This is a description of the Climate Control System from a 1999 factory service manual I have. To me it looks like the duct doors are controlled by vacuum. There is a electronically controlled actuator motor that I think controls the temperature blending door. I haven't worked on this before so I'm just trying to figure it out by what you see here.
System Air Flow
MAX A/C
View attachment 828468593
When MAX A/C is selected:
l The air inlet duct doors are at full vacuum, closing off outside air and admitting only recirculated air.
l The panel/defrost door is at full vacuum and the floor/panel door is in the no vacuum position, directing airflow to the instrument panel A/C registers (19893).
l The temperature is automatically set for maximum cold (60ºF) but may be raised if desired.
l Air will be picked up at the recirc opening by the blower motor (18527). With the A/C control set for maximum cold, airflow across the A/C evaporator core (19860) will be diverted past the heater core (18476) and then directed into the passenger compartment through the instrument panel A/C registers. There is also some airflow to the A/C side window demisters.
l If equipped with a center console, the rear seat passenger A/C damper door will be at full vacuum, directing airflow to the rear seat passenger A/C registers.
l The A/C compressor (19703) will be enabled when MAX A/C is selected.
l The blower motor is on.
Vent
View attachment 828468594
When VENT is selected:
l The air inlet duct doors, with no vacuum being applied, will block recirculated air and admit outside air. From there, air flows through the system to the instrument panel A/C registers.
l The floor/panel door is in the no vacuum position to block airflow to the heater outlet floor ducts (18C433).
l The panel/defrost door is at full vacuum, closing off airflow to the windshield defroster hose nozzles (18490). There is also some airflow to the A/C side window demisters.
l If equipped with a center console, the rear seat passenger A/C damper door will be at full vacuum, directing airflow to the rear seat passenger A/C registers.
l The temperature can be adjusted to heat the air but air cannot be cooled below outside temperature.
l The A/C compressor (19D629) will be disabled when "VENT" is selected.
l The blower motor is on.
l The A/C compressor will be disabled when "VENT" is selected.
l The blower motor is on.
OFF
View attachment 828468595
When OFF is selected:
l The A/C inlet duct doors are at full vacuum, closing off outside air and admitting only recirc air.
l The floor/panel door and the panel/defrost door are at no vacuum, closing off the passages to the instrument panel A/C registers and heater outlet floor ducts.
l If equipped with a center console, the rear seat passenger A/C damper door will be in the no vacuum position, closing off airflow to the rear seat passenger A/C registers.
l The blower motor and A/C compressor are off.
PANEL/FLOOR
View attachment 828468596
When PANEL/FLOOR is selected:
l The air inlet duct doors are set at no vacuum, blocking the recirc passage and admitting outside air.
l The floor/panel door is in the partial vacuum position, allowing airflow to both the heater outlet floor duct and the instrument panel A/C registers.
l The windshield defroster door is at full vacuum, closing off the majority of airflow to the windshield defroster hose nozzle, a small amount of bleed air to through the defroster nozzle is normal. Airflow will be directed to the instrument panel registers. There is also some airflow to the A/C side window demisters.
l If equipped with a center console, the rear seat passenger A/C damper door will be at full vacuum, directing airflow to the rear seat passenger A/C registers.
l The A/C compressor will be enabled when PANEL/FLOOR is selected.
l The blower motor is on.
FLOOR
View attachment 828468597
When FLOOR is selected:
l The air inlet duct doors are in the no vacuum position, blocking recirc air and admitting outside air.
l The floor/panel door is in the full vacuum position, directing all air flow to the heater outlet floor duct.
l The temperature can be adjusted to mix air flowing through and around the heater core to achieve the desired temperature level.
l The panel/defrost door is in the no vacuum position, blocking air circulation to the panel A/C registers.
l If equipped with a center console, the rear seat passenger A/C camper door will be in the no vacuum position, closing off airflow to the rear seat passenger A/C registers.
l The A/C compressor will be disabled when FLOOR is selected.
l The blower motor is on.
FLOOR/DEFROST
View attachment 828468598
When FLOOR/DEFROST is selected:
l The air inlet duct door is in the no vacuum position blocking recirc air and admitting outside air.
l The floor/panel door is in the partial vacuum position, allowing airflow to both the heater outlet floor ducts and the windshield defroster nozzle there is also some air flow to the A/C side window demisters.
l The panel/defrost door is in the no vacuum position directing airflow to the windshield defroster hose nozzle.
l If equipped with a center console, the rear seat passenger A/C damper door will be in the no vacuum position, closing off airflow to the rear seat passenger A/C registers.
l The A/C compressor will be enabled when FLOOR/DEFROST is selected to dehumidify the air and reduce windshield fogging, if the outside air temperature is above 10º C (50º F).
l The blower motor is on.
DEFROST
View attachment 828468599
When DEFROST is selected:
l The air inlet duct doors are in the no vacuum position, admitting outside air.
l Both the panel/defrost door and the floor/panel door are in the no vacuum position so that most of the incoming air is directed to the windshield defroster hose nozzle. There is also airflow to the A/C side window demisters.
l The temperature setting will determine the amount of air that is directed through the heater core and the amount that bypasses the heater core.
l If equipped with a center console, the rear seat passenger A/C damper door will be in the no vacuum position, closing off airflow to the rear seat passenger A/C registers.
l The A/C compressor will be enabled when DEFROST is selected to dehumidify the air and reduce windshield fogging, if the outside air temperature is above 10º C (50º F).
l The blower motor is on.
Climate Control System Electrical Components