GEM's guide to air suspension

Pneumatic suspension is fitted to a surprisingly large quantity of cars, including expensive off-road vehicles. Yet, like conventional suspension, air suspension requires annual maintenance. Consider also that neglect of one component can damage associated parts, leading to a much higher repair bill. If you drive a car with an air suspension fault, it also can be dangerous.

What is air suspension?

Air suspension replaces conventional coiled metal springs with air-filled springs that support the vehicle's weight.

Why do cars have air suspension?

Self-levelling is a major benefit. This not only stops the vehicle from looking as though its suspension has collapsed, when carrying heavy weights, but it also preserves optimum suspension geometry and handling characteristics. This can be especially useful, when towing. Some less expensive cars, such as the pre-2013 Citroën C4 Picasso, employ self-levelling air suspension on the rear axle alone.

Self-levelling systems can also vary the ride height, especially on performance cars. Aerodynamics and fuel consumption can be improved, by bringing the car closer to the ground automatically at motorway speeds. Off-road vehicles also benefit from the owner being able to raise the suspension to clear obstacles, without damaging the vehicle's underbelly.

The primary benefit, however, is a more comfortable ride. The cushioning effect of air pressure, contained within a flexible air spring, insulates the cabin more effectively against bumps, vibration and noise. More sophisticated systems incorporate active damping to reduce the front end from dipping under braking, or the car rolling to one side excessively under cornering.

Try not to confuse air suspension with hydraulic suspension. Some manufacturers employ both systems. For instance, Mercedes-Benz's AIRMATIC utilises pneumatics, whereas the faster-reacting ABC (Active Body Control) employs hydraulics.

How does air suspension work?

The air springs are flexible air bags, produced mainly from a special flexible rubber material. Air is pumped into them by an electric compressor and the vehicle height is regulated by electric sensors. Air pressure flows in and out of an electric valve block and the whole system is regulated by an Electronic Control Unit.

An air strut contains an integral shock absorber; an air spring does not.

What goes wrong with air suspension?

Air leaks are the most common issue. Pressure tends to escape from air springs, because the flexible material splits over time and mileage. This causes the compressor to work longer to compensate. This can result in overheating, which damages not just the compressor but also the air lines. In many cases, the suspension ECU detects a fault and cuts power to the compressor. However, this results in the suspension depressurising to its lowest ride height and looking as though it has collapsed.

A failed relay also causes the compressor to run continually and overheat. This explains why it is a good idea to replace the relay at the same time as a compressor.

Air suspension systems tend to utilise conventional shock absorbers ('dampers'), too. These fail in the usual manner. Not only do they become less effective over time but they also can leak.

When replacing leaking air springs and failed dampers, avoid replacing one at a time. They should be renewed in axle sets to preserve safe handling and braking.

The main way of avoiding an expensive repair is to ensure that regular checks are carried out. This includes a close annual assessment of the air springs. Should you

find that the rubber materials have started to perish, it is best to replace the units in axle pairs, before leaks start.

Our accompanying blog on air suspension shows the most common checks and repairs that you can conduct.

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