It is a common occurrence on the coldest days of the year to get a customer call or visit complaining that they have a warning message on their dash telling them the air ride suspension system is malfunctioning or there is a fault.
The customer may say that the vehicle is not riding low and that they do not notice any other problems. But, in extreme cases, the customer may see a message that the vehicle is too low. By the time it gets towed to your shop or sets for more than 30 minutes, the air suspension could be working just fine and the message could be gone.
To the untrained technician, a winter air ride problem might lead you down the path thinking there is a leak in the system. However, chances are there are no detectable leaks. These types of faults are usually set by parameters from the air ride module for a certain action to cause an expected result. Think of it as an EVAP system.
Codes might be related to the time it takes for the compressor to replenish the reservoir or the time it takes to carry out a trim adjustment. If it takes too long, the control module will assume there is a problem and puts the vehicle into a safe mode to preserve the compressor and airbags.
While these observations by the system could be the result of a leak, these faults more likely are the result of a weak compressor, sensor or a problem with the solenoids controlling the system.
There are three types of sensor information a vehicle uses to trim the ride height and regulate the compressor: temperature, pressure and ride height.
Temperature is a major parameter for air ride. Air density and pressure can vary widely due to temperature. This is called Charles’s Law.
If a driver with an air ride suspension drives their vehicle home from work with an outside temperature of 32º F, and the brakes also heat the air in the bladders, the air’s volume in the bladder expands. If the driver parks the car overnight and wakes up to a 10º F morning, the air inside the bladder will have contracted and there will be less volume and pressure to support the vehicle. When he walks outside, the vehicle will be riding lower. Air temperature also impacts the reservoir. As the temperature decreases, the amount of air in the reservoir also decreases. This means there is less air to trim the vehicle.
Charles’s Law also applies to compressors. If air intake temperature is lower, more dense air can be used by the compressor. In that case, the time to pressurize the reservoir should be reduced.
All air ride control modules regulate how long the compressor will run to keep temperatures inside a safe range. The control module will look at ambient temperature and other factors to determine when and how long the compressor should run to generate the correct volume and pressure to fill the reservoir or trim the vehicle.
Outside temperature can be determined by an ambient temperature sensor or the mass airflow sensor. Some systems will have a thermocouple connected to the compressor. Measuring air temperature is critical because it will determine how much time it will take the compressor to fill the air springs and reservoir.
Wheel travel is measured by Hall-effect sensors at the wheels. These sensors measure the ride height of the vehicle. But, the data generated is used to determine if a correction or adjustment was effective by measuring the time it takes to make an adjustment. Some systems will mount accelerometers on the strut towers to determine the quality of the ride.
Pressure sensors are typically mounted in the reservoir, compressor or control block. Most systems will only measure the pressure at the reservoir. Pressure sensor input is used to determine compressor output and not the pressures that go to the wheels. Most systems look at ride height instead of pressure to trim the vehicle.
The codes for cold weather problems will typically include the words “replenish,” “reservoir,” “overheat,” “no pressure increase” or “timed out.”
There are two main culprits for these codes: a weak compressor or a leak in the system. It is possible that a sensor in the reservoir is faulty, but this is extremely rare.
Codes may require a scan tool that can communicate with the air ride system, and the codes may be proprietary to the make and model. Also, a good scan tool can perform bi-directional tests and calibrate the system.
Also, most advanced air ride systems are on the CAN bus. It is possible that missing CAN signals due to a communication error can cause a malfunction message to be displayed.
One of the main operating parameters for the air ride system concerns controlling the temperature of the compressor. Modern systems are dry and do not use oil to lubricate the piston and bore. Therefore, excessive heat can destroy the piston seal and bore.
The control module will not immediately turn on the compressor when the engine is started and will trim the suspension from the air in the reservoir. The compressor will be turned on until the vehicle has reached a predetermined set of conditions. These rules are in place so moving air will cool the compressor and so the driver will not notice the compressor operating while the vehicle is stopped.
In some cases, the compressor will run during startup if the pressure is low enough. In some cases where the vehicle’s suspension is too low, the compressor will cycle to build up enough pressure to raise the vehicle. On some vehicles, this may take 20 minutes or more.
An air ride system performs a lot of work when the keys are not in the ignition. The system will trim the ride height to make sure the vehicle is not sitting on the ground as the air in the bladders cools and contracts. During winter, the temperature difference between when the car was parked and when it is started in the morning can cause a system to set a fault message on a healthy system.
Also, some systems will keep a level ride height if the vehicle is parked on an uneven surface, or if the vehicle is unlocked and a load is placed in the trunk. But, most systems will never turn on the compressor to make a key-off correction.
Most fault messages will prevent the system from being used to its full capacity to trim the vehicle. Corrections are filtered so the least amount of pressure is used but the suspension will stay in a safe zone.
Engineers will size the reservoir so it will carry twice the volume to lift the vehicle. But, just like tires, this volume can change as the vehicle cools.
Most systems have fail-safe modes that preserve the system until it can be serviced or the abnormal conditions have passed. Most units will maintain minimum pressure or height in order to prevent a bladder from fully compressing and the suspension from bottoming out. But, if the car sits long enough with a small leak, the reservoir will become depleted.
Water does the most damage to the system when it is extremely cold. Frozen water can block the lines, preventing the air springs from inflating and the valves in the control block/solenoid from functioning.
The dryer on most air ride systems contains a moisture-absorbing desiccant, such as silica or specialized clay. All desiccants have a limited lifespan; they can absorb only a certain amount of water. Once the desiccant is saturated, the air coming into the system is no longer treated, and water can then accumulate in the system.
The dryer is typically located after the compressor and before the reservoir. On the majority of import systems, the dryer is mounted on the compressor. The dryer can also act as a cooler to lower the temperature of the air so even more water drops out of the air.
The desiccant inside most dryers is not serviceable. In these units, the entire dryer assembly must be replaced. On units where the desiccant can be replaced, it is advisable to use new desiccant beads. Some beads will change color to a dark blue or purple, while some desiccants will not change color at all.
Removing water from an air ride system requires some disassembly. Removing a line at both ends and blowing it out with shop air is one method. Removing and draining the air reservoir can also help remove a large amount of water that may have accumulated in the system.
Do not try to use methyl alcohol to flush out the water. This can damage plastic and rubber components in the air ride system. It may also leave behind more liquid in the system than what was removed.
The best bet is to replace the dryer and let it do its job of removing the excess moisture in the system. Another possible occurrence if water enters the system is the freezing of the exhaust port. This may cause the vehicle to ride higher than normal. This failure is more common with older air ride systems that “trim” the vehicle using height sensors rather than pressure sensors.
It is critical to solve all fault codes and leaks before replacing the air struts or shocks since blindly replacing these units may not address the root causes of the issue.
Selling the customer an expensive repair only to have a fault message comeback is bad for your shop. Treat every air ride problem like an EVAP or driveability problem. Take time to look at the service information for clues to how the system operates. Also, be sure to review air ride manufacturer websites for additional sources of information.