system settings.

The Refrigerant Cycle 

During stabilized conditions (A/C system shutdown), the refrigerant pressures are equal throughout the system. When the A/C compressor is in operation, it increases pressure on the refrigerant vapor, raising its temperature. The high-pressure and high-temperature vapor is then released into the top of the A/C condenser core.

The A/C 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 condenser and enters the inlet side of the A/C receiver/drier. The receiver/drier is designed to remove moisture from the refrigerant.

The outlet of the receiver/drier is connected to the TXV. The TXV provides the orifice which is the restriction in the refrigerant system and separates the high and low pressure sides of the A/C 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 evaporator, it absorbs heat from the airflow passing over the plate/fin sections of the A/C 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 compressor again increasing the pressure and temperature of the refrigerant.

The evaporator temperature sensor (attached to the evaporator core fins) controls A/C clutch cycling. If the temperature of the evaporator core is low enough to cause the condensed water vapor to freeze, the A/C clutch is disengaged by the vehicle PCM.

The externally controlled variable displacement compressor is electronically controlled by the PCM. The PCM pulse width modulates the solenoid in the compressor to control the compressor displacement. The PCM changes the compressor displacement based upon the:

• evaporator temperature
• ambient air temperature
• engine rpm
• vehicle speed
• A/C high side pressure
• intake air temperature

The high-side line pressure is monitored so that A/C compressor operation will be interrupted if the system pressure becomes too high or is determined to be too low (low charge condition).

The A/C compressor pressure relief valve opens and vents refrigerant to relieve unusually high system pressure.

TXV Type Refrigerant System - GTDI 

Courtesy of FORD MOTOR COMPANY

TXV Type Refrigerant System - Ti-VCT 

Courtesy of FORD MOTOR COMPANY