By Jim Marotta
Ford introduced 4-Wheel Air Suspension (4WAS) on the 1995 Explorer, and has since made it available on all three generations of the Ford Expedition and Lincoln Navigator. These big trucks weigh about three tons, so they already have a relatively comfortable ride using conventional suspension. Air suspension smooths that ride even further, but it's also used to increase off-road ride height and for load-leveling, which enhances stability when towing.
Today, air suspension still costs more than conventional suspension, but thanks to modern materials and electronics, it is simpler, reliable and offers more than just a comfortable ride.
The 4WAS is basically the same as conventional suspension. Up front, air struts replace the standard strut assemblies. In the rear, air springs replace the standard suspension's coil springs. All four air units have a normally-closed solenoid valve operated by the control module for adding or venting air pressure. Changing pressure not only changes ride height, it also changes spring rate. You can replace the solenoid valves on the rear air springs, but not the valves on the front air shocks.
The air suspension system consists of front air struts (center) and rear air springs (left) with shock absorbers (right).
The electric air compressor is different from the compressor on the Rear Air Suspension (RAS) system. It includes a built-in vent solenoid valve and a replaceable air dryer. The solenoid valve is also a spring-loaded safety valve that opens at 260 psi. A solid-state relay mounted below the headlight supplies battery voltage to the compressor.
In addition to the compressor's vent solenoid and the solenoid valve on each suspension unit, there are four more solenoid valves in the system. The front fill solenoid isolates the front air shocks from the rest of the system, and the rear-fill solenoid isolates the rear air springs. The front gate solenoid isolates the front air shocks from each other so the pressure between them does not equalize as the vehicle rolls in a turn. On smooth roads, this valve opens for a softer ride. Lastly, there is a pressure relief valve mounted on the frame crossmember near the rear height sensor. The control unit can use it to vent the rear air springs without having to open the rear-fill and system-vent solenoids.
On 2006 and earlier models, there are two linear ride height sensors that have a total stroke of about 3 inches. Sensor output is 0.25 volts at full extension and 4.75 volts at full compression.
On 2006 and earlier models, there are two linear ride height sensors, one mounted between the rear suspension panhard rod and the crossmember just forward of the spare tire, and one between the frame and the left front upper control arm. Even with only two sensors, the control unit is still capable of limited left/right height adjustments. The sensors have a total stroke of about 3 inches, which is how far the panhard rod and control arm move though the full 10 inches of suspension travel. Sensor output is 0.25 volts at full extension and 4.75 volts at full compression. Adjusting the sensor or the static ride height of the truck requires a scan tool for storing the new settings in the 4WAS control unit.
Later models have four identical potentiometer-type sensors, one at each corner, but they're permanently mounted to left- and right-side brackets so they can't be interchanged. The bracket is bolted to the frame, and a small ball stud link connects the sensor arm to the suspension arm.
On earlier models use the same control unit for the 4WAS, the Rear Air Suspension (RAS) system and the EVO Steering Control Module. Mounted in the instrument panel above the radio, it supplies reference voltage to the height sensors and controls the ground circuit of the compressor relay and the power circuit of each solenoid valve. Inputs come either directly from sensors or from other modules, including the front Generic Control Module (GEM) and the PCM. Inputs include vehicle height, vehicle speed, steering wheel position, brake and accelerator pedal positions, transfer case gear selection, transmission gear selection and door ajar signals.
On later models the Vehicle Dynamics Module (VDM; mounted above the parking brake handle) controls air suspension.
A manual air suspension switch turns the system off for service or long-term storage. On 1997-02 models, the switch is behind the front passenger side kick panel. On newer models, it's behind an access panel aft of the third-row seat on the driver side. That switch controls power to the control unit. When the switch is OFF, a warning light appears on the instrument panel with the ignition switch ON.
All of the air lines in the system are nylon. Two lines connect directly to the drier mounted on the compressor, one leading to the front fill solenoid valve and the other to the rear fill valve. The system uses quick-connect fittings throughout. You cannot repair or splice the air lines.
When the ignition switch is first turned ON, the control module runs a self-diagnostic test, storing or displaying any electrical or electronic problems detected as B or C codes.Any time a door or the rear hatch opens, the system will remember the height sensor readings. If you add or remove weight, the system will operate as needed to maintain that height. Once the doors are all closed and vehicle speed exceeds 10 mph, ride height will readjust as needed. The system's first priority is load leveling, but that's not all it can do.
The control unit can select from three different programmed suspension heights. "Trim" height is the normal ride height for paved road. Defined as trim plus 1 inch, "off-road" height engages automatically when the ignition switch is ON, the transfer case is in 4X4 LOW and the vehicle speed is less than 25 mph. Defined as trim minus 1 inch, "kneel" engages when vehicle speed is zero, all doors are closed and the ignition switch is OFF. Obviously the kneel height is to ease vehicle entry, and while one inch doesn't seem like much, it can make a difference. If the system detects a malfunction, the system will default to trim height, either by operating the compressor or by venting pressure as needed.
As noted earlier, there is a switch to turn the system off for service. It should be turned off whenever you lift the wheels off the ground. Failure to do so can cause expensive damage, especially to the rear air springs. The only service required by the 4WAS itself is a visual inspection. Look for physical damage to the air springs, cut or crimped air lines or evidence of something rubbing the lines. There is no pressure sensor in this system, so the self-diagnostic test can't detect a leak. Symptoms of a leak include excessive compressor operation, failure to reach or maintain commanded height, and most commonly, a sagging rear end.
Leaking, dry-rotted air bags will cause excessive compressor operation, failure to reach or maintain commanded height, and most commonly, a sagging suspension.
A quick visual check for physical damage, like this torn airbag can make quick work of diagnosis.
Use soapy water to leak-check the fittings, and check the whole system with a smoke machine. Smoke is especially handy for finding a solenoid valve that's open when it shouldn't be, and vice-versa. Do not, under any circumstances, disconnect any fittings without venting the system first.
Check height sensors for damage and to make sure they're securely mounted. With the vehicle at trim height, sensor output should be 2.66 volts. Repairing, adjusting or replacing a sensor requires a scan tool to recalibrate the system.
Historically the most troublesome part of the system is the compressor. Failures tend to be gradual rather than catastrophic, and tell-tales signs are noise, rough running and excessive current draw. The sagging rear end noted earlier is what commonly drives owners to seek help. When replacing a compressor, techs often replace the solid-state relay too.
After replacing a sensor or the control unit, calibrate the control unit. This involves setting the mechanical ride height (with the system vented), and then using a scan tool to command specific functions through the control unit. The Ford NGS and the new IDS tools are best for the job, but there are some purpose-built aftermarket tools available from companies that sell air suspension parts.
This brings us to the final point. Since the compressor is the part that fails most often, techs often just install a new part and "get lucky." However if something else is wrong, the lack of proper tools, equipment, information, and parts can turn a potentially profitable job into an ugly situation. Should you decide to make the investment, there are several companies that offer everything you need to become an air suspension specialist.
While compressors (shown) are the part that fails the most frequently, a thorough diagnosis can save you money on replacing parts in the long run.
When you need to maintain or service a vehicle, get a valuable subscription to Chilton’s comprehensive service procedures, specifications, technical service bulletins, graphics, and wiring diagrams at www.ChiltonDIY.com or www.ChiltonPRO.com. A subscription, designed for professional technicians and for do-it-yourselfers, can save you many hours of frustration and pay for itself as soon as you use it.
A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.