Electronic Climate Control

Automatic Temperature Control System:

The automatic temperature control (ATC) system is a split case design, integral blower system and will automatically control the temperature, blower speed, and reduce the relative humidity of air inside the vehicle. Two control levers are provided to maintain the desired temperature and system functions. The system will deliver heated or cooled air to maintain the vehicle interior temperature and will maintain the set comfort level automatically. The blower speed is controlled automatically with the blower control lever in the AUTO position or can be set (LOW, MED, HIGH) for more or less air as desired. In the AUTO position the blower speeds will automatically be reduced as the vehicle interior temperature approaches the set point temperature selected.






Automatic control of the passenger compartment temperature is maintained in all function control settings, except when the system is turned off. In hot weather, it will cool the vehicle to the preset comfort level and then modulate cooling or heating to maintain the desired temperature. In cold weather, provision is made in the FLOOR position of the function lever to delay the operation of the system until engine coolant has warmed enough to minimize the discharge of cold air from the heater outlet. The system will then heat the vehicle to the desired temperature and level out to maintain the preset temperature.

During system operation, outside air is drawn in from the cowl vent just below the windshield at all times, except during maximum cooling when recirculated air is used.

The system utilizes what is called a reheat method to provide conditioned air to the vehicle interior. With this method all airflow from the blower passes through the evaporator core, where it is cooled and dehumidified. Temperature is then regulated by reheating a portion of the cooled dry air and blending it with the remaining cool air to the desired temperature. Temperature blending is automatically varied by the temperature blend door which controls the amount of cooled air that flows through or around the heater core where it is mixed and directed into the distribution plenum. The air is finally directed to the heater ducts, the defroster nozzles or the instrument panel registers according to the selected function lever position.

Maintaining Automatic Temperature Control
Automatic temperature control of the system is accomplished by a microprocessor located on a control module on the control assembly. Inputs from an ambient temperature sensor and an in-car temperature sensor coupled with the control head blower switch and temperature lever settings help determine output signals to the blend door actuator for positioning the blend door, and the variable blower controller for controlling the blower speed. A vacuum selector valve controlled by the function control lever, distributes vacuum to the various vacuum motor doors which in turn direct the airflow through the system.

System Airflow

System Airflow Schematic And Vacuum Control Chart:

System Electrical Wiring Diagram And Continuity Test:

The System Airflow Schematic And Vacuum Control Chart illustrates vacuum routing and airflow during the various system conditions. The System Electrical Wiring Diagram And Continuity Test shows the system electrical wiring diagram including test information.


OFF
In the OFF position the outside-recirculate air door is in the recirculating air position. The PANEL doors are in the no vacuum position, and the floor-defrost door is in the FLOOR (full vacuum) position. The blower is off and the temperature blend door will maintain the position it was in when the system was last turned off.


MAX A/C
With the function lever in the MAX A/C position, the outside air/recirculating door will go to the recirculating position. Air is drawn from the passenger compartment and through the evaporator core. When the passenger compartment begins to approach the selected temperature, the temperature blend door actuator is actuated to a position where part of the air leaving the evaporator core is directed through the heater core. Blended air (the combination of the cooled and heated air) is then mixed in the plenum and directed into the vehicle interior through the panel ducts and registers. The position of the temperature blend door changes automatically to maintain the selected temperature. If the in-vehicle temperature rises above the temperature level setting, the blend door moves to allow more cooled air and less heated air. If the in-vehicle temperature drops below the setting, the blend door moves to allow more heated air and less cooled air. The compressor is allowed to function normally if the outside temperature is above approximately 1° C (30° F).

NORM A/C
With the function lever in the NORM A/C position, the outside air/recirculating door will go to the outside air position. Air is drawn from outside the vehicle, through the cowl vent. The system will then perform as in the MAX A/C position. The compressor is allowed to function normally if the outside temperature is above approximately 1° C (30° F).

VENT
With the function lever in the VENT position, outside air is directed through the panel registers with automatic control of discharge air temperatures, however, refrigerated cooling is not available. VENT and FLOOR positions provide the most economical operations because the compressor does not operate.

FLOOR-(Maximum Heating)
With the function control lever in the FLOOR position and the vehicle interior temperature below the temperature setting, blower operation will be delayed by the Thermal Blower Lockout (TBL) until the engine coolant reaches approximately 49° C (12O° F). When the blower turns on, outside air will pass through the evaporator core and then is diverted by the blend door through the heater core before being distributed through the heater floor ducts into the vehicle interior. A portion of the air is diverted to the defroster nozzles.

FLOOR-(Modulated Heating)
When less heating is required to maintain the temperature setting, the temperature blend door will be positioned so that only part of the air leaving the evaporator core is directed through the heater core. Blended air (the mixture of air directed through the evaporator core and the heater core) is then directed into the vehicle interior. The compressor does not operate in the FLOOR position.

DEF/FLOOR
When the function lever is in the DEF/FLOOR position, the panel doors are in the DEFROST position (no vacuum) and the floor-defrost door is in the mid position (partial vacuum) so that incoming outside air is divided between the defroster nozzles and the floor heat outlets. The compressor is allowed to function normally if the outside temperature is above approximately 0° C (32° F). This helps dehumidify the air.

DEFROST
When the function lever is in the DEFROST position, the PANEL doors and the floor-defrost door are in the full defrost (no vacuum) positions so that all incoming air is directed to the defroster nozzles with a slight bleed to the floor. The temperature blend door modulates in this position, but maximum heat can be obtained by moving the temperature lever to the highest temperature position. The A/C clutch also operates in this position to dehumidify the air and reduce windshield fogging.