Heavy-Duty Air Suspension Failures: Air Springs, Leveling Valves and ECAS

In the yard on a cold morning, a tractor unit is sitting visibly low on its right rear corner. The driver's diagnosis is ready before anyone looks underneath: the air spring must have burst. Yet the chassis was level the night before, and there is no visible tear in the bellows. Once the ignition is on and pressure builds, the rear axle rights itself within a couple of minutes and shows no symptom for the rest of the day. This pattern is not the death of a single part; it is the early warning of a circuit that is losing air quietly overnight, and the leak could just as easily sit in the air spring as in the leveling valve, the solenoid valve block, or a fitting thread nobody suspected. In a heavy commercial vehicle, air suspension is not a single assembly; it is a chain that links the functions of producing, drying, metering, distributing and carrying the air. This guide maps that chain from end to end.

Why replace the leaf with air in a heavy-duty vehicle?

Air suspension is a suspension system that carries the vehicle's weight on air compressed inside a sealed volume instead of a steel spring, and continuously varies that air's pressure with load to keep the chassis height constant. A rubber-reinforced air spring carries the load, a shock absorber damps the oscillation, and a leveling device makes the position decision. What sets the system apart is not softness but its ability to behave independently of load.

A steel leaf spring has a fixed spring rate. An empty tractor on leaf springs rides hard; fully loaded, the same springs feel inadequate — no single spring pack can satisfy both conditions at once. The compromise is set for the loaded state, and the empty vehicle pays for it in road holding. In an air spring, pressure inside the bellows rises with load, so the spring rate scales with it; the vehicle behaves with a similar oscillation character whether empty or fully loaded.

The practical result is that chassis height stays constant, and that matters for more than comfort. A fixed ride height sets the driveshaft working angle, headlamp aim, the relationship between aerodynamic body panels and the road, and the fifth-wheel height on a tractor unit, all at once. On a leaf-sprung tractor, the rear sags when a trailer is coupled and rises when it is dropped, so kingpin height shifts with every load. On an air-suspended tractor, that height stays within the same band regardless of load, and the geometry of the combination stays predictable.

The second consequence is the impact on the road surface and on the load itself. Suspension systems that reduce the shock transmitted to the road surface are classed in regulation as road-friendly suspension; air suspension and systems recognised as equivalent fall into this class, and a somewhat higher axle load may be permitted on the drive axle as a result. Because the allowances vary by country, the vehicle's registration documents and the applicable national regulation govern. The same softness benefits the cargo too: for fragile loads and sensitive machinery transport, air suspension is not a comfort feature but a damage-prevention tool.

Leaf springs versus air suspension

Splitting the two systems into "old and new" is misleading. In heavy construction and off-road service, leaf springs are still preferred; in long-haul, distribution and passenger transport, air suspension has become close to standard. The choice follows the vehicle's duty cycle.

The most important row in the table is failure character. On a leaf-sprung vehicle, a fault is seen and heard; on an air-suspended vehicle, a fault is more often measured over time. A vehicle that settles overnight can behave completely normally once it is moving. Diagnosis therefore has to look at the circuit, not just the part.

System map: what does each part do, and what does a fault break?

Air suspension is made up of three layers. The supply layer produces, dries, protects and stores the air. The control layer decides how much air goes to which air spring; that decision is made either by a mechanical arm or by an electronic control unit. The carrying and damping layer holds the load, damps motion and locates the axle to the chassis. The table below brings all three layers together in one list.

Where the air comes from: when and how is the suspension circuit fed?

Air suspension does not generate its own air; it is fed from the same source as the brake system. The compressor produces the air, the dryer separates moisture and oil vapour, and the multi-circuit protection valve distributes the incoming air in a defined priority order. In that order, the brake circuits always come first; the suspension circuit only fills once the brake circuits have reached their defined threshold pressure. A vehicle not rising immediately on a cold morning is not a fault — it is proof that the priority order is working correctly. The real fault is the vehicle not rising once pressure has reached normal.

The dryer's effect on suspension deserves separate emphasis. Once the cartridge is saturated, or the purge valve fails to do its job, moisture and oil are carried into every circuit. Water collects inside the solenoid block, freezes there in winter, and one corner simply stops responding; oil reaches the inner surface of the air spring and chemically ages the rubber-reinforced body. In a vehicle with recurring suspension faults, the dryer's condition should be questioned before the air spring is.

How is ride height controlled? The difference between a mechanical valve and ECAS

The part that actually makes a system "air suspension" is not the air spring, but the leveling device that decides how much air enters it. That decision is made in one of two ways, and the difference defines everything the vehicle can do.

A mechanical leveling valve consists of a body bolted to the chassis, and an arm and link rod connected to the axle. When the vehicle is loaded, the chassis drops, the arm rotates upward and the valve opens the fill path; once the chassis reaches design height, the arm returns to neutral and the valve closes. There is no interpretation, no memory, no electricity; the arm's position is close to being the target itself. The adjustment logic and failure symptoms of this device are covered in detail in the levelling valve guide.

ECAS does the same job electronically. Height is measured by a sensor, compared against a target, and the difference is interpreted by the control unit. The unit can fill or empty each air spring separately by opening the solenoids in the valve block. The key difference is delay and threshold management: the system does not react to every small deviation, and it takes into account whether the vehicle is moving and at what speed. Air consumption drops as a result, and no unnecessary correction is made while driving.

Raising, lowering and kneeling: the system's additional functions

Every capability of air suspension beyond constant height comes from deliberately taking the same circuit out of its balanced state. Foremost among these is raising and lowering: the driver can command the chassis below or above design height from the control panel. This function is intended for loading and unloading while parked, and only for limited, low-speed situations while driving.

Traction assist (kneel-forward) temporarily shifts extra load onto the drive axle. The tag axle's air springs are partially or fully emptied, weight shifts to the drive axle, and traction increases. This is the function that saves a start-off on slippery ground, a snowy ramp or a muddy site exit. Because it temporarily pushes past axle-load limits, it is restricted in time and speed and disengages automatically above a set speed. The scope of the limits depends on vehicle type approval, so the owner's manual and applicable regulation govern.

Lift-axle control works in the opposite direction: when running empty or lightly loaded, the tag axle is lifted clear of the road; tyre wear drops, rolling resistance falls and manoeuvrability improves. Once loading begins, the system lowers the axle automatically. The most common complaint is the axle not lowering under load, or not lifting when empty; this usually points to a fault in the lift axle valve, pressure switch or lift air spring's own circuit, while the main suspension circuit can be perfectly healthy.

Dock ramps and load-height adjustment

The platform height of a trailer waiting at a warehouse dock and the height of the dock itself often do not match. Air suspension removes that mismatch: a command from the control panel brings the platform level with the dock, the forklift crosses a flat surface, and loading time is shortened. This is the capability the system sees the most use for day to day.

On electronically controlled systems, the process is faster with memory levels: the driver saves a frequently used dock height once, and on the next visit drops to that height with a single button; the return-to-ride-height button then brings the vehicle back to design height. Confirming that return before setting off is the driver's responsibility. Although the system pulls itself back to ride height above a set speed, the vehicle can run low until that speed is reached, and the underbody can contact the ground.

During loading, the vehicle settles after every pallet and the system tops up automatically to hold the level. These corrections use air; on long loading operations with the engine off, the tank can run down and the system quietly stops responding. A vehicle lowered at the dock is also not a vehicle to work under: the lowered position is not mechanically locked, and a valve command or a leak can move the chassis.

From symptom to cause: the air suspension fault-diagnosis table

Most faults are narrowed down by asking three questions in order. Is the problem at one corner or across the whole vehicle? Does it appear while driving or once parked? Does the system not respond at all, or respond incorrectly? The table below matches these three questions to field symptoms.

The most common mix-up is confusing the first two rows. A vehicle leaning to one side usually points to a single component, while a vehicle settling overnight points to a leak somewhere in the circuit; the real evidence is gathered while the vehicle is parked.

How is an air leak found? A methodical search

Leak-hunting is the most time-consuming job in air suspension, because a leak is not heard, is not seen, and is usually looked for at the place its result shows up rather than where it actually is. A methodical search puts the checks in order.

1. Put the vehicle on level, solid ground, apply the parking brake, chock the wheels and let the system reach full pressure before shutting off the engine.
2. Stop the engine, note the pressure value, and leave the vehicle undisturbed for at least several hours, ideally overnight; then read pressure and chassis height again. The rate of drop gives a first idea of leak size.
3. Measure chassis height at all four corners separately. Only one corner dropping points to that corner; all corners dropping equally points to the shared supply side.
4. Re-pressurise the system and sweep the suspect areas with leak-detection solution: the air spring's top cap, the lower skirt, the retaining nuts, the air inlet fitting and the last stretch of line before the air spring come first.
5. Check the solenoid valve block's exhaust port. If air keeps coming from this port while the system is at target height, the leak is not in the air spring but in the valve's internal seal. On mechanical systems, the same check is made at the leveling valve's exhaust port.
6. Follow the air lines by hand along the chassis; a clip that has slipped, a line rubbing on the chassis rail, or a section worn by vibration are the most common leak points. Do not over-tighten fittings on reflex — over-tightening plastic lines creates a new leak.
7. If the leak still cannot be found, isolate the circuits one by one. If the drop stops after the point where a circuit is cut off, the leak is in that section; if it does not stop, move the search further upstream.
8. Repeat the measurement after the repair. The search can only be closed once pressure and height hold over the same waiting period as before.

Air spring service life, causes of damage, and replacement

An air spring works by changing shape on every cycle through its rubber and textile-reinforced body. There is no defined replacement interval; its service life is set not by distance but by the conditions it is exposed to. Of two vehicles on the same route, one can complete high mileage without issue while the other needs a new air spring far sooner because of a misaligned air line rubbing against it.

The causes that kill an air spring are limited in number and nearly all preventable. Oil and fuel contact softens and swells the rubber; the source is most often oil vapour carried through the dryer. Ozone and ultraviolet exposure produce a fine network of surface cracking. Chafing is the most insidious: an air line with a loosened clip, or a loose wiring harness, wears against the air spring's side wall on every cycle and opens a hole within weeks. Overload and bottoming on the bump stop stresses the fold area outside its design range. An incorrect ride height keeps the air spring running either permanently over-extended or permanently over-folded. Continuing to drive on a deflated air spring can finish the body off within a few kilometres.

Replacement is not mechanically complicated; the real risk is safety, and mistakes made in preparation cannot be corrected afterwards.

1. Put the vehicle on level ground, apply the parking brake, chock the wheels and unload the vehicle if possible.
2. Support the chassis mechanically on stands of adequate capacity; never work underneath relying on air pressure or a jack's hydraulics.
3. Vent the suspension circuit's air in the manner specified by the manufacturer, and take the precaution that prevents the circuit from re-pressurising on its own.
4. Disconnect the air spring's air fitting, cap the end and check the line's condition; then remove the upper and lower retaining fasteners. Use a suitable release agent and penetrating oil on corroded fasteners, and do not score the mounting surfaces.
5. Remove the air spring and clean rust, dirt and old gasket residue from the mounting surfaces; surface flatness directly determines the new air spring's seal.
6. Fit the new air spring without twisting its body or forcing the fold area; an air spring installed already folded will be damaged on its first cycle.
7. Tighten the retaining fasteners to the manufacturer's torque and sequence, reconnect the air fitting cleanly, route the line so it cannot chafe, and fit new clips.
8. Pressurise the system gradually, confirm the seal with leak solution, and only then remove the stands; check ride height and recalibrate if required.

Shock absorber, anti-roll bar and chassis mounts

The air spring carries the load but cannot stop the oscillation. The part that turns the spring's energy into heat and damps the motion is the shock absorber, and its importance is greater on an air-suspended vehicle: in a leaf-spring pack, friction between the leaves produces damping on its own, while an air spring has no such internal friction. When the shock absorber stops doing its job, the air spring oscillates freely, the tyre loses contact with the road, and stopping distance increases. The factors that determine shock absorber life and replacement criteria are covered in the shock absorber guide.

The anti-roll bar limits the vehicle's roll through corners and over ramps; on high-bodied vehicles and loads with a high centre of gravity, this bar and its drop links are a direct part of driving safety. Play in the link joints shows up first as noise: a single sharp knock heard at low speed over a kerb transition usually comes from here.

Trailing arms and chassis-mount bushings hold the axle's position. When these bushings wear, the axle wanders both longitudinally and laterally; the result is one-sided tyre wear, a wandering feel on a straight road and slight pulling under braking. Air suspension can hide this wear for a while, because the system keeps correcting height. The bump stop, meanwhile, is the quiet safety part: a disintegrated bump stop allows metal-to-metal contact the moment an air spring bursts, and spreads the damage to air lines, brake hoses and the trailing arm.

ECAS electronics: sensor, control unit and fault codes

Diagnosis follows a different order on electronically controlled systems. The control unit bases its decisions on the signal produced by the height sensor; if the signal is wrong, every decision the unit makes is wrong too, and the system holds the vehicle at the wrong height while believing it is working perfectly. For this reason, the first place to look is not the valve block, it is the sensor's live data.

The height sensor measures the angle of rotation through an arm connected to the axle. Even with a healthy sensor, a bent arm, a link rod that has changed length, or play in a joint shifts the reading. A situation commonly seen in the field is the fault persisting after the sensor has been replaced, because the defect was never in the sensor but in the mechanical linkage that transmits motion to it. The sensor's wiring harness also works in a moving area; intermittent connections there can appear while the vehicle is moving and disappear once it is parked.

The valve block has two fault classes. An electrical fault is a coil going open-circuit or a driver stage failing, and it usually produces a clear fault code. A pneumatic fault, on the other hand, is a valve leaking or not fully closing while the coil itself is fine; this type of fault often produces no code at all. The absence of a code does not clear the system: continuous air consumption and one corner settling overnight are the typical signature of a codeless pneumatic fault. The failure symptoms of the solenoid valve and the calibration required after replacement are explained in the ECAS solenoid valve guide.

A third factor that gets overlooked is power supply and chassis grounding. Low battery voltage, a corroded chassis ground point, or a loose connector can distort the sensor signal and produce a large number of seemingly unrelated fault codes at once; when the code list grows crowded, supply and grounding should be checked before reaching for a parts replacement. Calibration is mandatory after a repair: the system has to relearn its ride height and level limits.

Winter conditions: moisture, freezing and the effect of cold on the system

The season when faults cluster is winter, and the cause is not the cold itself but the cold making the water already in the system visible. The compressor draws in ambient moisture along with the air it compresses; separating that moisture is the dryer's job. Once the cartridge is saturated, water passes into the circuits and collects at the lowest points and narrowest sections. This build-up stays silent through summer, then shows up on the first frost as a single corner failing to respond, clearing on its own once the day warms up. That explains faults present in the morning and gone by afternoon.

The second effect is the rubber body stiffening: at low temperature the air spring material becomes less flexible, and one frozen in a folded position can be damaged on the very first movement. The third effect is salt and road chemicals; salted water corrodes retaining fasteners, sensor arms, link joints and the valve block's connectors.

Maintenance checklist and general reference ranges

Air suspension has no separate maintenance schedule of its own; its upkeep is a handful of checks folded into the vehicle's regular maintenance. What these steps share is that all of them try to catch a fault before it appears.

• Ride height measurement: The distance between chassis and axle is checked periodically against the value in the vehicle's own documentation. An incorrect height is the most common hidden cause of later air spring, shock absorber and tyre faults.
• Overnight pressure and height tracking: The vehicle is left parked overnight, then pressure and all four corner heights are read again in the morning. This single check catches slow leaks early.
• Air spring body inspection: The side walls are checked for chafing marks, cuts, crack networks, oil contamination and bulging; any line or clip routing causing the chafing is corrected at the same time.
• Air line, fitting and dryer inspection: Lines are followed by hand along the chassis, loosened clips are renewed; the dryer cartridge is checked and water is drained from the tanks. This step is moved up ahead of winter.
• Shock absorber, link and bump stop inspection: The body is checked for oil leakage, bump stops for crushing, and anti-roll bar links and trailing-arm bushings for play; the bump stop's integrity is confirmed.
• Fault-code reading, function check and logging: The fault memory is read even if the warning lamp is off, and sensor values are compared; raise, lower, return-to-ride-height, traction assist and lift-axle functions are each tried in turn. The replaced part's position, date and calibration data are logged in the vehicle file.

Air suspension in a heavy commercial vehicle is not a part that can be understood on its own; it is a chain of function running from the supply side all the way to the electronics. The quality of the air leaving the compressor determines the air spring's service life; the moisture the dryer fails to remove freezes in the valve block come winter; a ride height set incorrectly is read months later as tyre wear. That is why a vehicle arriving at the workshop with an "air spring burst" diagnosis is at the beginning of the job, not the end of it: first the air is proven clean and sufficient, then the control device is proven to be deciding correctly, and only then are the carrying and damping parts proven sound. In every case, the vehicle's current OE service documentation for its specific chassis code is authoritative.

Shock absorbers, which work alongside the air springs, belong in the same inspection round; for their make-up and warning signs, see the guide what is a shock absorber and its failure symptoms.

Shop this part: Suspension

All our technical guides on this topic

• What Is a Shock Absorber? Bushing, Failure Symptoms, and Service Life
• ECAS Solenoid Valve: Faults, Replacement & Maintenance Guide
• ECAS Solenoid Valves: The Precision Control Point for Heavy-Duty Air Suspension
• Levelling Valve: Faults, Replacement & Maintenance Guide
• Cab Suspension (Shock Absorber, Air Bellow, Leveling Valve): Faults & Repair
• Cab Tilt Pump: Faults, Replacement & Maintenance Guide

VADEN ORIGINAL products

• Suspension — 255 products
• ECAS Solenoid Valves — 171 products
• Suspension (Cabin) — 44 products
• Cab Suspension Levelling Valve — 32 products