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Closed loop systems – a deeper look

By autotech-nath on May 30, 2023

Fun fact – not all automotive air suspension systems use the same working principle! Therefore, it is important for a professional workshop to know what system they are working on: a conventional open system or a so-called ‘closed loop’ air suspension system. The technical know-how for properly diagnosing possible system errors or defects, ensuring the correct working procedure is followed, correctly installing new air suspension components, and checking the system and making it operational again after an air spring leak is different from the conventional open air suspension system. In this article, Arnott will provide more insight into the alternative closed loop system.

Inflation and deflation processes…

Now that we know that specific technical knowledge and understanding is required, let’s take a look at the general differences between the two systems’ working principles before diving deeper into the subject.

Looking at the graph above, it shows the workings of the compressor are quite different in a closed loop system. In an open system the compressor compresses ambient air to fill the reservoir and/or air bellows, a closed loop compressor uses the pressurized nitrogen that is already existing in the system. Additionally, in a traditional air suspension system excessive air is released into the open but for a closed loop system the nitrogen stays inside the system and excessive pressure is led back to the reservoir. This is an important aspect to keep in mind when working on this system.

For inflation/additional pressure, the closed loop system uses two reversing valves that will open towards the valve block. The solenoid valve for the appropriate air spring opens and the additional pressure is transferred. When deflating, the excessive pressure is not discarded but cleverly routed back to the reservoir by opening the two reversing valves in the other direction. The excessive pressure is now transferred back into the reservoir. The pressure flow diagram below gives an indication of how the pressurised nitrogen is routed when an air spring is inflated or deflated.

The twist…

Although it is called a ‘closed loop’ system and the diagram above shows how the compressed nitrogen is transferred from one area to another to inflate/deflate the system, don’t be fooled! A closed loop system does have the capability to make up for small amounts of nitrogen loss by drawing in and compressing atmospheric air. When the system has a leak, it will try to retain its required operating pressure by opening the (ambient) air intake. However, it can only compensate for a small loss of nitrogen and when the compressed nitrogen mass gets too low the system will stop functioning.


Just like a conventional air suspension system, the system pressure will gradually drop when there is a big air leak present. Therefore, a symptom of an under-charged system is lack of operation. However, in a closed loop system, the ECU (Electronic Control Unit) continuously calculates the ‘nitrogen mass’ as an indication of normal operation. If too low, error messages like ‘max payload exceeded’ might appear on the dashboard (depending on the car manufacturer).

An indicator of an overcharged system is audible discharging of excess pressure. This only occurs when a mechanic has not followed the correct procedure when recharging the system after a leak repair. It goes without saying that an overcharged system can lead to damaged and defective components, alongside serious safety consequences.

This information is provided to you by Arnott – Suspension Products. With more than 30 years of experience in engineering, designing, and manufacturing high quality air suspension components for the aftermarket, Arnott is the technical expert when it comes to air suspension systems. Arnott’s products are produced with high-quality, OE components offering exact form, fit and function. Each product is extensively tested in our American and European facilities and custom-tuned to suit the specific vehicle make and model before being produced.



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