1. FIGURE 10-1 A heater and A/C system includes an evaporator, a heater core, a blower motor, and a series of doors to control air flow. (Courtesy of Chrysler LLC)

2. FIGURE 10-2 Fresh air enters through the grill below the front of the windshield; it becomes a high-pressure area when the car is moving forward. (Courtesy of Chrysler LLC)

3. FIGURE 10-3 Most HVAC control heads include a control for turning things on and setting the mode of operation, a control for adjusting the temperature, and a control for the fan speed.

4. FIGURE 10-4 Many air control doors swing on their upper and lower pivots. (Courtesy of Chrysler LLC)

5. FIGURE 10-5 Early Honda Civics used a conventional temperature door (left).This was changed to a rolling design in the 2001 Civic. (Courtesy of American Honda Motor Co., Inc.)

6. FIGURE 10-6 Many systems include ducts to move heated or cooled air to the rear seat area (a). A rear system in a van can have ducts to move air forward (b). (a. Courtesy of Toyota Motor Sales USA, Inc.; b. Courtesy of Chrysler LLC)

7. FIGURE 10-7 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.

8. FIGURE 10-8 The fresh air inlet for most vehicles is at the base of the windshield, with a plastic and/or metal screen (a). On this car, it is under the hood (arrow). Ductwork brings the fresh air to the HVAC case inlet (b). (b. Courtesy of Chrysler LLC)

9. FIGURE 10-8 (CONTINUED) The fresh air inlet for most vehicles is at the base of the windshield, with a plastic and/or metal screen (a). On this car, it is under the hood (arrow). Ductwork brings the fresh air to the HVAC case inlet (b). (b. Courtesy of Chrysler LLC)

10. FIGURE 10-9 This outside-recirculation (air inlet) door is moved from one position to the other by a vacuum actuator (motor); here, it is in the fresh air position (a). The actual unit is shown in (b). (Courtesy of Chrysler LLC)

11. FIGURE 10-9 (CONTINUED) This outside-recirculation (air inlet) door is moved from one position to the other by a vacuum actuator (motor); here, it is in the fresh air position (a). The actual unit is shown in (b). (Courtesy of Chrysler LLC)

12. FIGURE This cabin/clean air filter can be removed through the glove compartment. Some filters have a charcoal center layer to remove odors. (Courtesy of Toyota Motor Sales USA, Inc.)

13. FIGURE The temperature door swings to direct all of the cool air past the heater core (a), through the core to become hot (b), or to blend hot and cool air (c).

14. FIGURE 10-12 In a blend-air system, all of the air is cooled
FIGURE In a blend-air system, all of the air is cooled. Then some of it is reheated and blended with the cool air to get the right temperature (a). In a reheat system, all of the air is cooled and then reheated to the correct temperature (b). (Courtesy of Everco Industries)

15. FIGURE This dual-zone air distribution system has two temperature-blend-air-mix doors and air distribution sections, one for the driver and one for the passenger. (Reprinted with permission of General Motors Corporation)

16. FIGURE This split/two-way air-flow system can deliver fresh air at face level and recirculated air at floor level (a). The HVAC case and blower is divided as shown (b). (Courtesy of Toyota Motor Sales USA, Inc.)

17. FIGURE (CONTINUED) This split/two-way air-flow system can deliver fresh air at face level and recirculated air at floor level (a). The HVAC case and blower is divided as shown (b). (Courtesy of Toyota Motor Sales USA, Inc.)

18. FIGURE This unit uses mechanical cables to change the air door position. (Courtesy of Chrysler LLC)

19. FIGURE A typical vacuum control circuit starts at the vacuum source (engine), is controlled by a valve at the control head, and ends at the vacuum actuators or motors. (Courtesy of Everco Industries)

20. FIGURE This system controls the vacuum actuators with a series of solenoid valves, one valve and control solenoid for each actuator operation. (Reprinted with permission of General Motors Corporation)

21. FIGURE Three compact, electric actuators/servomotors operate the doors in this part of the HVAC case.

22. FIGURE Most systems route blower electrical feed through one or more resistors to obtain the slower speeds. (Courtesy of DaimlerChrysler Corporation)

23. FIGURE This vacuum control circuit includes a check valve and reservoir to maintain constant vacuum during acceleration. Most vacuum harnesses are color-coded to help in locating a particular hose during repair procedures.

24. FIGURE 10-21 A vacuum actuator
FIGURE A vacuum actuator. This unit uses an internal spring to return the diaphragm when there is no vacuum signal.

25. FIGURE 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).

26. FIGURE Most early HVAC systems had simple electrical circuits and placed the blower and A/C switch in the control head as they do today.

27. FIGURE 10-24 This blower resistor is just slightly larger than a credit card.

28. FIGURE This blower circuit routes the current through resistors R-1, R-2, and R-3 for low speed; through R-2 and R-3 for M1 speed; and through R-3 for M2 speed. High speed actuates the relay to bypass the resistors. (Courtesy of Everco Industries)

29. FIGURE The critical dimensions needed when replacing a blower motor are shown here. (Courtesy of Four Seasons)

30. FIGURE This rear HVAC assembly fits into the vehicle’s rear side panel (a). The air flow is shown in (b). (Courtesy of Chrysler LLC)

31. FIGURE Many ATC systems are a standard system plus these components (a). They are combined as shown in (b). (Courtesy of Chrysler LLC)

32. FIGURE (CONTINUED) Many ATC systems are a standard system plus these components (a). They are combined as shown in (b). (Courtesy of Chrysler LLC)

33. FIGURE This ATC system uses 12 major components, in addition to the control head, at various locations. (Courtesy of Visteon)

34. FIGURE 10-30 The resistance of ambient and in-car sensors varies directly with temperature.

35. FIGURE This ambient sensor is mounted onto the front of the radiator support; others are mounted at other locations under the hood or in the fresh air inlet section of the HVAC case.

36. FIGURE The aspirator causes an air flow past the thermistor (in-car sensor) when the blower motor operates. (Courtesy of Toyota Motor Sales USA, Inc.)

37. FIGURE This sensor group includes three DAT (discharge air temperature) or in-vehicle sensors (one with a fan to move air) (top left), an OAT (outside air temperature) (top right), and an evaporator outlet temperature sensor (bottom). (Courtesy of Delphi Corp., all rights reserved)

38. FIGURE 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.

39. FIGURE The solar sensor provides a signal to the control module when the sun shines on it. (Courtesy of Chrysler LLC)

40. FIGURE This electronic servomotor converts the signal from the sensors and control head and moves to adjust the temperature-blend door, mode doors, and blower speed. (Courtesy of Chrysler LLC)

41. FIGURE When the system is cold, the servomotor produces the operations on the right; when the system is hot, servomotor movement changes to produce the operations on the left.

42. FIGURE These mode door, blend-air door, and recirculation-air-inlet actuators are electric motors (a). The circuit for the blend-air door is shown in (b). (Courtesy of Chrysler LLC)

43. FIGURE The blower motor wiring diagram shows the transistor in the A/C control assembly controlling the blower motor control relay. (Courtesy of Toyota Motor Sales USA, Inc.)

44. FIGURE The power module is used to switch the current feed to the blower and produce infinite blower speeds. (Courtesy of Chrysler LLC)

45. FIGURE A rear window defroster routes electricity from the feed bus bar at one side through the grid to the ground bus bar to warm and defrost or de-ice the window.

46. FIGURE This circuit for a rear window defogger (RWD) includes an LED light in the instrument panel and three fuses.