1. Heating systems

2. Source of heat
99.9% of the cars on the road today use waste heat from the engine cooling system to provide cabin heat.
This heat energy would normally be lost to the radiator.
Electric heating grids are used on electric vehicles [Nissan Leaf] and plug in electric vehicles [Chevy Volt and Prius Plug-in].
Electric cabin heat uses energy from the battery, reducing the range the vehicle can travel in electric mode.
Exhaust heat was used for vehicles with air cooled engines [older Porsche 911 and VW Beetle]

3. Cooling system
Hot coolant from cylinder head
Radiator
Heater core
Coolant return to water pump
The cabin air is heated by the air passing through the heater core.
The heater core uses hot coolant that would otherwise flow through the radiator.

4. Heater core The heater core is a heat exchanger.
Heat from the coolant is transferred to the air passing through the fins.
The heater core shown here has an aluminum core and plastic side tanks.

5. Heater core The heater core is essentially a mini radiator.
Coolant tubes
Coolant flow
Fins
Air flow
The heater core is essentially a mini radiator.
Hot coolant passes through thin flat tubes.
Fins attached to the tubes provide additional surface area for heat to be transferred to the air passing between the tubes.

6. Brass heater core
Brass heater cores are normally found on older vehicles.
Solder is used to join the components of the brass heater core.
Unlike aluminum, brass heater cores can be repaired.
Foam rubber prevents air bypassing the heater core

7. Heater core – side tanks
Inlet
Side tank
Outlet
Baffle
Inlet and outlet tubes are located on the heater core’s side tanks.
If both inlet and outlet tubes are on the same side of the core there must be a baffle in the tank to separate the inlet and outlet sections.

8. Rear heater coolant lines
Heater hoses
The heater core is normally located inside a heater box on the cabin side of the firewall.
The inlet and outlet tubes extend through a hole in the firewall where the heater hoses can be attached.
The extra connections shown on this van are for a rear heater core.
Heater core tubes
Rear heater coolant lines

9. Heater hoses
Heater hoses are reinforced flexible rubber hoses used to connect the engine to the heater core.
The engine rotates slightly on the engine mounts as torque is applied to the wheels.
The heater hoses must flex to accommodate the rotation of the engine.
Metal pipes are used to conduct coolant in places where there will be no movement – such as lines to rear cabin heater cores.

10. Molded heater hose vs. replacement hose
OEM [Original Equipment Manufacturer] hoses a molded to custom fit the application.
Generic heater hose can be purchased in various inner diameters by the foot from a part store.
Generic heater hose cannot be bent through sharp angles – it will kink – restricting the flow of coolant if bent through too small a radius.
Wire wound generic heater hose will not kink.
OEM heater hoses purchased through a dealer is the best method of repair.

11. Heater shutoff valve
The heater shutoff valve blocks the flow of coolant through the heater core when the A/C system is set to A/C max or the temperature control is set to it’s coldest position.
The shutoff valve can be mounted on the heater core or under the hood

12. Heater shutoff valve
The shutoff valve must have a shaft seal to prevent coolant loss at the point where the shaft enters the valve housing.
The shutoff valve is operated by a cable or vacuum diaphragm.

13. Vacuum operated heat cutoff valve
Heater hose
Vacuum chamber
When vacuum is applied to the valve shown here the flow of coolant through the heater core is shut off.
This valve is open whenever there is no vacuum applied [fail safe position]
Vacuum Line
Blower motor resistor

14. Heater fan
Air is circulated through the heater core by an electrically driven ‘squirrel cage’ fan.
Air enters the center of the fan.
The rotation of the squirrel cage pushes the air outward by centrifugal force.
The housing catches the air and sends it on to the heater box assembly.
Centrifugal force pushes the air outward
Air enters through the center of the squirrel cage

15. Fan motor speed control
Medium speed
Base speed
Low speed
Diode
A series of wire resistors is used to control the speed of the electric motor.
Some electronic climate control systems will use a fan control transistor to regulate fan speed.

16. Resistor block
The blower motor resistors get very hot when the electric motor is running.
To keep the resistors cool the resistor block assembly is located in the heater box[heater case] directly downstream from the blower fan.
The resistor assembly is normally retained by two screws and is usually easy to service.
Removing the resistor assembly can sometimes allow the technician to get a peak at the evaporator without having to remove the entire heater box assembly.

17. Blower motor speed control
The electric motor that drives the heater fan is called the blower motor.
The ‘fan motor’ is the electric motor that drives the radiator fan.
Since 1977 all cars run the blower motor at very low speed whenever the ignition key is on.
This is intended to prevent injury from carbon monoxide poisoning when the vehicle is not moving but the engine is running to provide heat.

18. Blower motor
The blower motor is typically attached to the heater box by 4 screws and can be easily replaced.
You can easily test the operation of the motor using jumper wires.
Most blower motors draw between 10 and 30 amps.

19. Blower motor circuit The blower motor circuit is made up of:
Resistor block
Fan switch
Ignition switch
15 amp fuse
Blower motor
30 amp fuse
High speed relay
The blower motor circuit is made up of:
Fan switch
Ignition switch
Blower motor
Resistor block
High speed relay
2 fuses

20. Ignition on – fan switch off
Ignition switch
15 amp fuse
When the ignition switch is turned on power flows through R1 to the motor.
If R1 is a 10 ohm resistor the motor will receive 1.2 amps of current [about 5% of the motors capacity].

21. Ignition on – fan switch on ‘Low’
Ignition switch
15 amp fuse
When the fan switch is turned to low power flows through R2 and R3 then on to the blower motor.
If R2 is a 2 ohm resistor and R3 is a 1 ohm resistor the motor will receive 4 amps of electric current. [20%]

22. Ignition on – fan switch on ‘Med’
Ignition switch
15 amp fuse
When the fan switch is turned to medium power flows through R3 then on to the blower motor.
If R3 is a 1 ohm resistor the motor will receive 6 amps of electric current. [33%]

23. Ignition on – fan switch on ‘High’
Ignition switch
15 amp fuse
30 amp fuse
Note: both fuses are needed for high speed operation
1/4 amp trigger current
High speed relay
20 amp current flow
When the fan switch is turned to high, the fan switch energizes the electromagnet in the high speed relay.
The high speed relay allows full power to go to the blower motor.
Blower motors typically draw 20 to 30 amps of current.

24. Electronic blower motor speed control
HVAC control head
Fuse box
BCM
CAN network
Cabin air temperature sensor
Blower motor module [driver]
Blower motor
The blower motor speed is electronically controlled by the BCM [Body Control Module] on most late model cars.

25. Blower motor module On
10 mS
50% duty cycle
Off
10 mS
1 mS =1/1000 second
The blower motor module is a high speed electronic switch that turns the power to the electric motor on and off hundreds of times each second.
This type of electrical control is called a pulse width modulated duty cycle.

26. Blower motor module On
1 mS
5% duty cycle
Off
19 mS
1 mS =1/1000 second
If the motor is turned on half the time and turned off half the time the motor runs at 50% of it potential speed.
If we send power to the motor for 1 millisecond and then turn the power off for 19 milliseconds the motor will run at 5 percent of its potential speed.

27. Blower motor module On
19 mS
95% duty cycle
Off
1 mS
1 mS =1/1000 second
If we send power to the motor for 19 milliseconds and then turn the power off for 1 millisecond the motor now runs at 95 percent of its potential speed.

28. Blower motor control module
BCM to module communications
12 Volt + power feed
12 Volt + power feed
Some of the terminals on the blower motor module send cabin air temperature data back to the BCM

29. BCM and HVAC control head
CAN network
Network
Interface chip
Cabin air temperature sensor
When the driver presses the buttons on the HVAC control head a digital signal is sent to the BCM through the vehicle’s CAN network.
The BCM processes the request and sends the appropriate signals to the heating system actuators.

30. Electronic blower motor control
On initial startup in cold weather the fan is usually turned off so that cold air is not blown onto the passengers.
If the cabin is cold and defrost mode is selected the fan will normally be commanded to high speed by the BCM.
Electronic blower motor controller
The BCM may run the fan for a few minutes after engine shutdown to dry off the A/C evaporator. This helps prevent the formation of mold on the evaporator core.

31. Temperature control
There are two ways to manually control cabin temperature:
Heater valves regulate the flow of coolant through the heater core
Blend doors
Most cars use a blend door system that maintains a constant flow of hot coolant circulating through the heater core
Air temperature is controlled by a blend door that mixes heated and unheated air to achieve the desired cabin air temperature

32. Heater valve
Heater valves are normally located on the engine compartment side of the firewall where the heater hose meets the heater core inlet.
The heater valve is normally cable controlled.
Some times an electronically controlled stepper motor is used

33. Heater valves
The heater valve requires a seal where the valve shaft passes through the valve housing.
This seal is a potential source of coolant leakage
When the heater valve is located under the hood the cable is subject to corrosion which will cause binding and eventual cable failure.

34. Heater box
Defrost outlet
Fresh air inlet
Heater core
Face level outlet
Blower motor
Foot well outlet
The heater box is a plastic enclosure that contains the heater core, blower motor and mode doors.
The heater box is normally located on the cabin side of the firewall.
Fresh air from the cowl area enters the heater box through a large hole in the firewall.

35. Blend door – cold position
Defrost outlet
Blend door
Heater core
Face level outlet
Blower motor
Footwell outlet
The blend door controls the temperature of the air coming out of the air vents at the footwell, face level and defrost vents.
In the position shown here the air passage to the heater core is totally blocked by the blend door.
This forces all of the air to bypass the heater core.

36. Blend door – Hot position
Defrost outlet
Heater core
Face level outlet
Blower motor
Blend door
Footwell outlet
In the position shown here the bypass passage is blocked by the blend door.
This forces all of the air to pass through the heater core.

37. Blend door – mixed position
Defrost outlet
Heater core
Face level outlet
Blower motor
Footwell outlet
In the position shown here the blend door is in-between the heater core and bypass passage .
Some of the air goes through the heater core – some bypasses it.

38. Mode doors – Footwell
Defrost outlet
Heater core
Face level outlet
Blower motor
Footwell mode door
Footwell outlet
The footwell mode door diverts air to the defrost/face level duct or foot level outlet.
Most vehicles have a middle position allowing half of the air to be directed to the face level and haft to go to the footwell vents.

39. Defrost/face level mode door
Mode doors – Defrost
Defrost outlet
Heater core
Face level outlet
Defrost/face level mode door
Blower motor
Footwell mode door
Footwell outlet
When the footwell mode door is closed air is diverted into the defroster/face level duct.
A second mode door diverts air to either the defrost or face level vents.

40. Defrost/face level mode door
Mode doors – Face level
Defrost outlet
Heater core
Face level outlet
Defrost/face level mode door
Blower motor
Footwell mode door
Footwell outlet
The defrost / face level vents diverts air into the face level vents.
Most cars have individual valves and louvers on the face level vents so that air can be directed differently between the driver and passenger side.

41. Studs attach the heater box to the fire wall
Heater core
To gain access to the heater core and mode doors the entire heater box may need to be removed from the vehicle.

42. Recirculation mode door in fresh air position
Recirculating mode
Recirculation mode door in fresh air position
Recirculating mode doors are generally not found on vehicles that are not equipped with Air conditioning.
In recirculating mode the air entering the fan is taken from inside the cabin.

43. Recirculation mode door in ‘Recirc’ position
Recirculating mode
Recirculation mode door in ‘Recirc’ position
When the outside air temperature is high the air inside the cabin is cooler after the A/C has been running for a few minutes.
This reduces the amount of work the A/C compressor must do, which helps improve fuel economy while keeping the occupants comfortable.

44. Blend and mode door actuators
There are 3 different methods of operating the blend and mod doors.
Cable
Vacuum diaphragm
Electric motor
Electric motors have become the dominant type of control in the past few years because they allow the vehicle electronic systems a greater degree of control of the HVAC system.

45. Cable actuators
Mode door
Outer conduit
Inner cable
Clamp
HVAC control head
Clamp
Operating lever
Cable actuators work the same as the brake cables on a bicycle.

46. Cable actuators
One disadvantage of a cable is that it’s good at pulling but not so good at pushing.
If the blend door or mode door is binding the cable may be bent if the operator tries to force it.
Another disadvantage of cables is that they cannot operate through a sharp angle.
Cable systems are normally operated by horizontal levers in the control head.

47. Vacuum actuators
Rubber diaphragm
Vacuum chamber
Vacuum hose
Actuating rod
Spring
Vacuum actuators – sometimes called vacuum motors were very common in the 1970s and 1980s.
When no vacuum is applied spring tension pushes the actuating rod outward.

48. Vacuum actuators
5 psi intake manifold pressure
15 psi atmospheric pressure
Vacuum applied
When vacuum is applied to the actuator there is a 10 psi pressure difference operating on the diaphragm.
If the diaphragm has a surface area of 2 square inches 20 pounds of pulling force will be applied to the actuating rod.

49. Dual chamber vacuum actuators
Seal
Conventional vacuum actuators are generally either open or closed.
3 position vacuum actuators are often used for mode control.
This allows for an open, ½ open and a closed position.

50. Vacuum ball [reservoir]
Vacuum control
To intake manifold
The vacuum actuators are controlled by a rotary valve in the HVAC control head.
Check valve
A reserve of vacuum is needed for conditions when there is little or no manifold vacuum [hard acceleration, climbing hills etc].
A check valve in the vacuum reservoir prevents air from entering the reservoir when vacuum is low
Vacuum ball [reservoir]
Grommet
Rotary valve
Several rotary valves are combined into one assembly to control all the mode door actuators.
HVAC control head

51. Vacuum reservoir
The vacuum reservoir is typically a plastic sphere or canister that is located on the inner fender

52. Vacuum check valve
The vacuum check valve closes when the engine is run at high throttle openings. Without it the mode doors would switch to defrost every time you accelerated hard or went up a steep hill

53. Default no vacuum position
When there is no vacuum available to the heater control system the system should be in full defrost mode.
This is the fail safe position
Most manual heater controls are designed for left-max-defrost.
When all controls are moved to the farthest left the system has the heat at maximum, the fan speed also at maximum and the mode at full defrost.

54. Electric actuators
Most modern cars use an electric motor to operate the mode and blend doors.
There are three types of motors in common use:
Conventional DC electric motors
Stepper motors
Feedback loop control DC motors with a position sensor
The BCM controls sends electrical current to the motor when it receives a command from the HVAC control head.

55. Mode control using conventional DC motors
Worm shaft
Sector Gear
Mode door shaft
Two position mode doors can be controlled using a simple DC electric motor.
One end of the armature is a worm shaft.
The worm shaft is in mesh with a nylon sector gear.

56. Stepper motors A stepper motor normally has four electric terminals.
When power and ground are applied to 2 terminals the motor will turn a fraction of a turn then stop.
Power and ground must be applied to a different set of terminals to get the motor to another step.
The computer can keep track of the position the motor is at by counting the number of times it has changed the polarity of current.

57. Feedback motors
Feed back motors have a conventional DC electric motor with 2 terminals combined with a position sensor.
The position sensor is a potentiometer [variable resistor].
The sensor produces a voltage signal that varies from .5 volts with the door in one position to 4.5 volts as the door moves to the opposite position.
The BCM converts the voltage into a number [0-to 255] and stores it in RAM memory.

58. Feedback motor control
BCM
A 5 volt reference signal plus a ground is applied to the resistor.
A graphite brush slides back and forth across the resistor as the door opens and closes.
The voltage at the brush varies from .5 volts to 4.5 volts depending on how far open or closed the door is.
ref +5 volt
Resistor strip
Ground
Signal

59. Cabin intake air
Screen
Gasket
Fresh air enters the vehicle through a gap between the trailing edge of the hood and windshield.
A gasket located between the firewall and underside of the hood prevents engine fumes from entering the HVAC system.
Air inlet
Firewall
Inlet air plenum
Water drain channel

60. Cabin air filter
Note: plastic screen has been removed
Underhood / HVAC gasket
Many newer cars have an air filter for the HVAC system.
The filter is normally located ahead of the blower motor.
Image courtesy Robert Bosch GMBH