Electronic Climate Control

Automatic Operation

Automatic control of system temperature is accomplished with the use of a microprocessor located in a control module attached to the control assembly. The microprocessor analyzes inputs from the ambient temperature sensor, in-vehicle temperature sensor, the temperature selector lever and the blower switch. The microprocessor then operates the blend door electric motor to position the temperature door as required to obtain the desired discharge air temperature. If the blower switch is in the AUTO position, the microprocessor also determines and selects the blower speed automatically.

The function selector switch, operated by the movement of the function selector lever, directs vacuum to the air door vacuum actuators (motors) as required to discharge the air to the selected locations. The switch also supplies electrical power for blower operation in all positions except OFF and for A/C clutch operation when required.

System Air Flow

The previous illustration contains a system vacuum schematic and vacuum motor chart which illustrates vacuum routings and air flow during the various system conditions.

OFF

In OFF position the outside/recirc air door is in the recirculating air position. The PANEL door is in the no vacuum position, and the floor-defrost 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 turned off.

MAX A/C

With function selector lever in the MAX A/C position, the outside / recirc door will go to the recirculating position. Air is drawn from the passenger compartment and forced through the evaporator core. When the passenger compartment begins to approach the selected temperature, the temperature blend door actuator is moved 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 direct less air through the heater core allowing more cooled air to bypass the heater core. If the in-vehicle temperature drops below the setting, the blend door moves to direct more air through the heater core with less going around.

The A/C compressor will function normally it the ambient temperature is above approximately 50°F. This is controlled by the clutch cycling pressure switch which is operated by refrigerant pressure.

NORM A/C

With the function selector lever in the NORM A/C position, the outside/ recirc door will go to the outside air position. Air is drawn from outside the vehicle, through the cowl air inlet. The system will then perform as described under MAX/A/C.

The compressor will function normally if the outside temperature is above approximately 50° F.

VENT

With function selector lever in the VENT position, outside air is discharged through the instrument panel registers. The system will automatically heat the air if necessary but cannot cool the air. The VENT and FLOOR positions are the most economical operating positions because the compressor does not operate.

FLOOR (Maximum Heating)

With the function selector lever in the FLOOR position and the vehicle interior temperature below the temperature setting, blower operation will be delayed (by the Cold Engine Lock Out) until the engine coolant reaches approximately 49°C (120°F). When the blower turns on, the outside / recirc door will open to outside air. All air flow will pass through the evaporator core (not cooled) and then is diverted by the blend door thr9ugh the heater core before being distributed through the heater floor ducts into the vehicle interior. A small portion of the air is bled 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 and around the heater core) is then directed into the vehicle interior. The compressor does not operate in the FLOOR position.

DEF/FLR (Defrost/Floor)

When the function selector lever is in the DEF / FLR position, the panel door is in the DEFROST position (no vacuum, closed to panel) and the floor-defrost door is in the mid position (partial vacuum) so that discharge 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 50°F This helps dehumidify the air when the compressor operates.

DEF (Defrost)

When the function selector lever is in the DEFROST position, the panel door 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 providing the ambient temperature is above approximately 50°F or above.

Control Assembly

The control assembly consists of three main parts.
1. The control assembly frame includes the levers, face and knobs.

2. The function selector switch is mechanically connected to the function selector lever. This switch is similar to those used in Manual A/C-Heater systems. It directs source vacuum to the three vacuum actuators as required for the function selected. The switch also connects the thermal blower lock out (TBLO) vacuum circuit to the outside/recirc door when FLOOR is selected. Three single pole, single throw electrical switches are also an integral part of the switch and controlled by the position of the function selector lever. These switches supply electrical power for blower and A/C clutch operation.

3. The ATC control module is attached to the top of the control assembly and is connected to the blower switch with wires. The blower switch is attached to the back of the control assembly frame with the switch lever extending through a slot in the control face. The microprocessor within the control module analyzes inputs from the ambient sensor, in-vehicle sensor, the position of the temperature lever and the blower switch position. The microprocessor then operates the electric motor attached to the temperature blend door to achieve the selected in-vehicle temperature. If the blower switch is in the AUTO position, the blower speed is also automatically controlled through the blower motor speed controller by the microprocessor. When not in AUTO, blower speed is manually controlled through the microprocessor by the position of the blower switch. When the function selector lever is in FLOOR the TBLO interrupts the blower circuit until the engine coolant reaches or exceeds approximately 120°F.