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CHAPTER 2

HVAC SYSTEM

The heating, ventilation, and air-conditioning (HVAC) system is a fascinating series of components that all work together to help control the temperature and humidity of the cabin in a vehicle. These systems work by adding heat to the air in the duct box when the heat mode is chosen and removing heat from the air in the duct box when the air-conditioning (AC) mode is chosen. The information in this book will help you understand how the system works as well as how to troubleshoot and repair the system when it is not functioning properly.

Working Safely

Many precautions must be taken when working around the HVAC system on cars and trucks. This section provides information and procedures to help prevent injuries, accidents, and unwanted disposal of the fluids and chemicals within the HVAC system.

Always wear personal protection equipment (PPE) when performing repair and maintenance on a vehicle’s AC or heating system. Quality Z87-rated safety glasses will provide eye protection. Wearing quality latex or nitrile gloves protects the hands from potential frostbite.

Refrigerant

The substance inside the AC system that is used to flow through the components and remove heat from the cabin is called refrigerant. There are three main types of refrigerant that have been used by original equipment manufacturers (OEMS): R12, R134a, and R1234yf. All of them have some characteristics that need to be understood, and they will all be explained in greater detail later in this chapter.

All three of these refrigerants behave in a similar way. First of all, these substances are pressurized when in an enclosed vessel or when in a closed system, such as the AC system on your car. Care must be taken when working around this pressurized refrigerant because it can cause frostbite if liquid refrigerant gets on your body. A person could also get burned by the refrigerant while the system is in operation due to the high pressures and temperatures within the AC system. When working around refrigerants, do the following:

1. Wear safety glasses to protect from getting refrigerant in your eyes.

2. Wear latex or nitrile gloves to give a layer of protection for your skin.

3. Store refrigerant containers in a cool, dry location away from any heat source, such as a heater or direct sunlight.

4. Work with confident and meaningful actions when connecting or disconnecting service hoses to the service ports on the AC system.

Cooling System

The cooling system on cars and trucks must be dealt with in a safe manner. The liquid that transfers heat from the engine to the radiator as well as into the cab to the heater core is called coolant, and it is typically a mixture of antifreeze and pure water. Care must be taken to never attempt to repair a cooling system when the engine is at operating temperature or above due to the system operating under a pressure of 14 to 17 pounds per square inch (psi). This means time should always be taken to allow the engine to cool down before removing the cap that holds the coolant in the system.

In addition to hot systems being a safety concern, coolant should always be drained into a suitable container when it is removed from the engine for maintenance or repair. Used coolant should also be disposed of in a proper and legal manner. Many parts stores allow customers to bring in used fluids to be recycled. The store will typically record the names of people who bring in used fluids. Coolant should never be poured out on the ground or into a drain that connects to public sewer systems.

It is vital to use confident actions when connecting and disconnecting service hoses to the service ports on the AC system. The valve should also be in the released position when connecting and disconnecting it from the system.

Refrigerant should only be stored in areas that are free from direct sunlight or added heat. The pressurized refrigerant should also be stored away from areas where lots of movement and sharp objects are present to prevent one of the containers from being punctured.

A reliable drain pan should be placed under the drain valve when releasing coolant from the radiator. There is typically an open path for the draining coolant to reach the pan without running down the frame, but care should be taken to catch as much of the draining coolant as possible.

Belts

The drive belts on an engine are an area where caution must be used in order to prevent injuries when working on the AC system. Many drive belts on late-model vehicles are held in place by a spring-loaded tensioner that holds strong pressure on the belt at all times. The belt tensioner must be released with the proper tool to safely remove or install the drive belt.

Drive belts on older vehicles are typically a V-belt type and are adjusted by moving one of the accessories on a pivot to allow the belt to be loosened or tightened. The key point to remember when dealing with adjustable V-belts is to tighten the belt to the correct level but not overtighten it. Overtightening these belts adds stress on the pulley and bearing of the accessories and could lead to early failure. It is also a prudent action to recheck and possibly adjust the belt tension on brand-new V-belts after they have been installed for a few days.

V-belts are used on older-generation cars and trucks. These belts are adjusted by physically moving one of the driven components within an open groove in the mounting brackets or by installing an adjustment strut that allows the belt tension to be adjusted by shortening or lengthening the bar.

The accessories on most late-model vehicles are driven by a serpentine belt that is much wider than the belts on older vehicles. This belt system typically uses a spring-loaded tensioner that holds constant pressure on the belt during operation.

The belt tensioner on a serpentine belt must be released when removing or installing the drive belt. Some vehicles allow a ratchet or a socket to be used to release the belt tensioner, while other vehicles require a special belt-release tool to be used to service the drive belt.

A belt tension gauge is a valuable tool to use when installing a new V-belt or when adjusting the tension on a used V-belt. After this tool is inserted on a section of the belt, it measures the deflection of the belt and lets the user know if the tension is correct. If the tension is too loose, then the belt can be tightened up. If the tension is too tight, then the belt can be loosened.

Electrical System

The vehicle’s battery and electrical systems must be respected when working on the air-conditioning system. Disconnect the battery prior to performing work under the dash, such as replacing the heater core or the evaporator core. Repairing the HVAC system often requires electronic components to be disconnected, so this process should be handled carefully.

Airbags

All modern-era vehicles have airbags located on the steering wheel and passenger-side dash panel. In addition, airbag assemblies are also found in the side pillars, seats, and doors of many late-model vehicles. Extreme caution must be used when performing diagnosis and repair inside the vehicle and under the dash area. The first step that should be taken prior to performing extensive disassembly of the instrument panel is to disconnect the battery negative cable and secure it with a shop towel or similar item to prevent the cable from moving back into contact with the battery terminal. After removing an airbag assembly, it should be handled carefully and stored in a safe place facing upward to prevent the unit from being projected in case of an inadvertent deployment.

The negative battery cable should be removed and secured prior to performing extensive repairs on the AC or heating system. Having the power removed from the vehicle reduces the chance of shorting out a live circuit when performing repair work.

Extra caution should be used when connecting and disconnecting electrical connectors on cars and trucks. Most connectors have locking tangs that must be carefully released when disconnecting the device or module.

All modern vehicles are equipped with an airbag assembly that is mounted directly in front of the driver in the steering wheel. Care must be taken when working around the driver-side airbag assembly to prevent accidental deployment during service of the AC and heating system.

Passenger-side airbags are available on most late-model vehicles and must be carefully handled when working around the dash panel assembly. Yellow wire covering is used on most of the circuits that are used in airbag systems.

Airbag assemblies should be handled in a very careful manner by using a secure grip with both hands. The top of the airbag should be facing away from the body to add another layer of protection in case of accidental deployment.

Airbag assemblies being stored during a repair process should be kept in a safe place facing upward. This prevents the airbag from becoming a projectile if the airbag accidentally deploys.

Legal Issues

There are a variety of laws that must be followed when working on the HVAC system on a vehicle. The item that comes to the forefront is allowing refrigerant to be vented into the atmosphere. It is against the law to willfully allow refrigerant to be released from a vehicle’s AC system or from a container. Care should be taken to prevent the loss of refrigerant during any process being performed on the vehicle. In addition to not venting refrigerant, the lubricant for the AC system should be collected and stored in a safe container. The oil can be collected by a reputable company and disposed of properly.

Professional technicians who repair mobile AC systems are required to attain a Section 609 certification from a reputable entity such as Mobile Air Conditioning Society (MACS) or Automotive Service Excellence (ASE). These certifications cover how to safely and legally handle and store refrigerants and AC systems. As of January 1, 2018, a person must possess a 609 certification in order to purchase a container of refrigerant larger than 2 pounds. This means that the small cans of refrigerant are still available for the general public to purchase without any restrictions.

The engine coolant is another substance that must be handled in a safe and legal manner. If the coolant is not going to be reinstalled into the vehicle, then it must be recycled according to the laws in your area. Coolant should never be poured onto the ground or into a drain.

Types of Refrigerant

The three types of refrigerant that have been used by OEMs are known as R12, R134a, and R1234yf. Many people describe the refrigerant as Freon, but that is not accurate. Freon is a brand name of a product from the Chemours Company, but many people still use the term Freon to describe the refrigerants used in mobile, fixed, or even refrigeration systems.

R12 was first refrigerant used in mobile AC systems in the early 1930s. R12 was used widely until around 1992, when it began being phased out due to the chlorine in the chemical causing ozone depletion in Earth’s atmosphere.

R134a followed R12 as the refrigerant of choice in OEM installation beginning around 1993. This refrigerant performed very similarly to the previous R12. Systems that were built to use R12 can be converted to use R134a with a process called a retrofit. This process will be covered in great detail in chapter 6 of this book. R134a performs at an acceptable level and is still being used in new production vehicles at the time of this writing. It does have a negative effect on the environment because it is considered to be a global warming gas. Much research and funding was put into the development of the replacement refrigerant for R134a, and the result is R1234yf.

Many new model vehicles have already been switched over to R1234yf with more to follow as each year passes. Beginning with model year 2021, all new cars and trucks will have the new refrigerant installed in the AC system.

The vessels of R12 are 30 pounds and white in color. They have a shutoff valve on top as well as a threaded connection point that allows the yellow hose of a manifold set to be connected. R12 has not been manufactured since the mid-1990s, which has caused the supply and cost of the substance to rise greatly.

The 10-pound cylinders of R1234yf are white in color and equipped with a shutoff valve as well as a threaded connection point that allows refrigerant to be removed from the container. R1234yf also comes in 2-pound containers that have a connection point and a shutoff valve.

The lubricant required for the older R12 systems was a mineral-based oil that only came in one viscosity. The lubricant required for R134a and R1234yf systems is a polyalkylene glycol (PAG) type. This lubricant comes in several viscosity levels and extreme attention should be used when installing lubricant into a system. PAG oil is very hygroscopic, so it should be purchased in small quantities and resealed when not in use. PAG oil that has been open for longer than a few days should be discarded due to moisture absorption.

The 30-pound vessels of R134a are light blue in color and equipped with a shutoff valve as well as a threaded connection point that allows refrigerant to be removed from the container. R134a also comes in smaller containers that can be used to recharge the AC system.

R12 AC systems use a mineral-based refrigerant oil to lubricate the compressor. This oil comes in one viscosity and needs to be kept sealed when not in use. Mineral oil can be added into the system during the repair process; it can be added into the components as they are being replaced.

R134a uses a polyalkylene glycol (PAG) lubricant that is available in different viscosities depending on the manufacturer’s recommendations. The correct type and amount of PAG oil should be used during the repair of the AC system. Using the wrong type or amount of PAG oil can cause system damage and early failure of the compressor.

In addition to the three refrigerants mentioned, there are other refrigerant options that can be purchased in the marketplace. Examples of these options include Freeze 12, Free Zone, Hot Shot, and FrigC, just to name a few. All of these types of refrigerants are legal to install, but it is recommended to use one of the three types that the OEM engineers recommend.

The problem with these alternative refrigerant options is that they are typically not a pure substance, which means they have blends of more than one type of refrigerant. This causes a problem when it is time to recover the refrigerant into an air-conditioning recovery machine because the refrigerant storage vessel on the machine will be tainted with blended refrigerant. It is recommended that a refrigerant identifier be used on every system to protect against recovering a mixed refrigerant.

Another major consideration when choosing the refrigerant to use in the repair process is whether to use pure refrigerant or a type that has additives in the mixture. Additives that are considered acceptable by most professionals include dye or oil. There are times when it is appropriate to use the small can of refrigerant to add either dye for leak detection or oil for lubrication of the system.

There are many other options for the do-it-yourself (DIY) consumer that need to be addressed. A large section of the small can refrigerant market contains a stop-leak type of substance that sounds like a great choice when the cause of many refrigerant issues is that there is a leak. The chemical in this refrigerant is supposed to change from a liquid into a solid when it is exposed to air. The problem with using this type of product on a DIY-type repair is that there will very likely be moisture in the system that will cause the sealer chemical to turn into a solid on contact. This will cause damage to the compressor as well as block passages in the heat exchangers.

Small cans of R134a are available with dye added to the refrigerant. This is a good product to use when dye needs to be injected into a system that is still under pressure. If the system has a leak, the dye will exit at the leak point and can be detected by shining a black light on it.

Small cans of refrigerant with the PAG oil mixed with it are available as a method for adding oil with the system under pressure. Adding PAG oil directly into the system during the repair process is a more accurate way of keeping the system oil level near optimal levels.

Refrigerant that has unwanted additives, such as sealer, is widely available in the marketplace. Many of these products are equipped with a charging hose as well as a little gauge that gives some feedback on how much refrigerant to add to the system. This is a very inaccurate method of recharging that will be covered in greater detail in chapter 4.

Heat Transfer

The HVAC system uses hot engine coolant to move heat into the passenger compartment when the heat mode is chosen on the control head. The HVAC system uses refrigerant to remove heat from the passenger compartment when the AC mode is chosen on the control head. To understand how this transfer of heat takes place, an explanation of the three main types of heat transfer is needed. The three types of heat transfer include conduction, convection, and radiation.

Conduction is the transfer of heat directly from molecule to molecule. An example of conduction is how the fasteners on the exhaust manifold get hot because they are in direct contact with the exhaust. Another example is a person accidently touching the hot exhaust manifold while the engine is running. The extreme discomfort felt by touching the hot exhaust manifold would be a result of conduction.

Convection is the transfer of heat by using a substance, such as air, water, coolant, or refrigerant. The engine cooling system is a prime example of convection because the coolant absorbs large quantities of heat from the hot engine block and cylinder heads and is then pumped to the radiator where the heat is released into the surrounding air. The AC system is another example of convection because the refrigerant absorbs large quantities of heat from the warm air in the passenger compartment and is then pumped out to the condenser, where the heat is released into the surrounding air.

Radiation is the transfer of heat through rays without heating the air as it passes through. Sunlight is the best example of transferring heat through radiation. This concept can be understood due to the fact that it is possible to have a sunny day during the winter while the outside temperature can still be very cold. Radiant heat also greatly changes the temperature inside of dark-colored vehicles due to how the sunlight is absorbed by the dark color. Light-colored vehicles reflect the sunlight instead of absorbing the radiant heat. This concept can be seen by measuring the cabin temperature of a dark-colored vehicle on a hot day and comparing the findings with that from a light-colored vehicle.

Air Handling System Operation

The air handling system controls the speed, the temperature, and the distribution location of the air being discharged into the cabin of the vehicle. The types of systems include manual control and automatic temperature control (ATC).

Manual Control Systems

In a manual control system, the driver or the front-seat passenger has the option to pick the blower speed, the air temperature, and the distribution location by moving the selectors on the HVAC control head to the desired position. A large portion of the vehicles on the road today, as well as older vehicles, use this type of HVAC control system.

This diagram shows examples of heat transfer. Conduction is the transfer of heat directly through a material. Convection is the transfer of heat through air, fluid, or gas. Radiation is the transfer of heat through heat rays.

Manual climate control heads are found on many vehicles of every era. This type of control head has a knob for blower speed, air temperature, air distribution location, air-conditioning, and recirculated air. This control head will also have a rear defogger switch if the vehicle is equipped with that option.

Manual climate control heads have been a reliable method to control the blower speed, the air temperature, the mode of air distribution, the air source, and the operation of the AC system for many years.

The duct box is where the action happens when the levers and knobs are moved on the control head. The duct box is typically located under the dash panel and is fairly simple in operation. Items that are located in the duct box include the blower motor, the evaporator core, the heater core, the fresh/recirculated air door, the blend door, and the mode doors. Here is an explanation of how each of the main items inside the duct box functions.

Blower Motor: This device is a multispeed electric motor that has a round impeller that forces the air to flow through the duct box and be directed to the desired location by the mode doors.

Evaporator Core: This device is the heat exchanger for the AC system that the air in the duct box always passes through. When the AC system is turned on, the evaporator core surface is cold and causes the heat and moisture to be removed from the air that passes through it, resulting in cool and dry air.

An overhead view is shown of the duct box that was removed from a late-model vehicle. This duct box must be removed and taken apart in order to replace the heater core or evaporator core. The blower motor, door actuators, and blower resistor are typically serviceable without having to remove the duct box.

Heater Core: This device is the heat exchanger for the heating system. Once the engine is warmed up, the heater core has hot engine coolant flowing through its passages. Heated engine coolant is routed to the heater core through hoses or pipes. The duct box air only passes through the heater core when the blend air door is set to a position that allows this to happen. As the air passes through the heater core, heat is released from the hot coolant and absorbed by the air, which results in warm air.

Air is routed through the duct box with a blower motor and a series of doors and heat exchangers. The blower motor is the device that forces air to be moved within the duct box. The recirculation door causes air to be pulled into the cab from outside or to be recirculated from within the cab. The blend door (also known as the temperature control door) causes the air to be directed through the heater core or around the heater core, which controls the temperature of the air. The mode doors cause the air to be directed at either defrost, vent, or floor ducts.

Fresh/Recirculated Air Door: This door controls where the air is pulled from by the blower motor. The two sources for air in the duct box are fresh air from outside the cab and recirculated air from inside the cab.

Blend Door: This door is located between the evaporator core and the heater core. This door controls the amount of air directed to the heater core that controls the temperature of the air that will be distributed by the mode doors. This door is sometimes called the temperature control door. When the door is in full heat mode, all of the air is directed to the heater core, which results in maximum heating in the cab. When the door is in full cool mode, it blocks air from passing through the heater core, which results in maximum cooling in the cab. Many times, the blend door is positioned somewhere between the maximum points, which causes a blending action to occur and results in the desired air temperature being sent into the cab. This door is controlled by the position of the temperature lever on the control head.

Mode Doors: These doors control where the duct box air is distributed in the cab. The different available modes include defrost, vent, and heat modes that are all controlled by the mode doors. Many systems have bi-level options that allow air to be distributed at two locations simultaneously, such as defrost and heat or vent and heat.

The doors in the duct box can be moved using cables, vacuum actuators, or electric motors. Cables and vacuum were the methods used on all early AC-equipped vehicles, and electric actuators are always used on late-model vehicles. Vehicles with cable-operated mechanisms to move the duct box doors typically had a method to adjust the cable length in case the door needed an adjustment. Vacuum-operated systems used either engine vacuum or some type of vacuum pump to create the negative pressure needed to move the doors. The electric actuators used on all late-model and current vehicles receive a command from the control head or logic device when movement is needed. The electric actuators seem to fail more often than the other devices used to move the doors.

A cable is used to move this blend door as the temperature lever is moved. Cables were used on many older-model vehicles and provided reliable operation of these systems.

Vacuum actuators are widely used on vehicles as a method to move the duct doors. These devices can be used on all of the duct box doors except the blend air/temperature door. Vacuum is directed to these actuators from the control head as the different options are requested. Vacuum is supplied to these systems by the engine or by a vacuum pump.

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.