194 Auto Heating & Air Conditioning
Copyright by Goodheart-Willcox Co., Inc.
vehicle, usually with brackets formed into the sheath. The
wire is attached to the dashboard lever and the door crank.
Moving the lever causes the cable to move the door crank.
Vacuum Diaphragms
Vacuum diaphragms, Figure 12-19, are sometimes
used to operate various diverter doors. They are sometimes
called vacuum motors. The diaphragm consists of a sealed
chamber connected to the engine manifold vacuum through
hoses. The hoses are directed through the HVAC control
system, which applies or releases vacuum according to
driver input.
The chamber is connected to the door through link-
age. A spring inside of the diaphragm opposes movement
of the diaphragm. Applying vacuum causes the diaphragm
to move against spring pressure to move the door. When
vacuum is removed, the spring returns the door to its origi-
nal position. Some diaphragms have more than one spring
or a variable rate spring. This spring arrangement allows the
diverter door to be partially opened when a certain amount
of vacuum is applied, and to open completely when more
vacuum is applied.
Some diaphragms are double diaphragm types. Using
vacuum on both sides of the diaphragm allows it to be posi-
tioned exactly, or to perform more than one function. Vacuum
diaphragms are sometimes controlled by electric solenoids.
Solenoids will be covered in more detail in Chapter 13.
Figure 12-16. An HVAC Dual Zone air delivery system. The purpose of the extra doors is to better control the comfort levels between
the driver and front seat passenger. (General Motors)
Vacuum
Source
(Engine Running)
PPL
6
2
#2
Solenoid
Vent
#1
Solenoid
Vent
#5
Solenoid
Vent
#3
Solenoid
Vent
#4
Solenoid
Vent
Vacuum/
Electric
Solenoid
ORN 4 BRN 3 BLU 5 YEL 1 RED
Heater/Defroster
Mode Valve
Vacuum Actuator
A/C Bi-Level
Mode Valve
Vacuum Actuator
Air Inlet
Valve
Vacuum Actuator
Valves
shown at rest
Solid
State
LH Electric
Actuator
Outside Air
Inlet
Air
Inlet
Valve
B
A
Blower
Solid
State
RH Electric
Actuator
In-Car
Air Inlet
Passenger Air
Temperature Valve
A/C Evaporator
Core
Heater
Core
Cold
Mid
Warm
Warm
Mid
Cold
Driver Air
Temperature Valve
Defroster Valve
Driver/Passenger
Heater Outlet
Valve
C
B
A
A
B
C
A
B
C
Driver/Passenger
Vent Valve
Driver/Passenger
Heater Outlet
Driver/Passenger
IP Vent
Defrost
Outlets
Figure 12-17. A system with upper and lower mode doors. The
upper and lower door concept is widely used on General Motors
vehicles. (General Motors)
Defroster
door
Heater
core
Evaporator
Blower
Recirculation
door
Blend
door
Bi-level
doors
Heater
outlet