Heating and Air Conditioning: Description and Operation
HVACHow the Circuit Works
The driver's and front passenger's climate control switch assemblies, the display panel control unit, and the climate control unit work together to operate the blower controls, air delivery, and A/C compressor controls automatically. Battery voltage is supplied at all times through fuse 7 (in the under-dash fuse/relay box) to the display panel control unit. With the ignition switch in ON (II), battery voltage is supplied through fuse 30 (in the under-dash fuse/relay box) to the climate control unit.
AUTO mode
To put the climate control in fully automatic mode, press the desired side's AUTO button, and then set the desired temperature by using the temperature control buttons. AUTO will appear in the system's display.
The system automatically selects the proper mix of cooled and/or heated air that will raise or lower the interior temperature from its current level to the set temperature. The system also adjusts the fan speed and changes the direction of air delivery.
When the temperature is set to its lower limit (600 °F/18 °C) or its upper limit (90 °F/32 °C), the system runs at full cooling or full heating only. It does not regulate the interior temperature. When the temperature is set between the lower and upper limits, the system regulates the interior temperature to the set value.
Manual Operation
All of the functions of the climate control system also can be selected manually. Manual selections will not adjust to inside or outside temperature conditions.
Blower Controls
The fan speed is automatically controlled in the AUTO mode. The fan speed can be selected manually using the fan speed buttons. This takes the fan from AUTO mode to the manually selected speed.
Battery voltage is supplied at all times through fuse 21 (in the under-hood fuse/relay box) to the blower motor relay contacts. With the ignition switch ON (II), the blower motor relay is energized, which feeds battery voltage to the blower motor. The blower power transistor controls the blower motor in all speeds except the highest speed. Using inputs from the fan speed buttons, the climate control unit sends signals to the blower power transistor to control the blower motor speed. When the highest fan speed is selected, the blower power transistor grounds the blower motor, which connects the blower motor directly to ground, making the blower run at high speed. In cold weather, the blower motor will not come on automatically until the heater starts to produce warm air.
Air Delivery
The climate control unit controls the blower motor and supplies a 5 V DC reference voltage to the driver's air mix control motor and the front passenger's air mix control motor.
The air mix and mode control motors all receive inputs from the climate control unit. The driver's and front passenger's air mix motors regulate the mixture of cold and hot air by varying the position of the heater-evaporator doors. The driver's and front passenger's mode control motors control the direction and volume of outlet air. Use the MODE buttons to select the vents the air flows from, although some air will flow from the dashboard corner vents in all modes. Each time the MODE buttons are pressed, the display shows each of the modes selected. Press the buttons four times to see all of the modes. The climate control unit grounds the air mix control motors and the mode control motors.
The recirculation control motor receives battery voltage through fuse 30 (in the under-dash fuse/relay box) with the ignition switch in ON (II). The recirculation control motor regulates the position of the fresh/recirc door and is controlled by two position inputs from the climate control unit ("RECIRCULATE" and "FRESH").
A/C Compressor Controls
Battery voltage is supplied at all times through fuse 12 (in the under-hood fuse/relay box) to the A/C compressor clutch relay contacts. With the ignition switch in ON (II), battery voltage is supplied through fuse 30 (in the under-dash fuse/relay box) to the coil of the relay.
The A/C is turned on manually using the A/C button or automatically when needed in the AUTO mode. The relay control module detects that the A/C is ON when the climate control unit grounds the 12 V reference voltage sent through the A/C pressure switch. The relay control module communicates this information to the ECM/PCM through the B-CAN and F-CAN networks of the multiplex system. Upon receiving the A/C ON information, the ECM/PCM provides ground to the A/C compressor clutch relay coil, energizing it. Battery voltage is supplied to the A/C compressor clutch through the relay, which engages the clutch to begin turning the compressor.
A/C Pressure Switch
The A/C pressure switch is located in the condenser outlet line where refrigerant is in a high temperature/high pressure liquid state. When the refrigerant is at low pressure (below 196 kPa (28 psi)) or high pressure (above 3140 kPa (455 psi)), the A/C pressure switch opens, removing ground from the 12 V reference voltage. The relay control module communicates this information to the ECM/PCM through the B-CAN and F-CAN networks of the multiplex system. Upon receiving the A/C OFF information, the ECM/PCM removes ground from the A/C compressor clutch relay coil, deenergizing it. This removes battery voltage from the A/C compressor clutch, which disengages the clutch to stop turning the compressor.
Evaporator Temperature Sensor
The evaporator temperature sensor is located on the evaporator housing. If the temperature at the evaporator gets too cold, the evaporator temperature sensor sends a signal to the climate control unit to turn the A/C off. The climate control unit, in turn, removes ground from the 12 V reference voltage sent from the relay control module. The relay control module communicates the A/C OFF information to the ECM/PCM through the B-CAN and F-CAN networks of the multiplex system. Upon receiving the A/C OFF information, the ECM/PCM removes ground from the A/C compressor clutch relay coil, deenergizing it. This removes battery voltage from the A/C compressor clutch, which disengages the clutch to stop turning the compressor. This prevents condensation from freezing on the evaporator fins and blocking air delivery into the passenger compartment.