Electronic Climate Control

Automatic Temperature Control (ATC) Functions:

The automatic temperature control 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 and can also be set in manual override speeds (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 Temperature Control (ATC) System:

Automatic control of 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 the 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 ATC system is determined by the modulated vacuum produced by the sensor assembly which controls the servo vacuum motor. Temperature level changes are accomplished by means of a control cable between the control assembly and sensor assembly. System airflow is automatically controlled with the blower selector in AUTO or may be manually overridden at the control assembly. 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.

Sensor Assembly:

The sensor assembly contains a bimetal operated vacuum modulator, which directly converts air temperature into corresponding values of modulated vacuum. The bimetal senses in-vehicle temperature and is mechanically linked to an internal vacuum modulator to control and regulate a vacuum which is proportional to temperature. The bimetal and the vacuum modulator are placed within a plastic aspirator. Air is diverted by an air scoop near the high-pressure side of the blower (before the evaporator) through a tube to the sensor assembly. The air flows through the aspirator which draws vehicle interior air over the bimetal. This combined air is then exhausted from the sensor underneath the instrument panel where it will not be felt by the passengers or driver.

Modulated vacuum from the sensor is directly connected to the large calibrated vacuum motor portion of the servo assembly. The modulated vacuum positions the blend door attached to the arm of the servo motor assembly. The calibrated vacuum motor moves the blend door to mix various proportions of hot air from the heater core and cold air that has bypassed the heater core. Vehicle air temperatures produce compensating discharge air temperatures, i.e., high in-vehicle temperatures produce high sensor output vacuums which position the blend door to deliver cool discharge air. Low in-vehicle temperatures produce low sensor vacuum levels which cause warm discharge air. A vacuum diverter valve (part of the power servo assembly and also attached to the arm of the servo motor) controls the outside-recirculate air door for maximum cooling (recirculating air) conditions when the function control level is in the PANEL position.

System Airflow

System Airflow Schematic And Vacuum And Vacuum Control Chart:

Blower Switch Chart:

�V�LAutomatic Temperature Control Electrical Wiring Diagram:

The System Airflow Schematic And Vacuum And Vacuum Control Chart illustrates vacuum routings and airflow during the various system conditions. The Blower Switch Chart shows the blower motor voltage and information for each blower switch position. The Automatic Temperature Control (ATC) Electrical Wiring Diagram shows the wiring circuits of the system.

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 assumes a position depending on the modulated vacuum from the sensor to the power servo.

NOTE: A modulated vacuum is produced in ALL function control lever positions and is entirely dependent on the temperature lever setting and the existing in-vehicle temperature conditions.

VENT
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.

PANEL Position-Maximum Cooling-Recirculate
With the function lever in the PANEL position and the vehicle interior temperature considerably above the temperature setting, the modulated vacuum is high at the servo motor, causing:
1. The outside-recirculating door to go to the recirculating air position.
2. The temperature blend door to move to the maximum A/C position, allowing all air to bypass the heater core. Air is drawn from the passenger compartment, directed through the evaporator core where it is cooled and returned to the vehicle interior through the panel ducts and registers.

PANEL Position-Modulated Cooling-Outside-Air
When less cooling is required to maintain the temperature setting, the temperature blend door is actuated to a mid-position where part of the air leaving the evaporator core is directed through the heater core. Blended air (the combination of 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 admit more cooled air and less heated air. If the in-vehicle temperature drops below the setting, the blend door moves to admit more heated air and less cooled air.

HI-LO
Operation is similar to that described in the PANEL position, except that airflow is divided between the panel registers and floor ducts. The system will not recirculate interior air for maximum cooling.

FLOOR Position-Maximum Heating
With the function control lever in the FLOOR position and the vehicle interior temperature below the temperature setting, the modulated vacuum is low, causing the temperature blend door to fully open. Blower operation will be delayed until the engine coolant reaches approximately 49° C (120° F), then the air is forced by the blower through the evaporator and all of the air passes through the heater core before being distributed through the heater floor ducts into the vehicle interior. A portion of this air is diverted to the defroster nozzles.

FLOOR Position-Modulated Heating
When less heating is required to maintain the temperature setting, the temperature blend door has moved to the mid-position so that only part of the air leaving the evaporator core is directed through the heater core. Blended air (the mixture of outside air and heater air) is then directed into the vehicle interior. The A/C clutch does not operate in the FLOOR position.

MIX
When the function lever is in the MIX 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 the incoming air is divided between the defroster nozzles and the floor heat outlets. The A/C clutch operates in this position to 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.