Air-conditioning circuit
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 note
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 suitably qualified personnel only.
TABLE OF CONTENTS
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.
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:
| 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 |
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.
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