Chapter 10

Case and Duct Systems

After studying Chapter 10, the reader will be able to:1. Understand the function of the air control doors in the A/C

and heating duct system. 2. Understand how the temperature of the air entering the

passenger area is controlled.3. Be familiar with the methods used to control the blower

speed. 4. Be familiar with manual, semiautomatic temperature

control (SATC), and automatic temperature control (ATC) systems.

5. Be familiar with the sensors, actuators, and controls used with ATC and SATC systems and their function in controlling air temperature and flow.

OBJECTIVES

Objectives• Identify types of case/duct systems.• Discuss air distribution through the case/duct

system.• Describe the airflow through the case/duct

system for defrost mode, heat mode, and cool mode.

• Discuss how to control unpleasant HVAC odor.

• Explain the need for and location of cabin air filters.

• Discuss Mode Door Actuator operation.

Objectives• Discuss the removal and replacement of the blower motors.• Discuss the removal and replacement of the blower motor

resistor or power module.• Discuss the removal and replacement of the heater core.• Discuss the removal and replacement of the evaporator.• Odor control treatment of the case and duct system.• Test the vacuum system.• Discuss temperature control door adjustment.• Discuss the removal and replacement of the replace cabin air

filter.• Troubleshoot, service, and adjust the operation of the in-

vehicle mode circuits such as vent, HI-LO, MAX (cool/heat), and defrost.

Types• Independent• Compact or small cars• Upstream or

downstream blowers• All inside passenger

compartment• Split-case• Upstream or

independent blower• Larger cars• Both sides of firewall

Air Distribution• For simplicity of

understanding a generic case system will be used. It is divided into three sections:– Air Intake– Plenum containing heater

core and evaporator– Air distribution

• Basic airflows for all vehicles

• Multi-speed blower moves air through system

• Directed by blend doors

Blend Door

• The blend door regulates duct air discharge temperature.

Air Distribution Modes

• Defrost - windshield • Floor - heater• Vent - instrument panel (dash)• Bi-level - vent and floor• From outside vehicle - fresh air• Recirculation mode - from inside vehicle

(in-car)

Air Distribution Modes

• MAX cool - in-car air, through evaporator, out vents or bi-level

• MAX heat - in-car air, through heater, floor or bi-level, some windshield

• NORMAL cool - outside air, through evaporator, out vents or bi-level

• NORMAL heat - outside air, through heater, out floor or bi-level, some windshield

Basic Air Distribution System

• The blower motor draws air in and distributes based on mode door position.

Defrost - windshield

• DEFROST - outside air, through heater, out windshield, some to floor– A/C compressor may

be on in some conditions.

Floor - heater

• From outside vehicle - fresh air

Vent - instrument panel (dash)

• VENT - unconditioned outside air, through heater or evaporator (both off), out vents or bi-level

Bi-level - vent and floor

• From outside vehicle - fresh air

MAX cool - in-car air, through evaporator, out vents or bi-level

• Recirculation mode - from inside vehicle (in-car)

MAX heat - in-car air, through heater, floor or bi-level, some windshield

• Recirculation mode - from inside vehicle (in-car)

NORMAL cool - outside air, through evaporator, out vents or bi-level

NORMAL heat - outside air, through heater, out floor, some windshield

Mode and Blend Doors

• Mode doors control where air exits duct system.

• Blend doors control air temperature in duct system.

• Cable controlled• Vacuum-controlled actuators• Electronically controlled actuators

Mode Door Positions

Dual Zone System• The dual-zone duct

system has a separate driver- and passenger-side duct system.

• Both sides have a defrost/panel vent/ floor outlet that operate together.

• The passenger only has control of the passenger side temperature on most systems.

Dual Zone System

• The passenger manually controls the position of the passenger-side temperature air door.– Controls discharge

temperature of the passenger-side outlets between full hot (A) and full cold (B).

Dual Zone Rear Heat/Cool• Dual zone Front seats• Driver controls overall

system operation. Either heat or air conditioning.

• Passenger/driver select individual temperature

• Passenger cannot shut down system

Dual Zone Rear Cooling• Dual zone Front seats• Driver controls overall system

operation. Either heat or air conditioning.

• Passenger/driver select individual temperature

• Passenger cannot shut down system

• Rear Compartment Cooling– Second evaporator core near

rear of passenger compartment

– Some systems allow rear passengers to control temperature.

– Rear blower, controls in front, rear, or both

Dual Zone Rear Heating• Dual zone Front seats• Driver controls overall system

operation. Either heat or air conditioning.

• Passenger/driver select individual temperature

• Rear Compartment Heat– Second heater core near rear

of passenger compartment– Driver controls overall system

operation– Some systems allow rear

passengers to control temperature.

– Rear blower, controls in front, rear, or both

Odor Control• Moisture can accumulate in the

ductwork over time and evaporator drain tubes can become clogged.

• This dark damp environment becomes a breeding grounds for odor causing molds and bacteria.

• Commercially available antimicrobial deodorizer and disinfectants treatments can be used to chemically coat the evaporator and duct system.

• Chemical treatments are applied using a siphon-type sprayer and applied at the air intake with the blower running and the cabin air filter removed.

Cabin Air Filter• May contain a cabin air

filter– Designed to keep dust

and allergens from the air stream entering the passenger compartment.

– Some also contain an active charcoal elements to absorb odors.

– Requires periodic inspection and maintenance every 15K miles.

– Inspect for and remove debris

Cabin Air Filter

• The cabin air filter may be located and serviced from under the hood in the owl area or from the passenger compartment near the glove box area on most models.

Vacuum Control

• Check source vacuum.

• Inspect vacuum hoses.

• Inspect and test check valve.

• Inspect and test vacuum reservoir tank.

Mode Door Control

To play, click the video screen.

Electric Actuator Motors• Electric mode door

actuators may be used solely or in combination with cable or vacuum controlled mode doors.

• Some are two position type either fully open or closed.

• Others are variable-position actuators capable of positioning the mode door at any point from fully open to fully closed.– Temperature blend doors

are typically of this design.

Electric Actuator Motors• HVAC Control head does not

directly control actuators. Instead the control head sends command information to the BCM or HVAC control module which in turn activates the actuators to desired position.

• The control head uses a multiplexed switch which uses different resistance values for each function. The control module interprets this information by dropping a voltage through the circuit.

Electric Actuator Motors• Electric mode door actuators may

be five-wire, three-wire, or two-wire controlled.

• The 2- and 5-wire actuators use a driver circuit in the control module to control their movement.

• Giving bidirectional control depending on the polarity of the 2 motor wires. Which driver is negative and positive controls the rotational direction of the motor.

• The control module determines door position through feedback circuits.

– On 2-wire actuator, the control module counts the actuator commutator pulses to determine door position.

– On the 5-wire actuator the control module determines mode door position through the potentiometer feedback voltage signal.

Electric Actuator Motors

Electric Actuator Motors• Control of the 3-wire actuator

is through a logic module built-in to the actuator –a smart motor.

• 3-wire actuators are self-calibrating.

• Electric mode door actuators are not adjustable and must be replaced if faulty.

• It is necessary to initialize a calibration procedure in order for the control module to relearn mode door position if the actuator or control module is replaced or otherwise loses its memory.

Blower motor

• http://images2.advanstar.com/search-autoparts/eNews/Technician2010/alert_02272013_hapCT_alert.html

Cables

• Inspect and adjust mode and temperature door cables.

Diagnosis

• Some systems allow scan tool usage for diagnosis and actuator control.

• Trouble codes may be set.

• http://www.youtube.com/watch?v=guuQwZnRRqs

Testing & Diagnosis

To play, click the video screen.

INTRODUCTION

• A system that contains the HVAC plenum, ducts, and air doors, called the air management system or air distribution system, controls the airflow to the passenger compartment. – Air flows into the case that contains the

evaporator and heater core from two possible inlets: outside air , often called fresh air, and inside air, usually called recirculation.

– From the case, the air can travel to one or more of three possible outlets.

CASES AND DUCTS

• AIR INLET AND CONTROL DOOR• AIR FILTRATION• PLENUM AND CONTROL DOOR• AIR DISTRIBUTION, CONTROL DOORS,

AND OUTLETS• DUAL-ZONE AIR DISTRIBUTION• HORIZONTAL-SPLIT AIRFLOW

TECH TIP

• The outside air inlet screen must be kept in good condition to prevent debris and small animals from entering the HVAC case. Leaves and pine needles can enter, decay, and mold. Mice have been known to enter and build nests and/or die; a small snake entered a system and was partially chopped up by the blower. Any of these conditions can create bad to awfully bad smells and are very difficult to clean up.

CONTROL HEAD

• VACUUM CONTROL CIRCUIT• ELECTRICAL CONTROL CIRCUITS AND

BLOWER MOTORS

REAR AIR DISTRIBUTION SYSTEMS

• Some larger vehicles (vans and small buses) have dual heat and A/C units (front and rear systems) with an air distribution system that is completely separate from the front heat–A/C unit.– The unit is usually mounted in a rear side

panel or in the roof. – It normally contains its own evaporator, heater

core, blower, and doors to control temperature and airflow.

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.46

AUTOMATIC TEMPERATURE CONTROL AND SEMIAUTOMATIC TEMPERATURE CONTROL

• ATC SENSORS• CONTROL DEVICE• ACTUATORS

AUTOMATIC TEMPERATURE CONTROL AND SEMIAUTOMATIC TEMPERATURE CONTROL

• Automatic temperature control (ATC) is essentially a system of sensors and controls that allows the driver to set a desired temperature at the HVAC control head and let the system take care of maintaining that temperature and selecting the proper air discharge location and blower speed.

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.48

Are All ATC Systems Electronically Controlled?

• Some early systems use a mechanical vacuum sensor that uses a bimetallic strip to measure temperature. This strip bends in response to temperature change; this bending action is used to control a vacuum valve. In one system, the bimetal sensor is positioned in a unit with both ambient and in-vehicle air passing by it. The driver control is attached so it acts on a pivot bar of the bimetal sensor. This unit provides a variable vacuum signal that is sensitive to ambient temperature, in-car temperature, and the driver’s temperature setting. The signal is zero vacuum when heat is called for, engine vacuum when full A/C is called for, or somewhere between.

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.49

What Is the ATC Default Position?

• Most systems are designed so they return to a full heat position when turned off. This way, if they fail, they will fail to that position. A person in the southern United States would probably be uncomfortable if the A/C and heat system failed, but in the northern states a heat and defrost system failure could be life threatening in the winter.

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.50

REAR WINDOW DEFROSTER

• The rear window defroster is an electrical resistance heater designed to clear fog and frost from the rear window. • The wires of the electric grid are visible in the rear

glass. • When current from the car’s electrical system flows

through the grid, it heats up and warms the glass.

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.51

• R134A Counterfeit Concerns • http://

images2.advanstar.com/search-autoparts/eNews/Technician2010/ct_02142013.html?cid=59610

• http://images2.advanstar.com/search-autoparts/eNews/Technician2010/alert_02132013_hapCT_alert.html

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.52

SUMMARY1. The HVAC case contains a blower, A/C evaporator, the

heater core, and doors to control the air temperature and flow.

2. The control head allows the driver to change blower speed, adjust the temperature, turn A/C on or off, and direct the airflow.

3. Older control heads transferred motion to the HVAC case through Bowden cables; more modern systems use vacuum control; and modern systems use electronics and electric motors.

4. Manual systems are adjusted by the driver’s hand.5. ATC systems use electronic sensors to measure air

temperature and quality; they use an ECM to evaluate the input and direct the actuators to adjust the HVAC system.

6. Many HVAC systems include a filter to remove particles and odors from the air.