186 Auto Heating & Air Conditioning
Copyright by Goodheart-Willcox Co., Inc.
You cannot understand the refrigeration and heating
systems without knowledge of the air delivery system.
Some of the most common problems affecting the HVAC
system are caused by defects in the blower, ductwork, and
air doors. The parts discussed in this chapter are used when
the HVAC system is set to ventilation, heating, air condi-
tioning, and window defrosting.
Air Handling Systems
The delivery system that sends the cooled or heated
air to the passenger compartment is as important as the
operation of the refrigeration system and heater. These parts
are sometimes called the air handling system. This system
generates airfl ow, ensures the air is heated or cooled as
necessary, and delivers the air to the proper place in the
passenger compartment.
Note: The terms upstream and downstream
are used to locate HVAC air handling
components in relation to other compo-
nents. A component receiving incoming air before
another component is upstream of that component.
A component receiving incoming air after another
component is downstream of that component.
Blowers and Motors
The HVAC system could not function properly without
a blower fan and motor. The blower fan is usually placed
upstream of the evaporator and heater core. On a few
vehicles, the blower is downstream from the heater core
or between the heater core and evaporator. Some vehicles
with rear heating/cooling systems may have a second
blower and motor located at the rear of the passenger
compartment.
Blower Wheels
The blower fan consists of a blower wheel resembling
a hamster exercise wheel. These blowers are referred to as
squirrel cage blowers. Figure 12-1 shows the shape and
blade design of a typical squirrel cage blower wheel. The
blower wheel is actually a centrifugal air pump. Blower
wheel blades are shaped so the rotating wheel pulls
air into its center and drives it outward. The shape of
the blower housing directs the air into the center of the
blower wheel and causes it to exit in the proper direction.
The blower wheel may be bolted, press-fi t, or held to the
motor by a clip.
Blower Motors
A direct current motor turns the blower wheel. The
blower motor is connected to the electrical system through
the ignition switch and HVAC controls. When stationary
windings in the motor are energized, current is delivered
to the movable armature. The interaction between the
magnetic fi elds set by the armature and stationary wind-
ings causes the armature to rotate. The armature shaft is
directly attached to the blower wheel. See Figure 12-2.
Some blower motors have a shaft at each end, and turn two
blower wheels. Since the motor operates for long periods
of time, it is usually vented to cool the windings. The motor
must turn in the proper direction, and the fan blades must
be pointed in the proper direction. If the motor or blades
turn backward, air will be pulled out of the passenger
compartment.
Motor Speed Control
For good system operation, it is necessary to be able
to control the speed of the blower. In many cases, three or
four electrical resistors are used to control blower motor
speed. This assembly is usually referred to as a resistor
pack. Selecting the blower speed causes the current to
Figure 12-1. The blower wheel, or squirrel cage, delivers air to
the other HVAC passages.
Power lead
Ground
lead
Motor
Blower
wheel
Figure 12-2. The blower motor drives the blower wheel.
(General Motors)
Blower case
Blower wheel Blower motor
Screws