Air suspension: repair it, or convert to coil springs?

The failure chain

Almost every air suspension complaint is one story told at different stages.

Stage one: the bellows perishes. It is rubber, it flexes millions of times, and it lives under a wheel arch. Ozone, road salt and age crack it, usually along the fold. The leak starts small.

Stage two: the system compensates. The controller sees a corner sitting low and runs the compressor to fix it. From the driver's seat nothing is wrong yet — the car sits level, because the system is winning.

Stage three: the compressor works itself to death. These compressors are rated for a low duty cycle, seconds of running per hour under normal use. Fighting a leak they run for minutes at a time, get hot, and fail.

Stage four: the car sits on its bump stops and everyone blames the compressor.

Finding the leak properly

Soapy water and patience. Inflate the system, then work systematically:

• The bellows themselves, with the fold pulled open where you can safely do so.
• The crimped joints at the top and bottom of each bag.
• The air line fittings, which are usually push-to-connect and fail at the O-ring.
• The valve block, which is where a leak that affects more than one corner will be.
• The compressor's own outlet and its dryer connection.

A car that drops overnight has a leak large enough to find. A car that drops over a week is harder, and worth leaving inflated overnight with the system disabled so the drop can be measured rather than guessed at.

The dryer, which nobody replaces

The compressor draws in ambient air, which contains water. A desiccant dryer removes it before the air goes into the system, and it regenerates each time the system vents.

Once the desiccant is saturated — which happens after a long period of over-working, i.e. exactly the scenario above — water goes into the lines. Then two things follow: corrosion in the valve block, and in cold weather, ice in the lines. An air suspension that misbehaves only below freezing is telling you the dryer is done.

So the dryer is replaced with the compressor. Kits that include it are the ones to buy, and the ones that do not are cheaper for a reason.

Ride height sensors

The controller has no idea how high the car is except through these. Each is a rotary sensor on the chassis with a linkage arm down to the control arm.

They fail in ways that look like something else: a car that sits crooked, a car that raises one corner overnight, a car that refuses to leave a low setting. The linkage ends are plastic and they break; the connectors sit low and corrode.

Before condemning any air spring, read live ride-height data at all four corners and compare it against a tape measure. A sensor reporting a height the car is not at makes the whole system behave irrationally.

Repair or convert

Repair when the vehicle relies on levelling — it tows, it carries load, it has a third row, or it has adaptive damping tied into the same controller. Repair also when the vehicle is worth keeping and you want it to behave the way it was designed to.

Convert when the vehicle is older, the whole system is tired rather than one corner, the parts cost approaches the value of the car, and you are content to lose the levelling.

What a conversion actually costs you:

1. 1Load levelling. The car will now squat under load like any coil-sprung vehicle.
2. 2Adaptive damping, sometimes. If the dampers are electronically controlled and the kit replaces them with passive units, that function is gone.
3. 3Ride quality. It changes. Whether it is worse depends on the kit and on what the bags were like — a good conversion on a car with three perished bags is an improvement.
4. 4A fault light, unless the kit handles it. The controller will notice the sensors are no longer reporting movement. Kits that include a CAN module or a sensor emulator keep the dash clean; kits that do not, do not.

Buy a conversion kit that names the fault-light solution explicitly. That is the difference between a good one and a set of springs in a box.

Whatever you do, do it in pairs

Front pair or rear pair, together. The remaining original component on the same axle has the same age and the same mileage as the one that failed, and mismatched spring rates on an axle is a handling change as well as a levelling problem.