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Car air-conditioning

Here you will find useful basic information and handy tips relating to air-conditioning systems in vehicles.

The air-conditioning system used to be something reserved for the upper classes. Today it is standard equipment in all vehicle classes. But how does it achieve cooling and which components are actually involved? Here you will find not only find answers to these questions, but also tips on the maintenance, troubleshooting, and repair of air-conditioning systems.

Important safety information
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.

 

AIR-CONDITIONING CIRCUIT: FUNCTION

Refrigerant and the cooling circuit are required to control the climate inside the vehicle. A blend of cold and hot air makes it possible to create the desired climate conditions ─ irrespective of the conditions outside. This makes the air-conditioning system a key factor in improving safety and driving comfort.

 

The individual components of the refrigerant circuit are connected by hoses and/or aluminium pipes and thus form a closed system. Refrigerant and refrigerant oil circulate in the system, driven by the compressor.

How the air-conditioning system with expansion valve works

The circuit has two sides:

  • The section between the compressor and the expansion valve is the high-pressure side (yellow/red).
  • The section between the expansion valve and the compressor is the low-pressure side (blue).

 

The gaseous refrigerant is compressed by the compressor (thereby significantly increasing its temperature) and pressed under high pressure through the condenser. This removes heat from refrigerant - it condenses and changes its state from gas to liquid.

 

The filter dryer, the next unit, removes contaminants and air from the liquid refrigerant as well as humidity. This ensures system effectiveness and protects the components from damage caused by contaminants.

COMPONENTS OF THE AIR-CONDITIONING SYSTEM: OVERVIEW

Compressors

The air-conditioning compressor is usually driven by the engine via a belt or ribbed V-belt. The compressor compresses and transports the refrigerant in the system. There are different designs available.

The refrigerant is sucked in as a gas at low temperature from the evaporator and then compressed. It is then forwarded in a gaseous state at high temperature and high pressure to the condenser.

The compressor can be dimensioned depending on the size of the system. The compressor is filled with special lubricating oil. Part of the oil circulates with the refrigerant through the air-conditioning system.

INFO BOX

Insufficient lubrication caused by leaks and related refrigerant and oil losses, as well as insufficient maintenance, can lead to compressor failures (leaking shaft seal, leaking housing seal, bearing damage, the piston getting stuck, etc.).

Condensers

The condenser is needed for cooling down the refrigerant that has become heated during compression in the compressor. The hot refrigerant gas flows into the condenser, discharging heat to the surroundings via the pipe and fins. As it cools down, the state of the refrigerant changes again from gaseous to liquid.

 

Operating principle
The hot refrigerant gas flows on top into the condenser and transfers heat to the surroundings via the pipe and fins. As a result of cooling down, the refrigerant exits the condenser at the lower connection in liquid state.

 

Effect of failure
A defective condenser may exhibit the following symptoms:

  • Poor cooling performance
  • Failure of the air-conditioning system
  • Continuously running condenser fan

 

Causes for occurring faults can be:

  • Leaks at the connections or caused by damage
  • Insufficient heat exchange due to soiling

 

Troubleshooting
Test steps for fault elimination:

  • Check condenser for contamination
  • Check for leaks
  • Pressure test on the high-pressure and low-pressure sides

INFO BOX

Due to the special installation location, failures of environmental nature can occur caused by contamination or stone chipping. Defects caused by front-impact accidents occur particularly often.

Filter dryers

The filter elements of the air-conditioning system are either referred to as filter dryers or accumulators, depending on the type of system. The task of the filter dryer is to remove impurities from the refrigerant and also to dehumidify it.

 

Operating principle
The liquid refrigerant enters the filter dryer, flows through a hygroscopic drying medium and leaves the filter dryer again as a liquid. The upper part of a filter dryer simultaneously serves as a compensation chamber, and the lower part as a refrigerant store, in order to compensate fluctuations in pressure in the system.

 

Depending on its design, the filter dryer can only remove a certain amount of humidity – then the drying medium is saturated and no longer in a position to absorb further humidity.

 

Effect of failure
A failure of the filter dryer may exhibit the following symptoms:

  • Poor cooling capacity
  • Failure of the air-conditioning system

 

Causes for the failure of the filter dryer can be:

  • Excessive aging
  • Defective filter pad inside
  • Leaks at the connections or caused by damage

 

Troubleshooting
The following steps are to be considered during troubleshooting:

  • Check maintenance intervals (every 2 years)
  • Leak test/correct fit of the connections/damage
  • Pressure test of the high-pressure and low-pressure sides

INFO BOX

Therefore, the filter dryer must normally be renewed every 2 years or each time the refrigerant circuit is opened. Excessive aging of the filter dryer can lead to severe defects in the air-conditioning system. Filter dryers can be integrated in the condenser. Sometimes it is not possible to replace the filter dryer separately.

Expansion/throttle valve

The expansion valve represents the point of separation between the high pressure and low pressure sections in the refrigerant circuit. It is installed upstream of the evaporator. To achieve optimum cooling capacity in the evaporator, the refrigerant flow is controlled by the expansion valve depending on the temperature. As a result, complete evaporation of the liquid refrigerant is ensured so that only gaseous refrigerant arrives at the compressor. Expansion valves are available in a variety of designs.

 

Operating principle
The liquid refrigerant - arriving through the filter dryer from the condenser - flows through the expansion valve and is injected into the evaporator. The evaporating refrigerant releases evaporation cooling. This causes the temperature to drop. In order to achieve optimum cooling capacity in the evaporator, the refrigerant flow is controlled by the expansion valve depending on the temperature. At the end of the evaporator, the refrigerant is transported through the expansion valve to the compressor. If the refrigerant temperature increases at the end of the evaporator, it expands in the expansion valve. This results in an increase of the refrigerant flow (injection quantity) to the evaporator. If the refrigerant temperature lowers at the end of the evaporator, the volume in the expansion valve decreases. As a result, the expansion valve reduces the refrigerant flow to the evaporator.

 

Effect of failure
A defective expansion valve can manifest itself as follows:

  • Poor cooling capacity
  • Failure of the air-conditioning system

 

There are a number of possible causes of failure:

  • Temperature problems due to overheating or icing
  • Contamination in the system
  • Leaks at the component or the connection pipes

 

Troubleshooting
The following test steps should be followed in the case of a malfunction

  • Visual inspection
  • Acoustic test
  • Check connection pipes for tight and correct fit
  • Check components and connections for leak-tightness
  • Temperature measurement on the line system
  • Pressure measurement with the compressor switched on and the engine running

INFO BOX

Humidity and contamination in the air-conditioning system can severely impact the functional capability of expansion valves/orifice tubes and lead to malfunctions. This means regular maintenance is important!

Evaporator

The evaporator is used to exchange heat between the ambient air and the refrigerant in the air-conditioning system. The expansion valve and/or orifice tube injects the highly pressurised liquid refrigerant into the evaporator. The refrigerant expands. The evaporation cooling produced by this process is discharged to the environment via the large surface of the evaporator and routed to the vehicle interior through the blower air flow.

 

Effect of failure
A defective evaporator exhibits the following symptoms:

  • Poor cooling performance
  • Failure of the air-conditioning system
  • Poor ventilation performance

 

Causes for failure of the evaporator can be:

  • Pipes blocked in the evaporator
  • Evaporator leaking (at the connection, caused by damage)
  • Evaporator contaminated (air passage disturbed)

 

Troubleshooting
The following test steps should be considered during troubleshooting:

  • Check evaporator for contamination
  • Inspect evaporator for damage
  • Check connection pipes for correct fit
  • Leak test
  • Pressure measurement with the compressor switched on and the engine running
  • Temperature measurement on the input and output line

INFO BOX

Temperature problems, contamination, humidity, and insufficient maintenance can lead to evaporator defects. In order to avoid this, the air-conditioning system must be maintained and/or disinfected on a regular basis.

Pressure switches and switches

Pressure switches are responsible for protecting the air-conditioning system against damage caused by pressures that are too high or too low. A distinction is made between low-pressure switches, high-pressure switches, and the trinary switch. The trinary switch includes the high-pressure and low-pressure switch as well as an additional switch contact for the condenser fan.

 

Operating principle
The pressure switch (pressure monitor) is normally installed on the high-pressure side of the air-conditioning system. If the pressure is too high (approx. 26-33 bar), it switches off the power supply to the compressor clutch. If the pressure is reduced (approx. 5 bar), it switches the power supply on again. If the pressure is too low (approx. 2 bar), the power supply is interrupted as well in order to avoid compressor damage due to insufficient lubrication. The third switch contact in the trinary switch controls the electrical condenser fan in order to ensure optimum refrigerant condensation in the condenser.

 

Effect of failure
A defective or failing pressure switch can manifest itself as follows:

  • Insufficient cooling performance
  • Air-conditioning system not working
  • Frequent switching on and off of the compressor clutch

 

There are a number of possible causes of failure:

  • Contact fault at electrical connections
  • Contamination in the system
  • Damage to the housing caused by vibrations or accidents

 

Troubleshooting
Test steps for troubleshooting:

  • Visual inspection
  • Check connector for correct fit
  • Inspect component for damage
  • Pressure measurement with the compressor switched on and the engine running
  • Component test in the disassembled condition with nitrogen gas cylinder, pressure reducer, and multimeter

INFO BOX

Pressure switches may fail due to contacting problems or contamination. Regular system maintenance prevents failures. Additional air-conditioning system switches, such as on-off switches, round off the range.

Blower fans

The blower fan is used for ventilation of the passenger vehicle. It ensures fog-free windows and a pleasant climate inside the vehicle. Major pre-requisites for safety and driving comfort.

INFO BOX

Failure of the fan results in an uncomfortable interior climate, which has a negative impact on the driver's concentration. This is a significant reduction in safety. Lack of ventilation can also cause the windscreen to fog up. Vision limited by misted up windows is a major safety risk.

Fittings and hoses

Fittings and hoses connect the individual components carrying refrigerant. The fittings are pressed onto the hose end using a special tool. This tool is available in a variety of designs.

INFO BOX

Hoses from Hella Gutmann Solutions are characterised by high flexibility, low weight, and minimal refrigerant loss.

Condenser fan

The condenser fan helps to ensure the optimal liquefaction of the refrigerant no matter what operating state the vehicle is in. It is mounted upstream or downstream of the condenser and/or engine radiator as an additional or combination fan.

INFO BOX

Condenser fans may fail due to electrical or mechanical damage. As a result, the refrigerant is no longer sufficiently liquefied. The air-conditioning system performance is reduced.

AIR-CONDITIONING SERVICE, RECHARGE AND CHECK: USEFUL INFORMATION

Alternating check and service

The air-conditioning system is a system requiring maintenance. It should therefore be inspected at regular intervals in order to prevent damage. The air-conditioning check and air-conditioning service are comparable to a small and large inspection:

INFO BOX

Behr Hella Service recommends: Perform the air-conditioning check every 12 months and the air-conditioning service every 2 years.

What should be done when?
What? Air-conditioning check
When? Every 12 months
Why? The interior filter filters dust, pollen, and dirt particles out of the air before it flows clean and cooled into the interior. As with every filter, it has a limited absorption capacity. There is an evaporator in every air-conditioning system. Condensation forms in its fins. With time, bacteria, fungi, and micro-organisms settle here. The evaporator must therefore be regularly disinfected.
What does it involve? ➔ Visual inspection of all components
➔ Function and performance test
➔ Replacement of cabin filter
➔ Where necessary, disinfection of evaporator

 

What should be done when?
What? Air-conditioning service
When? Every 2 years
Why? Up to 10 % of the refrigerant escapes per year, even from a new air-conditioning system. This is normal, however it reduces the cooling performance and increases the risk of damage to the compressor. The refrigerant is freed from humidity and contaminants by the filter dryer.
What does it involve? ➔ Visual inspection of all components
➔ Function and performance test
➔ Replacement of the filter dryer
➔ Where necessary, disinfection of evaporator
➔ Replacement of the refrigerant
➔ Leak test
➔ Replacement of cabin filter

AIR-CONDITIONING AND COOLING UNIT: OVERVIEW

Consider air conditioning and cooling as a unit

Although the air-conditioning system and the engine cooling system are two separate systems, they influence one another. Air-conditioning system operation places additional load onto the engine cooling system and the coolant temperature rises. The additives contained in the coolant not only protect against frost, but also against engine overheating. The proper coolant composition increases the boiling point of the medium to above 120 °C. An enormous performance reserve. This is particularly important in the summer, when the air-conditioning system and cooling system are heavily burdened by ambient temperatures and long trips. The best approach is to check the coolant during air-conditioning service as well.

CHECKING THE AIR-CONDITIONING SYSTEM: TROUBLESHOOTING

Testing the cooling capacity

In addition to test tools and special tools, every garage requires the relevant specialist knowledge, which can be acquired through training. This applies in particular to air-conditioning systems. Due to the different systems, these instructions can be used as guidelines only.

Correct evaluation of the pressure manometer display is particularly important.

Air-conditioning systems with expansion valve
Low pressure High pressure Outflow temperature at the center vent Possible causes
high high higher, up to ambient temperature engine overheated, condenser contaminated, condenser fan defective/incorrect direction of rotation, system overfilled
normal to occasionally low high, occasionally higher, possibly fluctuating expansion valve stuck, occasionally closed
normal high slightly higher filter dryer aged, condenser contaminated
high normal to high higher depending on bottleneck line from compressor to expansion valve narrowed
normal normal higher too much refrigerant oil in the system
normal, but inconsistent normal, but inconsistent higher humidity in the system, defective expansion valve
fluctuating fluctuating fluctuating expansion valve or compressor defective
normal to low normal to low higher evaporator contaminated, lack of refrigerant
high low higher, almost ambient temperature expansion valve stuck in opened position, compressor defective/td>
low low higher, up to ambient temperature lack of refrigerant
low pressure and high pressure are the same low pressure and high pressure are the same Ambient temperature Lack of refrigerant, compressor defective, fault in the electrical system

 

Air-conditioning system with orifice tube
Low pressure High pressure Outflow temperature at the center vent Possible causes
high high higher, up to ambient temperature engine overheated, condenser contaminated, condenser fan defective/incorrect direction of rotation, system overfilled
normal to occasionally low high higher system overfilled, condenser contaminated
normal normal to high fluctuating humidity in the system, orifice tube occasionally blocked
high normal higher orifice tube defective (cross section)
normal normal higher too much refrigerant oil in the system
normal to low normal to low higher lack of refrigerant
low pressure and high pressure are the same low pressure and high pressure are the same Ambient temperature Lack of refrigerant, compressor defective, fault in the electrical system

 

CAR AIR-CONDITIONING REPAIR - REMOVAL AND INSTALLATION INSTRUCTIONS: WORKSHOP TIPS

Spare part inspection

Before removing or installing a spare part, check whether the connections, mountings, and other installation-relevant characteristics are identical.

Use new O-rings

When replacing components, always use new O-rings suitable for the refrigerant.

Keep the system closed

The compressor oil has a pronounced hygroscopic effect, which is why the system should remain closed as much as possible and the oil only added shortly before the refrigerant circuit is closed.

Grease the O-rings and seals

Prior to the installation, O-rings and seals are to be greased with refrigerant oil or special lubricants in order to facilitate installation. No other greases or silicone spray may be used as this results in immediate contamination of the new refrigerant.

Renew the dryer

Every time the refrigerant circuit is opened, the dryer must be replaced due to its pronounced hygroscopic effect. If the dryer or accumulator is not regularly replaced, the filter cushion may become separated and distribute silicate particles throughout the entire system, which can cause severe damage.

Seal off connections

The connections of the system should never remain open for an extended period and instead be immediately closed, or sealed off, with caps or plugs. Not doing so will introduce the humidity of the air into the system.

Use two wrenches

In order to avoid damage to connection pipes and/or components, always use two wrenches when loosening and fastening the connections.

Watch out for damage due to vehicle edges

When routing hoses and cables, take steps to avoid causing damage by contacting vehicle edges and other moving components.

Ensure correct oil quantity

When replacing a component of the air-conditioning system, the correct oil quantity in the system is to be ensured. Oil must be refilled or drained as needed.

Before filling, check for leak-tightness and evacuate

Before refilling the system, the system must be checked to ensure that it is leak-tight. The system must then be sufficiently evacuated (approx. 30 minutes) to ensure that all humidity is removed.

Before filling, check for leak-tightness and observe the pressure values

After filling using the quantity of refrigerant specified by the vehicle manufacturer, check the system to ensure that it operates as intended and is leak-tight (electronic leak indicator). At the same time, the high-pressure and low-pressure values must be monitored using pressure manometers and compared against the prescribed values. Compare the outlet temperature at the centre nozzle with the values defined by the manufacturer.

Affixing the service label

After the service connections are fitted with protective caps, affix a service label sticker on the front crossrail to verify that the maintenance work has been carried out.

VIDEO ON THE SUBJECT

AC filter replacement

Replacement of the filter dryer including removal and installation

 

01:55 min

Information regarding the installation of air-conditioning compressors

Ensure that all contamination and foreign material has been removed from the refrigerant circuit. To this end, flush the system before installing the new compressor. Refrigerant R134a or a special flushing solution is suitable for flushing, depending on the contamination level. Compressors, dryers (accumulators), expansion valves, and throttle valves cannot be flushed. Since system contamination (abrasion, chips) must always be assumed in the case of a compressor defect and cannot be ruled out, it is absolutely essential that the system be flushed when replacing this component. Ensure that no residual flushing solution remains in the system. Dry the refrigerant circuit using nitrogen if necessary.

 

Replace the filter dryer or accumulator and the expansion valve or throttle valve (orifice tube).

 

As one and the same compressor can potentially be used for different vehicles or systems, the oil filling quantity and viscosity must be checked and/or corrected according to the manufacturer's specifications before installing the compressor. For this purpose, all the oil must be siphoned off and collected. The compressor must then be refilled with the entire oil quantity specified by the vehicle manufacturer (system oil quantity).

 

To ensure that the oil is distributed equally, the compressor must be turned 10x by hand before installation. When installing the drive belt it must be ensured that it is aligned. Some compressors are designed for so-called "multiple applications". This means that they can be installed in different vehicles. Except the number of grooves on the magnetic clutch, there is 100% agreement with the "used part".

 

After compressor installation and refilling of the refrigerant circuit, the engine should first be started and operated for several minutes at idling speed.

 

Further specifications (instruction leaflets, manufacturer's specifications, run-in specifications) are to be observed separately.