Chapter 12 Air Delivery Systems 187
Copyright by Goodheart-Willcox Co., Inc.
fl ow through one of these resistors before reaching the
blower. Each resistor reduces the amount of current fl ow-
ing into the blower by a different amount, causing blower
speeds to be different in every speed selection. On a
few HVAC systems, energizing different windings in the
motor controls motor speed. Most HVAC systems have four
blower speeds.
The highest blower speed is usually a direct connec-
tion that uses no resistors. High speed is usually selected
through a blower relay. A simplifi ed electrical schematic of
the blower control system is shown in Figure 12-3.
On the three lower speeds, power travels through the
switch and resistor assembly to reach the blower motor.
When high blower is selected, a relay is energized, sending
current directly to the blower, and bypassing the resistor
assembly. On systems that do not use a blower relay, cur-
rent is sent directly through the high blower switch contacts
to the blower motor.
Automatic Motor Speed Control
Some modern vehicles use an on-board computer to
energize the blower. Blower speeds are selected according
to computer inputs. Instead of using mechanical contacts,
the computer uses internal electronic circuitry with power
transistors and resistors. To reduce current fl ow through the
computer, the control system may use a high blower relay
to control the blower motor.
Afterblow Module
The evaporator core on some vehicles may have a
tendency to retain condensed moisture. This moisture
causes these evaporators to develop mold on their sur-
faces. Mold spores are always present in the air and may
form large colonies on the evaporator surface if moisture
does not dry between system uses. Mold can cause a
musty odor in the evaporator case. These odors will enter
the passenger compartment when the HVAC system is
used. To prevent this, the evaporator must be allowed to
dry after each use.
To dry out the evaporator, an afterblow module ener-
gizes the blower motor for a set period of time after the
vehicle’s engine has been shut off. Some afterblow mod-
ules allow the blower to run as soon as the engine is turned
off. Others energize the blower motor after the vehicle has
been parked for awhile, usually anywhere from 30-60 min-
utes. Air movement over the warmed evaporator core helps
to remove any remaining moisture, reducing the chance for
mold formation.
Exhaust Gas Purging
On all vehicles made since 1977, the blower motor
runs when the engine is running, whether or not the HVAC
system is being used. Running the blower at all times
provides a constant fl ow of air through the passenger
compartment. This purges any carbon monoxide that may
enter from a leaking exhaust system. The ignition switch
energizes the blower through the resistor assembly, causing
the motor to turn at its slowest speed.
Most modern vehicles have a temperature switch
installed between the ignition switch and the blower motor.
The switch opens to de-energize the blower motor when
coolant temperatures are low. This reduces driver discom-
fort when the vehicle is fi rst started in cold weather. Modern
engines heat up quickly and the blower is energized before
signifi cant amounts of carbon monoxide can form.
Air Ducts and Heater/Evaporator
Cases
Air ducts are tubes or passageways the HVAC system air
passes through. The sets of ducts in a vehicle are called the
ductwork. Air also passes through the heater and evaporator
cases on its way to the passenger compartment. Note many
of the components are combined into a single module. For
purposes of explanation, this discussion will treat the various
components as though they were separate units.
Figure 12-3. Simple schematic for the blower circuit. (General Motors)
+12V ALT
RED
BLOWER
RELAY
BLACK
BLOWER
MOTOR
PURPLE
M
DARK
BLUE
RESISTOR
BLOCK
TAN
LT GREEN
LT BLUE