The Ups and Downs of Air Suspensions

A vehicle suspension system features springs and dampers (shocks). The spring supports vehicle weight and determines ride height. The dampers (shocks and struts) provide control of the springs, dampening compression and rebound.

Coil or leaf springs provide non-adjustable support. When overloaded with additional weight, the springs compress and, depending on the amount of weight (fore/aft or lateral), reduce ride height. Air suspensions utilize inflatable airbags, in addition to or in place of steel springs. The addition of rear “air shocks” allowed owners to compensate for rear weight by simply swapping out the original shock absorbers with shocks that incorporate built-in airbags.

Today, many vehicle manufacturers offer air suspension systems (either as standard or an option) that replace steel springs with strut-style air spring/damper units (pneumatic) that are controlled by an on-board air compressor and control modules, regulating ride height on demand (either by driver selection or automatically, depending on design). So when a vehicle owner loads the rear of a Dodge Ram or Chevy Suburban, for example, the vehicle is equipped with air suspension, the ride height sensors detect the change in ride height; the control module commands the compressor to activate, providing additional inflation pressure to the air springs; and the rear ride height is restored to original specs.

Air suspensions also are available (again, depending on make/model/year/options) that allow the driver to raise or lower ride height to suit a driver’s preference in terms of vehicle handling. Maintaining original ride height serves not only to compensate for unwanted vehicle lean front/rear, but to maintain critical steering and suspension geometry, such as wheel alignment.

Vehicles with rear air suspension have improved towing capabilities. Drivers can increase the capacity and firmness of the system’s air springs when towing to help raise the rear of a vehicle with heavier loads. Simply having steel springs could cause the vehicle to bottom out with larger loads, causing damage to the vehicle or trailer. Additionally, overloading steel springs could cause safety concerns with the front of the vehicle, causing the headlights to point up instead of straight ahead.

An example of a replacement air spring. Shown here is an application for the 2014-2020 Land Rover Range Rover. (Courtesy Arnott)

System components

The air suspension compressor supplies the entire system with compressed air to inflate the air spring and is generally mounted on the vehicle’s frame or in the trunk. The compressor assembly consists of an electronic pump; a dryer filled with a desiccant to absorb moisture; mounting hardware, including rubber isolators and brackets; and thermal overload protection to help prevent the unit from overheating (burn-out) due to overuse.

Burn-out is normally caused by a small leak in one or more of the air springs, or less commonly, other components that cause the compressor to continually cycle in its effort to maintain the proper vehicle ride height. Signs of compressor failure include symptoms such as longer run time, a louder noise when activated or dashboard warning lights. They alert the driver and repair shop to trouble and signal that underlying issues will need to be fixed. Compressor warranties are often voided when burnout is indicated. Here are system components:

Air lines: They carry compressed air to the air springs or air shocks. The lines are typically high-pressure air lines and are routed along the frame of the vehicle protected from heat and pinch points. Unless original equipment (OE) lines have been damaged by an accident, harsh off-road driving or other abuse, they normally don’t wear out. They could, however, wear or become brittle at the ends where they connect to other components.

Valve blocks: They allow air to enter the system. Valves isolate and control where air is directed and how. While valve blocks are rugged, air line fittings have rubber seals that might dry rot and fail, causing damage to the rest of the valve block.

Solenoids: They are used in electronically controlled air suspension systems to fill and release air from each air spring. As the air suspension system adjusts for conditions, it tells each solenoid to open or close, adjusting the amount of air in each of the springs. While solenoids can typically last longer than the associated air spring, you would still replace them when replacing the air springs.

Control module: Electronic air suspension systems are managed through a control module. The controlling software can be very basic, offering not much more than an analog on/off switch, or it may be more sophisticated, monitoring pressure and ride height in real-time. These advanced modules receive data through a variety of inputs to turn the compressor on and off, as needed. These systems generally remain separate from the vehicle’s onboard modules and communications and might provide an error code, if bad.

An example of a replacement air strut assembly to directly replace the original equipment (OE) unit. The application shown here is the 2005-2009 Land Rover Discovery. (Courtesy Arnott)

Citing another specific example (comparable to other manufacturer systems), the 2005 Toyota Sequoia features an optional rear air spring system that allows rear ride height adjustment in three ranges, including low, normal and high. This allows body height adjustment for road surface clearance needs (high during off-roading, for example, or low for easier passenger access, etc.). The system also activates based on speed sensing, raising the height during slow operation or lowering at freeway speed for increased aerodynamic efficiency.

The Toyota system features a rear-mounted height control compressor, a height control valve and sensor, suspension control electronic control unit (ECU) and two rear air springs (pneumatic cylinders).

As with any system that features added parts and complexity, things can often go awry.

If the system develops an air leak, the compressor may run over time or continuously in an attempt to maintain pressure, likely resulting in the compressor motor overheating and failing.

Bear in mind that the air compressor pump and the rear air spring assemblies directly affect each other. If the pump goes bad, the air spring may not be activated, resulting in an un-inflated air bag. As the suspension compresses and rebounds, this can flex the air bag to the point where the bag becomes cracked and fails. By the same token, if the air spring is bad (leaking, for instance), the pump struggles to keep up and this added load can eventually cause the pump to fail. As with any spring and/or shock issue, it’s always best to replace it in axle pairs.

When running diagnostics with a scan tool, by looking at the air ride suspension system, you may run into a code that will lead you in the wrong direction. Using a 2009 Chevy Suburban as an example, you may find diagnostic trouble code (DTC) C0895-00 (device voltage), which may lead you to believe that the air system circuit is the issue. However, this may simply be the result of a low battery, which has nothing to do with the air suspension system.

During a system test, check the vehicle’s ride height, per the service specifications. Lower the vehicle and note the ride height. Then attempt to raise to the limit spec. If the compressor is heard running but it struggles to raise the vehicle, this is an indication that there might be a leak. Check the air bags and all air-line connections using soapy water to look for signs of air leakage.

Rear height control sensor: The height control sensor system attaches to both the frame and the rear-axle housing. If you plan to remove the sensor, first place match marks on the sensor link and bracket to maintain sensor adjustment during reinstallation, as these links can feature-length adjustment.

Winter tip: If the compressor motor continues to operate but the vehicle’s ride height does not change, in addition to inspecting for air leaks in the system, another suspect area (especially in cold climates) involves potential moisture in the air lines, which can result in freeze-clogging.

An example of a coil spring conversion kit to replace and to eliminate the original air suspension units. This is a popular choice for those who wish to eliminate a troublesome air system (Courtesy Arnott)

Diagnosing air suspension problems

Many consumers miss the early warning signs of an air suspension issue. Unfortunately, these problems only get worse and more expensive over time. The main signs of air suspension problems include:

The dashboard warning light. One of the obvious signs of an air suspension problem is when a dashboard warning light comes on. Even if the light doesn’t stay on, the vehicle should be checked.

Suspension sagging. One of the first signs of an air suspension problem is when a car sags in a corner or entire side. Typically, this is due to the rubber air spring developing tiny cracks or holes because of dry rot or road debris. If a customer notes that their vehicle drops in height overnight or after they park, they most likely have leaks in their air spring or strut.

Air suspension compressor constantly working. If there is damage to the air springs, the compressor will constantly be pumping air to keep the air bladders inflated. If the compressor is constantly running, the system should be examined as soon as possible.

Compressor not working at all. The air suspension system cannot function without the compressor. If the compressor does not come on at all, that may be a sign that it was overworked and has burned out or it could be as simple as a fuse or relay problem.

Compressor making noises. The owner may also notice abnormal noises during compressor operation, such as loud clicking, whining or grinding. This could be a sign that the compressor has been overworked, or there could be excess moisture in the system because the compressor’s dryer is saturated. Once the compressor has been replaced the rest of the system, including the air springs and struts, should be tested.

Other suspension issues also might apply to air suspension, including:

The vehicle bottoms out over bumps or rides roughly;
It nose-dives when stopping;
The vehicle pulls to one side or steering is difficult;
The car continues to bounce after hitting a bump;
Uneven tire tread wear;
The shock portion of the strut appears to be oily or damaged.

If the customer complains about any of these symptoms and if the vehicle is more than five years old or has more than 50,000 miles, you should inspect it for signs of air suspension damage.

If the vehicle’s dashboard suspension warning light comes on, but the vehicle seems to be maintaining its height, you should use a scanner to diagnose the fault code. Typically, this is an electrical problem such as a relay or fuse.

If the vehicle sags at one corner or side, the easiest way to diagnose the air suspension problem is to put the vehicle on a lift and do a visual inspection of the air struts and springs, the ride height sensors, air lines and connections and the compressor. To locate leaks in the air suspension system, spray the air bladders, fittings and lines and top seal with a solution of dish soap and water and look for bubbles.

An example of an air system’s compressor unit. The example shown applies to the 2005-2016 Land Rover Discovery. If original air spring bags or connections leak, a compressor can run over time, potentially overheating the compressor. If a leak has existed for a long period of time, a replacement compressor may be needed. (Courtesy Arnott)

The fix alternatives

There are multiple options for repairing and replacing a leaking air spring or strut. New OE springs, struts and compressors are expensive at dealerships, but aftermarket vendors offer options that cost far less. Arnott, for example, offers new OE quality compressors, as well as brand new replacement air springs that are often easier to install than the OE and built with heavier duty rubber. They also offer both new aftermarket replacement struts that they design and assemble and remanufactured OE struts.

While remanufactured struts maintain the vehicle’s electronic or active damping, as well as provide auto-leveling at a fraction of the price of a new OE strut, be aware that some suppliers may only repaint the external core parts and replace the leaking air bladder with a new one, leaving a shock and strut that has an unknown number of miles and problems.

An example of a ride height sensor. A failed sensor can cause the air system to not function properly, possibly resulting in a side-to-side lean condition. (Courtesy Arnott)

Quality-minded suppliers will recharge the damper by replacing the old, worn oil that has gone through literally millions of cycles with new high-performance shock oil, while also replacing worn seals, wear bands and top caps. They also computer test the damper and active damping coil to assure proper functionality and ride quality.

For older vehicles and those without active damping, new aftermarket struts are available that include a brand-new shock assembly with zero miles. Manufacturers like Arnott designed new struts that are built from the ground up with all new components, including a name brand air bladder, new strut body, isolators, bottom mount brackets, dust boots, mounting bolts and top caps, many of which are machined by using aircraft quality aluminum. New struts are typically pre-assembled for easy installation, don’t require a core deposit or core processing and provide a responsive, OE-like ride.

An example of a solenoid valve block. This is the junction block from which all of the system’s air lines are fed. (Courtesy Arnott)

Eliminating issues: The budget fix

If a vehicle has multiple air suspension problems or has had a long history of issues, it might be more cost-effective to replace the air suspension with a coil conversion kit. Some kits cost about the same as a single air strut at a dealership, but should provide many years of trouble-free driving.

Granted, the “luxury” of having an air system that alters ride height as needs change, and simply converting from the air system in favor of coil-over replacements completely eliminates the potential problems that can be exhibited by a problematic — and often complex — and expensive air system.

The bottom line is air suspension repairs are not difficult to diagnose or fix. You should be inspecting the systems after 50,000 miles and customers should be encouraged to repair their air suspension problems quickly before one problem turns into multiple, more expensive, problems.

Inspect the airbag’s condition. Brittleness, cracks and/or road debris damage can result in leaks, which cause the compressor to overwork and to overheat.

Typical component cost

Price estimates are for typical components, depending on the application:

Air suspension control module ($180-$800)
Air compressor aftermarket ($225-$500)
Air compressor OE ($800 or more)
Height sensors (typically $75-$100)
Valve blocks (typically $145-$300)
Air plumbing lines ($30-$50)
Air spring/shock assembly OE ($600 or more)
Air spring/shock assembly aftermarket ($270-$300)

Note that again, depending on make/model/year, the cost of OEM replacement parts can be much higher.

When checking for causes of improper air system operation, inspect the compressor connection for connector contamination, moisture intrusion or bent pins.