Heating and Air Conditioning: Description and Operation

AIR DELIVERY

Electronic A/C Control Assembly

With the Ignition Switch in "RUN," voltage is applied through Fuse 3 to the Electronic A/C Control Assembly. The operating mode of the system is determined by buttons on the front panel. Ground signals determine which air valves are to be operated as well as the position of the Temperature Valve.

Solenoid Module

The Solenoid Module contains the solenoid valves that control the vacuum actuators. When the solenoids are de-energized, the valves vent the vacuum actuators, allowing the vacuum actuator to return to the Vent position.

Recirculation Valve

The Recirculation Valve closes off the outside air inlet so that the air from the inside of the vehicle is recirculated through the A/C System for maximum cooling. The outside air inlet is closed when vacuum is applied to the actuator. When the vacuum actuator is vented, the Recirculation Valves close off the in-car air inlet which allows outside air to be drawn into the vehicle.

Temperature Valve

The Temperature Valve determines how much air flows through the Heater Core, which determines the temperature of the air to the outlets. It is moved by the Air Mix Door Motor. The position of the valve is determined by the temperature selected at the Electronic A/C Control Assembly.

Heat/Def

The Heat/Def Valve determines whether air flow will be directed to the windshield defrost outlets. With no vacuum applied to the actuator, the valve is positioned to allow air flow to the windshield defrost outlets.

Upper/Lower Mode

The Upper/Lower Mode Valve can assume three positions and is operated by a bi-directional vacuum actuator which can move the valve to either of two positions. If no vacuum is applied to either vacuum actuator, the valve is in midposition. Air flow with the valve in midposition will be delivered to the panel outlets and the floor outlets. When the Bi-Level Solenoid (#3) (red vacuum hose) is operated, the valve is positioned to direct air flow to the floor outlets. When the A/C Solenoid (#5) (tan vacuum hose) is operated, the valve is positioned to direct air flow to the panel outlets.

Heater Shut-Off Air Valve

The Heater Shut off Air Valve is used to block any air attempting to exit the Heater Core when temperature control is set to 16°C (60°F). Otherwise, it rotates out of the way so as not to block any air flow.

TEMPERATURE CONTROL

Three temperature variable resistors (sensors) are connected to the Electronic A/C Control Assembly to provide in-car and outside temperature information. These devices are thermistors which decrease in resistance as the temperature of the device increases. They are connected to fixed resistors in the Control Assembly to provide a voltage input which varies according to the change in resistance. Ground reference for the sensors is provided by the Control Assembly.
Circuits within the Control Assembly use the input from the In-Car Sensor and the Sun Load Sensor and the set temperature selected by the driver to determine whether the air should be heated or cooled to bring the inside of the vehicle to the selected temperature. The input from the Ambient Temperature Sensor is used to determine the amount of heating or cooling required to bring the temperature inside the vehicle to the selected value as quickly as possible.
The Air Mix Door Motor controls the position of the Temperature Valve. The motor can be operated in either direction by the polarity of the voltage applied to it. When the solid state switch grounds terminal "F" in the Electronic A/C Control Assembly, then terminal "5" of the Air Mix Door Motor is positive. If the condition of the solid state switches is reversed and terminal "G" is grounded, then terminal "6" of the Air Mix Door Motor is positive and the motor runs in the reverse direction.
The Position Feedback Pot indicates the position of the Temperature Valve at any point of its travel. As the Temperature Valve moves between fully open and fully closed, the voltage input at terminal "6" on the Electronic A/C Control Assembly will vary between zero and 5 volts.
When a problem is detected with the temperature control system, certain malfunction codes may be set.