Electric actuators are used to move the duct box doors to the correct location when the controls are activated in the control head. These actuators are usually serviceable without having to remove the complete duct box assembly. These actuators may need to be recalibrated when they are replaced or at times when the battery has lost power or has been disconnected.
Automatic Temperature Control and Dual Zone Systems
Electric actuators are widely used on late-model vehicles to move the duct box doors. These actuators are used to move the blend door, the mode doors, and the fresh/recirculated air door. These actuators are typically mounted with very small fasteners in a location that allows them to be replaced without having to remove the whole duct box.
Many late-model cars, trucks, and sport utility vehicles (SUVs) have more complex HVAC systems that require less input from the occupants of the vehicle. With an increase in computing power, high-tech systems can now control the blower speed, air temperature, and air distribution point automatically by using electronic control heads or smart interfaces with touch screen capabilities along with many sensors and logic devices.
These automatic temperature control (ATC) systems operate in automatic mode by letting the operator set the desired temperature in the cab, and then the sensors, computers, and actuators operate to deliver the correct results. These systems most often have dual zone capabilities that allow the driver and the passenger to choose different temperature settings. The duct box is more complex with these dual zone systems because it is able to deliver air at different temperatures to each side of the vehicle. These systems also have some diagnostic capabilities built into the logic that will often display messages or codes when a problem is present in the system. The codes need to be researched in a technical database to determine the action needed to correct the problem.
ATC systems use modules, sensors, and actuators to control the temperature and humidity in the cab. The HVAC control module receives inputs from several sensors while communicating with the powertrain control module (PCM) over a data network. Both of these modules have control over the output devices that determine the temperature and humidity in the cab. The data link connector allows a scan tool to be connected to the vehicle to retrieve trouble codes and live data from the electronic systems on the vehicle.
Vehicles with ATC are equipped with electronic control heads that allow the driver or the passenger to choose the desired temperature, and then the system automatically functions to achieve that temperature. Many of these ATC control heads also incorporate other functions into the platform, such as the audio system and the mobile phone connectivity.
The duct box on vehicles with dual zone climate control is more complex because it can be configured to distribute cold air on one side and warm air on the other side of the vehicle. These duct boxes must have two air mix doors (temperature control doors) to accomplish this task. The doors on dual zone climate control systems are moved with electric actuators.
Cabin Filter Replacement
Many modern vehicles use a cabin air filter that is mounted in the HVAC duct box to clean the air that is distributed to the passenger compartment. The steps to replace the cabin air filter are listed below.
1 The cabin air filter is typically located on the back side of the glove box. The glove box should be opened and then the contents of the glove box will need to be carefully removed and stored in a safe place.
2 Find the release clip that allows the glove box door to completely fold down and out of the way.
3 Once the glove box door is moved out of the way, find the cover for the cabin air filter.
4 Release the clips on the cabin filter cover and then pull on the cover to remove the filter from the duct box housing.
5 Gently pull the cabin filter from the duct box housing, paying attention to the orientation of the filter so that the new filter will be correctly installed.
6 Once the old cabin filter is removed from the vehicle, remove it from the cabin filter carrier.
7 Install the new cabin filter onto the cabin filter carrier while making sure that the arrow on the filter is pointing the same way as the old cabin filter. Typically, there is an arrow on the filter that should point in the direction of airflow through the duct box. Once the old cabin filter is removed from the vehicle, remove it from the cabin filter carrier.
8 Carefully reinstall the new cabin filter back into the duct box housing.
9 Continue pushing the new cabin filter into the duct box housing until it snaps into place. Reconnect the linkage for the glove box door and then repack the glove box with its contents and close the door.
CHAPTER 3
AC REFRIGERATION SYSTEMS
To understand the physics of how the temperature and humidity is controlled in the cabin of cars and trucks, it is important to expand on the discussion about heat transfer from chapter 2. The heat transfer process of convection is used to move heat into and out of the cabin depending on how the control head is set. The AC system will be explained in this chapter and the heat system will be covered in more detail in chapter 8.
Heat Transfer
In the AC system, refrigerant is the substance that is used to move the heat out of the cabin. In a similar way, coolant is the substance that is used to move heat away from the engine. A main difference between these two systems is that the refrigerant changes from a liquid to a gas during the process of moving the heat, but the engine coolant maintains its liquid state when the system is operating normally.
To understand how the heat is moved and how these state changes take place, more explanation is needed in the area of sensible heat and latent heat.
Sensible Heat
Sensible heat is the measure of heat that can be felt and measured on a thermometer.