Air suspension may seem complex but it is surprisingly Diy-friendly. Working in conjunction with Arnott Europe, Rob Marshall gives an overview of diagnosing and replacing air springs, using a Mercedes ML W164 (2005-11) as an example.
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Modern air suspensions seem immensely intricate but, while considerable engineering goes into their design and construction of the individual components, their layouts are generally straightforward. While you should research the specification of your particular car model, the majority of systems comprise a relayactivated air compressor, which inflates two or more ‘airbag’ springing media. The system is controlled via a dedicated
ECU and, while ride-height sensors tend to be fitted, the system is likely to receive and process information provided by other devices on the vehicle that are not associated directly with the air suspension.
As air springs tend to be made from a flexible, rubber-based material, fatigue introduces cracks and perishing courts splitting, both of which create leaks.
For this reason, air springs should be renewed between every six and ten years, regardless of condition. Should this be ignored, the compressor works overtime, which curtails its life expectancy. Should you buy a car with defective springs and you suspect that they have been less than air-tight for some considerable time, budget for the replacement of both the compressor and its relay.
Thankfully, a confident DIYER should be able to conduct the work at home but bear in mind the need to keep the car body supported with axle stands, whenever you venture beneath it.
The most apparent indication of a problem is an air suspension malfunction lamp/ message on the fascia. Even if this is not the case, walk around the stationary vehicle and verify that the ride-height is level on all air-sprung wheels. Check also that any selflevelling function operates, by loading the vehicle and assessing the suspension’s response. 1
Should you identify leaking springs (or if they are aged 6+ years), depressurise the system, disconnect the pneumatic connections and remove the units. Faulty air springs are in demand by suppliers that remanufacture them to OE standards, so do not scrap them.
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With a working compressor, inspect the air springs for leaks. Spray a soapy solution onto the rubber body and seek escaping air bubbles. Consider also that faulty height correctors can overextend the spring, stressing the flexible material. Check for leaks at every possible setting – splits may become apparent only at specific ride heights. Support the vehicle with axle stands – never venture beneath the vehicle, because it could lower itself onto you without warning.
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This strut incorporates both an air spring and a damper that a DIYER cannot strip and repair. Should the damper fail, hydraulic oil can leak into the air spring assembly, reducing its lifespan.
In either case, renew the entire strut. As with conventional springs and dampers, you should replace air springs in axle pairs. Exact removal and replacement procedures differ between makes and models, which is why Arnott’s website lists instructions for most popular vehicles: Before fitting the new parts, ensure that all the connections are clean. 7
Any dashboard warning message represents a fault code within the suspension ECU that needs to be diagnosed with suitable equipment through the OBD port. Record the codes before resetting the memory. Once done, restart the engine, dismount your diagnostic hardware and verify that the compressor activates. Take the car for a short run but be aware that resetting the codes alone does not mean that you have resolved a fault. Check if the initial code reappears. 2
Should you decide to replace the compressor, the good news is that it tends to be attached by only a few bolts. Take care not to strain any airlines and pneumatic connectors and separate them carefully. Replace the relay as well. Should it be faulty, the compressor will either not function, or will run continually. Arnott told CM that, for this reason, warranties on replacement compressors are not honoured, when the relay is not replaced at the same time. 4
If the compressor fails to start, check the basics. These include fuses and electrical wiring for continuity and any connectors for corrosion. Consider that the compressor is designed to work intermittently, not continually. Pumping air into leaking springs, shortens its working life considerably. 3
With the new air springs fitted, support the relevant side of the vehicle with a jack, so that the wheel dangles. Start the engine and note that the compressor operates to inflate the spring. If you start the engine with the car’s weight on the road-wheel, there is a risk that the spring will be folded as it inflates, causing it not to seat correctly and, potentially, tearing the material. Check the inflated spring for leaks and that air pressure is maintained. Lower the jack slowly. 8