chap49 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 49, the reader should
be able to:

•
•
•
•

Identify the type of HVAC system being used
on a vehicle.

Describe how dual-climate and rear airconditioning systems work.
Discuss vacuum and electric motor controls.
Explain how both heated and cooled air is
blended.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
afterblow • ambient air temperature sensor • aspirator
tube • automatic temperature control (ATC) system
cabin filter
discharge air temperature (DAT) sensor • dual-position
actuator

hybrid electric vehicle (HEV)
photo diode • three-position actuator
variable-position actuator
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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AIRFLOW MANAGEMENT
All automatic heating, ventilation, and air­conditioning (HVAC) 
systems use a combination of the following components to control 
airflow into the passenger compartment:
Vents
Ducts
Air doors (also called flap doors or valves)
The use of these components allows the system to provide airflow 

under the following conditions:
1.
2.
3.
4.

Fresh outside air or recirculated air
Air conditioning
Defrost
Heat

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Continued
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Figure 49–1 The three major portions of the A/C and heat system are air inlet, plenum, and air
distribution. The shaded portions show the paths of the four control doors.

Continued
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Typical settings for a manual or automatic air­conditioning system.
Heat

Temperature set to the desired setting
Air  intake—select  outside  air  (for  faster  heating,  select 
recirculation for the first few minutes)

Air conditioning set to off
Set airflow to flow
Fan speed to desired speed

Continued
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Typical settings for a manual or automatic air­conditioning system.
Air­Conditioning

Temperature set to the desired setting
Air  intake—set  to  outside  air  (for  faster  cooling,  select 
recirculation for the first few minutes)

Airflow—select dash vents (also called panel vents)
Air conditioning set to on
Fan speed set to desired speed

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Typical settings for a manual or automatic air­conditioning system.
Ventilation

Temperature set to lowest temperature
Air intake—select outside air
Airflow—set to dash (panel) vents

Air conditioning set to off
Fan speed set to desired speed

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Typical settings for a manual or automatic air­conditioning system.
Defogging or Defrosting the Inside of the windshield

Temperature set to high temperature
Air intake set to outside air
Airflow set to windshield
Fan speed set to desired speed

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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What is Afterblow?
Afterblow is a term used to describe the operation of the blower motor
after the ignition has been turned off. The purpose of afterblow is to dry the
evaporator to help prevent the formation of mold and mildew in the
evaporator case. The operation of the blower motor after the ignition is
turned off has created some customer complaints.
Check service information to be sure that the condition is normal or
not on the vehicle being investigated. For example, in a typical
General Motors system, the following conditions must be met for

afterblow to occur:
1. The engine has been off for 30 minutes.
2. The outside air temperature is 70°F (21°C) or higher.
3. The battery voltage is 12 volts or higher.
If the above conditions exist, the afterblow is commanded to be on
for 20 seconds, off for 10 seconds, and back on for another 20
seconds.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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AUTOMATIC AIR-CONDITIONING
Automatic temperature control (ATC) systems are similar to a 
normally adjusted system but with additional sensors.

Outside Air Temperature (OAT) Sensor  Usually located at the 
front of the vehicle behind the grille but in front of the radiator. 
This sensor, commonly 
called the ambient air 
temperature sensor and
is also supply temperature 
information for the driver 
on a display. 

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 49–2
The ambient temperature sensor in
this system is located in the fresh air
intake duct for the HVAC system.
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Inside Vehicle Temperature Sensor  Older ATC systems used a 
sensor located behind the instrument panel. Air to the sensor was 
forced to flow past the sensor by using an aspirator tube, which 
was connected to the blower motor case.
Discharge Air Temperature Sensor (DAT) Located at the outlet of 
the vents. The purpose of this sensor is to inform the controller of 
the actual temperature at the discharge ducts.
Evaporator Outlet Temperature Sensor  Used to control the AC 
compressor to keep the evaporative temperature within the specified 
temperature range for most efficient operation.

Continued
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Sunload Sensor  Mounted on the dash, they adjust temperature and 
fan speed to match increased heating through the windows from the 
sun. A common type of sunload sensor is a photo diode.
NOTE: Some vehicles are equipped with a dual­zone sunload sensor that
has two sensors included. This sensor allows the system to automatically
adjust the airflow and air temperature based on the actual sun intensity
experienced by both the driver and the passenger.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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ACTUATORS
Actuators move vanes or valves. Actuators in air­conditioning 
systems are electric or vacuum operated. Here are three types:
Dual­Position Actuator  Able to move either open or closed. An 
example is the recirculation door, which can be open or closed.
Three­Position Actuator  Provides three air door positions, such 
as the bi­level door, to allow defrost only, floor only, or a mix.
Variable­Position Actuator  Capable of positioning a valve in any 
position. All variable­position actuators use a feedback 
potentiometer, which is used by the controller to detect the actual 
position of the door or valve.
See Figure 49–3.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 49–3 A block diagram showing the inputs to the electronic control assembly and the
outputs; note that some of the outputs have feedback to the ECM.

See the chart on
Page 567 of your
textbook. 

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CABIN FILTERS

Most late­model air­conditioning systems include a cabin filter, 
which is an air filter in the outside air inlet.
The purpose of the cabin filter is to filter dirt and dust from the air 
before it enters the interior of the vehicle.
Cabin air filters can be accessed either in the dash, usually behind 
the glove box, or from under the hood.
Cabin air filters should be replaced regularly, usually every two 
years during normal service and more often if the vehicle is driven 
in dusty areas.
See Figure 49–4.
Continued
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Figure 49–4 A typical cabin filter being removed from behind the glove compartment.

NOTE: Some cabin filters contain activated charcoal which absorbs

hydrocarbons and helps to deodorize the air as it enters the interior. For
best results, use the designated replacement filter.
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VACUUM CONTROL CIRCUITS
Vacuum control circuits use 
vacuum created in the intake 
manifold of the engine. 
Because vacuum decreases

close to zero during heavy 
acceleration, a vacuum 
accumulator is used to
store vacuum during short 
periods of acceleration. 
Figure 49–5 With no vacuum signal, the spring
extends the actuator shaft to place the door in
a certain position (top). A vacuum signal pulls
the shaft inward and moves the door to the
other position (bottom).

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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ELECTRIC SERVOMOTOR CIRCUITS
Most HVAC systems use electric motors to move valves and doors. 
Each servomotor contains a feedback potentiometer, which is used 
by the air conditioning control unit to indicate the actual position 
of the valve or door.
If the commanded position and the actual position are not the same, 
then most systems are designed to store a diagnostic trouble code 
indicating which door is out of calibration.
See Figures 49–6 and 49­7.

Continued
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Figure 49–6 Three electric actuators can be
easily seen on this demonstration unit.
However, accessing these actuators in a
vehicle can be difficult.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 49–7 The feedback circuit
signals the AC control unit with the
blend door position.

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BLOWER MOTOR CONTROL
Blower motors are used to move air. The air is directed by the 

doors of the HVAC system.
Most blower motors use resistors to control speed of the motors 
by dropping the amount of current flow through the motor at the 
lower speed. The resistor lowers voltage and current to the motor.
The control allows full system voltage to be applied to the motor 
during high­speed operation. The blower motor resistor is always 
located in the plenum near the blower motor so that airflow past 
the resistor can help keep it cool.
See Figures 49–8 and 49–9.
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Figure 49–8 A typical blower motor assembly with attached squirrel cage blower. A

replacement motor does not include the squirrel cage blower so it needs to be switched to the
replacement.

Figure 49–9 A “credit card” resistor used
in the rear blower assembly in a Dodge
minivan.

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DUAL-ZONE AIR CLIMATE CONTROL
Dual­zone climate controls allow the driver and the passenger to 
select different temperatures, as much as a 30°F (17°C) difference.
In a dual­zone climate control system, the ducts and airflow are 

split and two air mix doors are used, with each door being 
controlled by its own actuator.
See Figure 49–10.

Continued
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Figure 49–10 A dual climate control system showing the airflow and how it splits.
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REAR AIR-CONDITIONING SYSTEM
Many larger trucks, vans, and sport utility vehicles (SUVs) are 
equipped with rear heat and air­conditioning units. Many vehicles 
are equipped with ducts that route heated or cooled air to rear­seat 
passengers. 

Figure 49–11 A typical dualzone climate control panel
showing left and right side
temperature control levers.

Continued
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