Mercedes-Benz C400 Active Suspension Air Spring Replacement Cost

Air springs are a more advanced alternative to traditional coil springs for vehicles. Instead of relying on set tension from a coil spring, they rely on air pressure to hold the vehicle at a desired height. They are more versatile in that they can set the vehicle to different ride heights, level the vehicle on unlevel terrain, and allow for greater or lesser ride stiffness. They are typically more comfortable for the passengers of the vehicle, but they require more maintenance.

Active air suspension refers to the ability of the air springs to adjust to features of the driving or parking surface, as well as driver inputs. This is accomplished by an air compressor sending pressurized air to the air springs, which allows them to raise the vehicle or stiffen the ride. A solenoid will open an air valve to let air into the air spring or relieve pressure when needed. When air is let into the air spring, a rubber bladder will prevent the air from escaping, and lift the vehicle on that corner. Likewise, when the relief valve or solenoid releases air from the air spring, the vehicle will lower or become more soft on that corner. This is very comparable to sitting the vehicle on top of four very strong balloons.

A faulty or leaking air spring, or airbag, is very noticeable, but it will display its fault in a few different ways. If the spring is leaking, it will allow that corner of the vehicle to sag while parked or driving, and will constantly make a hissing noise. An airbag with a defective relief valve will fail to depressurize when needed, and the ride may become overly rigid with time. Lastly, an airbag which cannot receive are from the inlet valve will not make a hissing sound, but will likely decrease in pressure over time causing a sagging of that corner of the vehicle. In all cases, handling characteristics will likely become undesirable.

Just like a coil spring, a failed airbag should be replaced right away. Not only is driving with a faulty airbag difficult or dangerous, but there is a risk of harming other components - the shocks at the very least. Also, if the vehicle is burdened by any kind of a load, bumps will likely cause the vehicle to lose control at even moderate speeds.

Most vehicles with airbag suspension will require maintenance at some point, and few will keep the same air springs throughout their useful service life. Since the air springs are composed of rubber bladders, they tend to crack and dry rot much like other rubber components on a vehicle. To prevent the airbags from degrading early, the best practice is never to leave the air springs deflated, especially when the vehicle is being stored, and keep them clean of dirt and other contaminants.

When diagnosing active suspension air springs, the technician must first inspect the vehicle to locate the suspected component of the active suspension system. If the air spring is suspected of leaking, the pressure relief valve, air spring control solenoid, or both are in working condition. If they are both functional, and the air spring has a feed of compressed air, the technician will then look for signs of leaking and degradation. If a leak is found, which would be likely in this example, the air spring will be declared failed and replaced.

Some air springs are installed on struts, and some are installed separately of shocks absorbers. These two types have different replacement procedures, but, generally speaking, they are simply connected to the chassis and lower control arm. In the worst case, the wheel must be removed, followed by removal of the lower ball joint from the steering knuckle or wheel spindle. This is of course done after disabling the compressor and relieving air pressure from the suspension system. Once the lower control arm is free from the sway bar, tie rods, radius arms, or anything else preventing it from swinging away from the spring, the lower control arm will be lowered for removal of the air spring. For strut mounted air springs, or air struts, the top of the air strut must be removed from the upper strut mount. This is located under the hood, in the trunk, or behind interior side-wall cover panels. After installation, all fasteners should be torqued to specification, and the alignment should be checked as well.

Any time the air suspension system is opened, it should be purged of air before reclosing the system. This will help rid the system of moisture caused by humidity and condensation. Also, this will allow the technician to monitor for debris, and take action if needed.

When diagnosing any active suspension system, the suspension must be deactivated before raising the vehicle with a jack. When the vehicle is lifted, the active suspension will attempt to level itself if left on, and this may result in damage to the active suspension system, or unstable lifting with the jack. If deactivating the system is not possible, disable the compressor.

Replacing the air spring or air strut requires a bit of assumed knowledge of working with pneumatic systems, leak detection suspension system components, and operation of the electrical portion of the system. Without this prior knowledge, this job is not only difficult or impossible, but dangerous. In most cases, diagnosis of several components related to the air spring or air strut is necessary before determining if the air spring or strut has failed. Only the most well equipped and knowledgeable DIYer should attempt this repair.