Circuit Description
HVACHow the Circuit Works
The climate control unit controls the blower controls, air delivery, and A/C compressor controls either automatically or semi-automatically. With the ignition switch in ON (II), battery voltage is supplied to the climate control unit through fuse 3 (in the driver's under-dash fuse/relay box). The control unit is grounded at G401.
Fully-automatic Operation
To put the climate control in fully automatic mode, press the AUTO button and set the desired temperature by turning the temperature control dial. FULL AUTO will be displayed in the system's display.
The system automatically selects the proper mix of cooled and/or heated air that will, as quickly as possible, raise or lower the interior temperature from its current level to the set temperature. The system also adjusts the fan speed and changes direction of air delivery.
When the temperature is set to its lower limit (60 degrees F/18 degrees C) or its upper limit (90 degrees F/32 degrees 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.
Semi-automatic Operation
Various functions of the climate control system can be manually selected when it is in FULL AUTO. All other features remain automatically controlled. Making any manual selection causes the word FULL to go off.
However, when A/C OFF is selected, the system cannot regulate the inside temperature if the dial is set lower than the temperature outside.
Blower Controls
The fan speed is automatically controlled in the fully-automatic mode. The fan speed can be manually selected by turning the fan control dial clockwise, just past the AUTO position. This takes the fan from automatic mode to its lowest manually controlled speed. Turning the dial further clockwise increases the fan's speed, which increases air flow.
Battery voltage is applied through fuse 56 (in the under-hood fuse/relay box) to the front blower motor relay contacts at all times. With the ignition switch in ON (II), the front blower motor relay is energized, which feeds battery voltage to the front blower motor. The front blower power transistor controls the front blower motor in all speeds except HIGH. The front blower power transistor is controlled by the climate control unit. When the control unit requests HIGH blower speed, it connects the front blower motor directly to ground, making the blower run at high speed.
Battery voltage is applied through fuse 52 (in the under-hood fuse/relay box) to the rear blower motor relay contacts at all times. With the ignition switch in ON (II), the rear blower motor relay is energized, which feeds battery voltage to the rear blower motor. The rear blower power transistor controls the rear blower motor in all speeds except HIGH. The rear blower power transistor is controlled by the rear heater-A/C control panel-unit. When the control panel-unit requests HIGH blower speed, it connects the rear blower motor directly to ground, making the blower run at high speed.
In cold weather, the blowers will not come on automatically until the heater starts to produce warm air.
Air Delivery
The climate control unit controls the front blower motor and supplies a 5 VDC reference voltage to the front air mix control motor.
The front air mix and front mode control motors each receive inputs from the control unit. The front air mix motor regulates the mixture of cold and hot air by varying the position of the heater-evaporator door. The front mode control motor controls the direction and volume of outlet air. Use the MODE button to select the vents the air flows from. Some air will flow from the dashboard corner vents in all modes. Each time the MODE button is pressed, the display shows the mode selected. Press the button four times to see all the modes. Both the front air mix control motor and front mode control motor are grounded by the control unit.
The recirculation control motor receives battery voltage through fuse 3 (in the driver s under-dash fuse/relay box) when the ignition switch is in ON (II). It regulates the position of the fresh/recirc door, and is controlled by two position inputs from the control unit ( "Recirc" and "Fresh" ).
The rear heater-A/C control panel-unit controls the rear blower motor and supplies a 5 VDC reference voltage to the rear air mix control motor.
The rear air mix and rear mode control motors each receive inputs from the control panel-unit. The rear air mix motor regulates the mixture of cold and hot air by varying the position of the heater-evaporator door. The rear mode control motor controls the direction and volume of outlet air. Use the MODE button to select the vents the air flows from. Both the rear air mix control motor and rear mode control motor are grounded by the rear heater-A/C control panel-unit.
A/C Compressor Controls
Battery voltage is provided at all times to the A/C compressor clutch relay contacts through fuse 59 (in the under-hood fuse/relay box). With the ignition switch in ON (II), battery voltage is supplied to the relay coil through fuse 3 (in the driver's under-dash fuse/relay box).
With the ignition switch in ON (II), the climate control unit grounds the A/C ON input of the PCM through the A/C pressure switch. The PCM then grounds the A/C compressor clutch relay coil. This energizes the relay, which closes the relay contacts and provides voltage through fuse 59 (in the under-hood fuse/relay box) to the A/C compressor clutch. The clutch then engages and begins turning the compressor. By default, when the climate control system is on, the A/C is also on.
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 off the A/C compressor clutch. This prevents condensation from freezing on the evaporator fins and blocking air delivery into the passenger compartment.
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. The switch will sense abnormally high or low pressure, and open the circuit. This removes ground from the PCM, and turns off the compressor.