Many vehicle manufacturers have tried air suspension over the decades. No one has dominated the category like Ford and Lincoln. The Town Car, Crown Victoria and Grand Marquis models use a rear-wheel air suspension that is legendary. Durability with a great ride is the promise and with a little care these vehicles really deliver.
Supporting a vehicle’s weight
Engineers use many types of springs to support vehicles. Leaf springs, coil springs and torsion bars are popular. Each offers advantages and all have shortcomings. Softer spring rates provide a smooth ride. Unfortunately, soft springs bottom out when loaded. Achieving a great ride is difficult when the load in a vehicle must vary. The answer is a spring that can adapt to the load applied.
The air suspension
Maintaining level suspension height is critical for wheel alignment and handling. The Lincoln Town Car uses durable rubber bags, filled with air, for rear springs. A computer-controlled system allows air pressure, from an onboard compressor, to control height. Several inputs influence the proper pressure and a sensor maintains height when weight changes.
An example is a vehicle with a driver only, low fuel tank and no luggage, compared with a loaded vehicle on vacation. Twenty-six gallons of fuel add more than one-hundred sixty pounds. Four people and luggage may add another 850 pounds. One thousand pounds push down on the vehicle’s springs.
Air suspension senses the lower vehicle height. The vehicle dynamics module or VDM powers a relay, which turns the compressor on. Around the same time, it opens the solenoid valves in the air bags and the vehicle returns to the proper height.
Once we achieve the proper height, the VDM closes the air bag solenoids and turns the compressor off. Later we remove the load. The system adjusts by releasing air and maintains the same height.
Two sides to the system
Over the years, Ford has refined the system and made changes, but all systems have many things in common. Ford rear air suspension has a pneumatic side that does the work and an electrical side to controls things. The remainder of this article concentrates on the electrical side and how it functions. We cover the pneumatic side of the system in the next installment.
The brains of the operation
The vehicle dynamics module or VDM is the microprocessor that controls the system. They program this computer to maintain a set vehicle height. Engineers program the desired height into firmware and it is not adjustable. To maintain height, the VDM turns the compressor on, and vents air through solenoids. Steering angle, door openings and other factors are considered. For instance, when a door opens, the system reasons someone may be entering or exiting the vehicle. It waits to see if the height changes before attempting to control the system.
The VDM also stores diagnostic trouble codes to help find problems in the system. The computer store codes for 50 key-cycles and then clears them, if the problem does not recur. A Ford scan tool is extremely useful to read these codes and to operate bay tests. Most code readers cannot access this memory and will be of little use.
When a problem occurs, the VDM illuminates the Check Suspension message. Without a proper scan tool, finding the cause of the problem is difficult. We can get some information by noting how long the system takes to illuminate the message. For instance, a message that immediately comes on after we start the vehicle, suggests circuit problems.
When the VDM is powered up, it checks for input from the sensors and basic operation. A bad or unplugged height sensor or a blown fuse will cause the check-suspension message to come on immediately. Depending on the year model, multiple fuses feed the system. Some year models have two and others may have three or more fuses. A blown fuse will cause the Check-suspension message to appear, when the vehicle starts.
Another common problem is the on/off switch in the trunk, on the left side. When we turn the switch to off, the system shuts down and the message comes on when the vehicle starts. We may accidentally bump this switch and turn the system off. When the switch is off the check-suspension light is on.
Cut or unplugged wires, and solenoids shorted or open may also cause an immediate check-suspension message. A bad suspension computer will also cause an immediate message display, but this is rarely the case.
Each time we turn off the vehicle and then restart, the system resets. With the next key cycle, the process begins again. A check-suspension message, which takes time to come on, suggests problems in the raising or lowering of the vehicle.
The light comes on after driving a minute or more
A message that is off after starting and comes on while driving for more than a minute, often shows a vehicle unable to achieve the correct height. One common cause is leaking air bags. The compressor is not able to maintain pressure with the leak and the computer shuts the system down after 90 seconds of attempting to rise.
Weakened suspension compressors may show the same symptom though we have replaced the bags. If we allow the leak to exist, it may over work the compressor. After replacing the bags, the compressor is not able to build pressure. This causes the check-suspension message to come on after driving for more than a minute.
Another cause might be a bad height sensor that does not show the vehicle rising. Cut hoses or solenoids not allowing air into the bags will cause the same problem. Things that keep the vehicle from achieving the proper height in 90 seconds will result is a check-suspension message.
Height that is too high in the rear may also set a check-suspension warning after driving. Turning off the ignition and opening and closing the driver’s door should cause the vehicle to vent. The system will not attempt to lower while any of the doors remain open. A faulty door sensor can cause the vehicle to fail to lower. A door-ajar message would be another symptom of a faulty door sensor.
Normal operations with the key turned off
When we turn the vehicle off, the system maintains power to the VDM for sixty-minutes. In this period, the vehicle will vent and lower itself, if the doors are closed. A door that is open will stop the system from venting. After one hour, the system will also try to raise the vehicle to trim height if needed. It will attempt this for about 30 seconds and then stops. At one minute past the one-hour cycle, the system shuts down and attempts no further adjustment until we restart it.
In the next article, we cover the pneumatic side of the system.