Chapter 35

Heavy-duty Heating, Ventilation,

and Air Conditioning Systems

Objectives (1 of 4)

• Understand the basic theory of heavy-duty truck air conditioning systems.

• Outline the requirements of the Clean Air Act that apply to a heavy-duty truck air conditioning system.

• List the five major components of a heavy-duty air conditioning system and describe how each works in the operation of the system.

• Explain how the thermostatic expansion valve or orifice tube controls the flow of refrigerant to the evaporator.

Objectives (2 of 4)

• Identify the refrigerants used in heavy-duty truck air conditioning systems.

• Describe the function of the main components in a typical heavy-duty air conditioning system.

• Recognize the environmental and personal safety precautions that must be observed when working on air conditioning systems.

• Identify air conditioning testing and service equipment.

Objectives (3 of 4)

• Test an air conditioning system for refrigerant leaks.

• Outline the procedure required to service a heavy-duty air conditioning system.

• Perform some simple diagnosis of air conditioning system malfunctions.

• Outline the advantages of connecting air conditioning management electronics to the chassis data bus and explain how to access the system.

Objectives (4 of 4)

• Explain how a truck cab ventilation system operates.

• Describe the role a liquid-cooled heating system plays in a truck cab heating system.

• Describe some types of auxiliary heating and power units.

How Air is “Conditioned”

Principles of Refrigeration

• States of matter

• Absolute heat, heat movement, and measurement

• Pressure and heat

• Latent heat

Principles of Heat

Summary of Air Conditioning Principles

• Heat always moves from a warmer area to a cooler area.

• When liquids are heated and evaporate to a vapor state, heat is absorbed.

• When a gas condenses from a vapor to a liquid state, heat is released.

Air Conditioning System Components

• Compressor

• Condenser

• Receiver/dryer or accumulator

• Expansion valve or orifice tube

• Evaporator

Location of Components

Refrigerant Flow Cycle

The Refrigeration Cycle

Shop Talk

• Refrigerant containers for R-12 and R-134a are color coded. – R-12 containers are white, and R- 34a

containers are light blue and clearly marked.– In addition, R-134a containers use 1/2-inch 16

acme threads, which cannot be connected to an R-12 gauge set or recovery machine.

The A/C Compressor

• Piston-type compressors

• Rotary vane Compressor

• Swashplate-type compressors

Piston Compressor

Vane Compressor

Swashplate Compressor

Clutch

Condenser

Receiver Drier

Binary/Trinary Switch

Accumulator

Two Systems

TX Valve

FOT

Thermostat

Hose Fittings

Shop Talk

• Remember that just one drop of water added to refrigerant will lead to corrosion and refrigerant breakdown.

• Corrosive hydrochloric acid can be produced in the older R-12 system when trace drops of water are added.

• Also, the smallest amounts of air in the refrigerant system can start chemical reactions that result in system malfunctions.

Caution

• Refrigerant should never come into contact with skin or eyes.

• Liquid refrigerant, when exposed to the air, quickly evaporates and will almost instantly freeze skin or eye tissue. – Serious injury or blindness could result.

First Aid

• Refrigerant splashed in the eyes should be treated as follows:– Flush eyes with cold water to raise the temperature

above freezing point. – Avoid rubbing eyes. – If available, apply an antiseptic mineral oil to the

affected area. • This will form a protective film over the eyeball to reduce

the possibility of infection. Then rinse with a weak boric acid solution.

– Call an eye specialist or doctor immediately and receive medical treatment as soon as possible.

Caution

• Avoid working in areas where refrigerant may come into contact with an open flame or any burning material, such as a cigarette. – When it contacts extreme heat, refrigerant

breaks down into poisonous phosgene gas, which, if breathed, causes severe respiratory irritation.

Caution

• Under current federal Clean Air Act regulations, refrigerants must be recovered and recycled by all users to protect the environment and never released into the atmosphere.

• Under these regulations, service facilities not having the required recovery and recycling equipment and properly trained and certified personnel are not permitted to perform any refrigeration service work.

Odor Diagnosis

• See Table 35-1 on page 114 of the textbook.

Caution

• Never pressure or leak test R-134a service equipment or vehicle air conditioning systems with compressed air. – Some mixtures of air and R-134a have been

shown to be combustible at elevated pressures.

Shop Talk

• Electronic-type detectors must be correctly calibrated before each use to detect the lowest permissible leak rate.

• Trace the refrigerant system in a continuous path.– If a leak is found, always continue to test the

remainder of the system. • The probe should be moved around the location at

a rate no more than 1 to 2 inches/second and no more than 1/4 inch from the surface completely around the position.

• It also helps to place the probe lower than a suspected leak because the refrigerant is heavier than air.

Manifold Gauge Sets

Caution

• Never open the high-side hand valve with the system operating and a refrigerant source at the center hose connection. – This will cause refrigerant to exit the A/C

system under high pressure into the source container, which could cause it to burst.

– The only time both hand valves should be open is when evacuating the system.

Shop Talk

• Atmospheric pressure reduces as altitude increases.

• The gauge pressure must be corrected using an OEM correction chart when checking system low-side pressures.

Shop Talk

• A vacuum pump is unable to remove moisture in liquid state from a system.

• It lowers the system pressure and, therefore, the boiling point of liquid moisture.

• It then removes the vaporized moisture. – This is why it is necessary to pull a vacuum on

a system for an extended period to ensure that no moisture is left in it.

Air Conditioning Service Procedures

• System recovering and recycling

• System flushing

• Compressor oil level checks

• Evacuation

• System recharging

Refrigerant R and R Stations

Purging Station

Shop Talk

• The efficient operation of the air conditioning system greatly depends on the correct amount of refrigerant in the system.

• A low charge will result in inadequate cooling under high heat loads, because of a lack of reserve refrigerant, and can cause the clutch cycling switch to cycle faster than normal.

• An overcharge can cause inadequate cooling because of a high liquid refrigerant level in the condenser. – Refrigerant controls will not operate properly, and compressor

damage can result. – In general, an overcharge of refrigerant will cause higher-than-

normal gauge readings.

Caution

• Accumulator systems do not have a sight glass. – This is because bubbles are always present in

the liquid line, even with a full charge. – If the refrigerant is added until the bubbles are

gone, serious damage and injury can result from overcharging the system.

A/C Schematic

Performance Testing

Typical Electrical Circuit

Heater Core and Air Duct Locations

Coolant Valve

Diesel Fuel-Fired Heater

Summary (1 of 5)

• Heavy-duty heating and air conditioning systems are designed primarily to keep the cab comfortable despite the outside weather.

• Liquids absorb heat when changed from a liquid state to a gas.

• Gases release heat when changed from a gaseous state to a liquid.

• Heat always moves from a hotter area to a cooler area.

Summary (2 of 5)

• The temperature at which a liquid changes state to a gas depends on the pressure acting on it.

• Refrigerants are required to have a low boil point and to change state quickly.

• The basic refrigeration cycle consists of compression, condensation, expansion, and evaporation.

• Compression heats up refrigerant gas and changes the state of the refrigerant from gas to a liquid.

• Expansion reduces the pressure of the liquid refrigerant and changes the refrigerant from a liquid state to a gas.

Summary (3 of 5)

• There are two types of refrigerant lines: suction lines and discharge lines.

• Air conditioning systems are sensitive to moisture and dirt. Clean working conditions are extremely important.

• The electrical system must be checked periodically to prevent the truck air conditioning system from failing unexpectedly.

• Some trucks use electronic climate and A/C controls. These systems can either be standalone or connected to the chassis data bus.

Summary (4 of 5)

• Current A/C system controllers are networked to the chassis data bus. – This enables them to communicate with other MIDs to

optimize system operation.

• When the A/C controller is networked to the chassis data bus, the system is accessed by the J1939 connector. – This allows the system to be read by any software

that can interpret MIDs, PIDs, SIDs and FMIs. To perform operations other than simply read the system, proprietary software is usually required.

Summary (5 of 5)

• The A/C electronics can respond to a request off the data bus to put the A/C compressor into dropout mode. – An example would be a temporary request for

full power from the engine electronics, when it would be desirable to eliminate as many parasitic loads as possible.