Air ride systems always used to have two diagnostic paths: either the vehicle was riding low or the compressor blew a fuse. Today’s modern vehicles have computer-controlled air ride units at all four corners that can pose a unique diagnostic challenge when it comes to weak or marginal air compressors.
Today’s air ride suspensions seem to have a mind of their own when it comes to alerting the driver of a problem. Unlike engine-related trouble codes, most air ride codes are cleared every key cycle or when the vehicle has been sitting for a specific period of time.
Codes are not always a sign of a leak, as they can be set by components like the compressor, sensors or reservoir. These codes can appear to be intermittent, but once you understand how the system operates and the criteria for setting a compressor-related code, a diagnosis can be easy.
Modern air compressors do not directly fill the air shocks or struts. Most systems use the compressor to fill a reservoir. The air in the reservoir is then used to trim the pressure in the air ride units. When the reservoir is depleted, the system turns on the compressor.
One of the most common compressors used on four corner air ride vehicles is a WABCO air compressor, which is a single piston high-pressure air supply pump. The compressor assembly has an intake air filter, an integral regenerative air drier element and an air exhaust solenoid valve. These types of WABCO compressors can be found on Ford, Jaguar, Land Rover, Audi, BMW, Mercedes-Benz and many other brands.
You need to become familiar with the numbers of the compressor. These numbers can change depending on the vehicle and can be found in the service information. The typical compressor can generate more than 200 psi, and there is a pressure relief valve on the compressor set at approximately 250 psi. Most of these compressors also have a valve to keep the pressure at a minimum of 40 psi. Most air springs have a burst pressure above 450 psi.
The heart of an air ride system is the piston, piston ring and cylinder.
WABCO compressors have a fixed piston that tilts in the cylinder. The cylinder is sealed with a ring made of phenolic resin or similar material. The ring is not lubricated and is designed to wear into a barrel shape. Over time, the compressor will not generate enough pressure to recharge the reservoir in a specified time period.
What hastens the failure of the piston ring is heat. All air ride control modules regulate how long the compressor will run to keep temperatures in a designated range. The control module will look at ambient temperature and other factors to determine when and how long the compressor should run to manage the heat.
It’s not uncommon for the compressor to wear out before one of the air ride units start to leak. A weak compressor can still keep a vehicle at level and within an acceptable tolerance, but the system will go into a safe or inhibit mode and operate with the system in one setting that will not allow certain adjustments or modes to be initiated. The customer may complain of a stiff ride and imprecise handling.
The modern air ride vehicle could have several different modes depending on a few variables. As each mode is initiated, the computer will bleed off or add pressure to the corners to trim the vehicle. Switching between some modes can deplete the reservoir.
Here are some examples of the modes that may be found on an air ride vehicle.
Preliminary/Wake: This mode is used when a door is opened or a door/trunk is opened. This mode does some housekeeping with the sensors and valves. Only air from the reservoir is used to trim the vehicle.
Sleep Mode: This occurs when some vehicles sit for 30 minutes. In sleep mode, the only lowering corrections that are permitted balance the vehicle height to the lowest corner.
Post/Park Mode: This mode is used when the car is stationary with the key off. The ride height adjusts over time depending on the temperature.
Stance Mode: This mode is used when the vehicle is running and stationary. In this mode, the system will react quickly to changes in ride height.
Speed Lowering Mode: This mode engages at higher speeds to improve handling and improve aerodynamics for fuel economy. Some vehicles will have different speed thresholds attached to this mode.
Jacking Mode: In this mode, the system monitors height changes at the corner being jacked up. If the system lowers the air suspension, but no reduction in height is achieved, the system will time out quickly. If all four height sensors change by preset values, the system will recognize the vehicle is being lifted on a hoist and stops adjusting the pressures. This mode typically will stop when the vehicle starts to move.
Additional modes are out there depending on the vehicle. Some modes, like sleep and park, will work while the vehicle is off. When the key is off, an air ride system will periodically wake up to check the ride height and go back to sleep to conserve power.
Finding leaks is easy; finding the source of intermittent problems can be difficult. Accessing these codes on some vehicles requires a scan tool that can talk to the module controlling the air ride.
One of the most frustrating problems to diagnose is a weak or marginal compressor. The codes for this type of problem will typically include the words: “replenish,” “reservoir,” “overheat,” “no pressure increase” or “timed out.”
To the untrained technician, a weak compressor might indicate a leak in the system is preventing the compressor from fully recharging. However, there are no leaks in the majority of problem vehicles. These codes are usually set by how long it takes a compressor to replenish the reservoir. The control module looks at algorithms that compute how long the compressor should run, and if the compressor can’t fully fill the tank, a code will be set.
Some vehicles have test procedures that can be used to determine the health of the compressor. When and how long the compressor runs depends on inputs from a variety of sensors over serial data buses.
Most systems factor in the ambient temperature. The outside temperature can affect compressor run time because of the relationship between temperature to air density. Some systems might use the temperature sensor mounted on the front of the car for the HVAC system and others use the engine’s mass airflow sensor.
Another factor is vehicle speed. With engines and interiors becoming quieter, the drivers will notice the sound of a compressor while the vehicle is parked. Most systems will turn on the compressor above 25 mph unless certain criteria are met. This also allows for moving air to cool the compressor.
You may get customers in your shop complaining that they are getting an air suspension fault message, but it only comes on sporadically, or they may say it comes on consistently during their drive to work. When they drive to your shop, the light may not come on at all.
These codes and fault messages can typically be traced to a weak compressor. Since the compressor on some vehicles likes to work when the vehicle is in a state of motion, the trigger may be a lengthy run time when the compressor is recharging the system in the morning or when the car is coming out of a speed-lowering mode.
The secret to diagnosing these problems comes down to knowing what criteria the system uses to regulate the compressor and having a scan tool that can interpret the information.