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
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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
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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-thannormal 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.