Air Distribution System
Air enters the heater and air conditioning evaporator module where it may be cooled for air conditioning or defrost modes, and then heated as necessary before being sent to the proper outlets for discharge into the passenger compartment. Within the heater and air conditioning evaporator module, valves (doors) guide the airflow in response to the position of the mode control.
Other components of the air distribution system include the plenum, the blower motor, the evaporator and the heater core. The plenum provides a source of outside air to the system. The blower motor draws air from the plenum or from the vehicle interior into the heater and air conditioning evaporator module and forces it through the air distribution system to the outlets. The evaporator is a part of the refrigeration system. It cools the air from the blower motor when operating in an air conditioning or defrosting mode. The heater core provides heat which can be used to warm the air downstream from the evaporator.
Air Inlet Valve
The first valve in the air distribution system is the air inlet valve. This two-position valve controls the two air entrances which feed air to the blower motor. When "MAX" A/C mode is selected, vacuum is applied to the air inlet vacuum actuator and the air inlet valve opens the entrance which supplies inside air from the passenger compartment In this position, the air inlet valve partially closes the entrance for outside air so only a slight amount can enter. Re-circulating the cooled air inside the passenger compartment when outside temperature is very high gives a faster cool down to a more comfortable temperature.
When any other operating mode (including "OFF") is selected, the air inlet vacuum actuator is vented to normal air pressure and the air inlet valve completely closes off the entrance for inside air. Only fresh outside air is used in the system in these modes.
Blower Motor and Evaporator
In any operating mode except "OFF," the blower motor draws in air from the plenum and pushes it through the evaporator. When any air conditioning or defrost mode is selected, the air conditioning refrigeration system chills the core of the evaporator to slightly above freezing temperature. This cools the airflow and causes excess humidity to condense on the fins of the evaporator. The condensed moisture drips to the bottom of the case and drains out through a hole that lets the water fall on the road surface below the vehicle. From the evaporator, the air flows toward the temperature valve and heater core.
Temperature Valve and Heater Core
The temperature control in the control is linked to the temperature valve by a flexible control cable. When the temperature control is rotated fully counterclockwise to the full "COLD" position, the temperature valve is held snugly against the air entrance to the heater core. This causes all of the airflow from the evaporator to bypass the heater core and there is no heat transfer.
As the temperature control is moved clockwise away from the full "COLD" position, the temperature valve begins to direct some air to the heater core to allow some air to flow through it. The farther the temperature control is rotated clockwise, the more the temperature valve directs air through the heater core. Heating a greater proportion of the total airflow in this manner results in a warmer discharge of air because the heated and unheated air flows join and are mixed together thoroughly beyond the heater core.
When the temperature lever is rotated fully clockwise to the full "HOT" position, the temperature valve blocks off the passage that allows air to bypass the heater core and causes all of the airflow to pass through the heater core.
Bi-Level Valve
The bi-level valve (8) at its at rest (no vacuum) position, allows no air to the instrument panel outlets. The bi-level valve (8) can be opened by either the bi-level valve vacuum actuator or the upper mode valve vacuum actuator. When opened by the bi-level valve vacuum actuator, as in "BI-LEVEL A/C" air will exit the instrument panel outlets, floor outlets and a small amount will exit the defroster outlets.
When the bi-level valve (8) is opened by the upper mode valve (9) vacuum actuator, as in "MAX" A/C, normal "A/C" and "VENT" modes both valves move at the same time, directing all airflow through the instrument panel outlets.
Heater and Defrost Valve
The heater and defrost valve (7) is operated by a three-position heater and defrost vacuum actuator. At its "at rest" (no vacuum applied to either side) position, the heater and defrost valve (7) is positioned to allow airflow through both the floor outlets and defroster outlets, as when in "DEFROST BLEND" mode During the "DEFROST" mode, vacuum is applied to one side of the heater and defrost vacuum actuator, causing the heater and defrost valve (7) to direct all airflow to the defroster outlets. A small passage within the heater and air conditioning evaporator module (11) will allow some air to bleed out the floor outlets. During the "HEATER" mode, vacuum is applied to the opposite side of the heater and defrost vacuum actuator causing the heater and defrost valve (7) to direct all airflow to the floor outlets with a small bleed to the defroster outlets.
Ducts and Outlets
There are adjustable outlets in the instrument panel. In addition, there are two side window defrost outlets, one at each end of the instrument panel. All outlets are fed by the air distributor.
Heated or cooled air can be directed to the windshield through the air distributor and through the floor air outlet duct.
Vacuum System
All valves in the heater and air conditioning evaporator module except the temperature valve are controlled by engine vacuum drawn from the vacuum tank, mounted to the right side body rail, just behind the radiator support.
The vacuum is routed by a vacuum selector valve in the control to spring-loaded vacuum actuators on the heater and air conditioning evaporator module. Vacuum stored in the vacuum tank is prevented from bleeding back to its source during conditions of low engine manifold vacuum (acceleration, hard pulling, etc.) by a check valve in the vacuum source hose.
The vacuum selector valve in the control is controlled by the mode control. For each mode control position, the vacuum selector valve directs vacuum to specific hoses to overcome the force of the spring within the appropriate vacuum actuators. The vacuum selector valve also vents the hoses to each remaining vacuum actuator to normal air pressure, allowing the actuator spring to expand. This results in the valves (doors) being properly positioned for the selected operating mode.
The heater and defrost valve uses a three-position vacuum actuator. A three-position vacuum actuator has a diaphragm with a spring on both sides. At rest (no vacuum applied) the diaphragm will be in a mid-position, allowing for a "blend" mode. When vacuum is applied to either end of the diaphragm, the diaphragm will move the heater and defrost valve to either stop, which will direct airflow to either the defroster outlets or the floor outlets.
When air is directed to the floor outlets, as in "HEATER" mode, a small amount of air still bleeds through the defroster outlets to help eliminate re-fogging of the windshield.
Ventilation
The air distribution system provides fresh air ventilation for the passenger compartment in all operating modes. A small amount of outside air is still provided in the "MAX" A/C mode while most of the air is re-circulated. Even in the "OFF" mode, fresh outside air is forced into the air distribution system by the forward movement of the vehicle.
After being warmed or cooled to the desired temperature, the air is discharged from the outlets. When all windows are closed, the addition of outside air builds a slight positive pressure within the passenger compartment. The pressure forces stale air to leave the vehicle through a pressure relief valve, near the left side quarter panel, behind the rear fascia.