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.