Necessity being the mother of invention that it is, innovators worked for decades on ways to insulate the body of vehicles from the sudden movement of the axles as they bounced through potholes or over rocks. Smoothing the roads helped a ton, but there are still rough places that have to be negotiated when the roads deteriorate or when we leave the pavement and potholes still abound in some places.
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Axles have to be braced and hinged so they remain in place under the vehicle but can still move up and down. That lets the wheels on the ends of those axles respond to uneven ground independently
In the muscle car era, some of my buds bought and installed air shocks, which allowed them to raise the rear car body by simply adding air to the bladders on the shocks via tire pressure-type Schrader
valves mounted on the lower side of the bumper. Plastic lines were snaked from the Schraders to the shocks. Fancier systems had a small on-board compressor and a dash-mounted control panel to handle this from the driver’s seat.
According to one source, Cowey Motor Works of Great Britain developed an air spring in 1909 that consisted of a cylinder, which contained a diaphragm made of rubber, and cord that was filled by using a bicycle pump. But due to the sub-standard materials available in those days, that arrangement was leaky and tended to need re-inflating a lot. There were earlier stabs at air suspension, but automobile engineers toyed with the idea for more than half a century before manufacturers picked it up for mass production. The 1957 Cadillac Eldorado had air suspension as standard equipment.
Air Suspension Basics
The cool thing about air suspension is that it is self-leveling; anybody who has loaded their trunk with heavy stuff and driven at night has experienced the headlights-in-the-trees syndrome. Air suspension more or less works the same on just about every vehicle, but the components are different from car line to car line. Air ride is most commonly used on the rear axle alone, but somehigh end cars tended to have it on all four corners, and for awhile, the compressor on some Lincolns was mounted in the engine compartment in plain view with a Schrader valve mounted on the compressor so air could be added to the springs with a conventional air hose if the compressor went bad. That was fine, but if there was also a leaky spring or a bad solenoid on the spring, the suspension would go flat in short order anyway and the ride is mighty rough and low with flat air springs.
Trim height on some late 1990’s Mark VIII vehicles can be adjusted by measuring from the bottom of the rim to the fender well, putting those numbers into a Ford-friendly full function diagnostic tool and then you can watch the scan tool trigger the module to re-calibrate correct ride height based on the information you provided. At Ford factory school, just for the fun of it, the instructor deliberately put wrong numbers into the WDS and activated recalibration so we could watch the module jack the car all askew.
Space won’t permit us to delve into every kind of air suspension that’s out there, but we’ll have a look at a couple of different systems here.
GM Minivans GM had a very sensible system in place beginning on their late-90s minivans that consisted of simple air shocks a lot like the ones Bubba used on his 1969 Chevelle, a smart little ride height
The ride height sensor on GM minivans like the one we’re discussing here is a small square box with a seven terminal connector. It has an internal shaft connected to a lever that is monitors rear suspension via a small tie rod. As the suspension changes position, the tie rod acts against the lever, which twists the shaft this way and that as the suspension changes position and the Automatic Level Control (ALC) sensor acts accordingly, using its internal circuitry to operate the compressor and whatnot. This smart little box is only operational when the ignition is on,
and won’t do anything to correct the ride height unless it sees too high or too low suspension in a static state for about 22 seconds (give or take five seconds) and it directly controls the system. And it won’t keep anything energized for longer than 5.5 minutes so as not to overwork the components.
If the ALC sensor sees that the sensor arm is above the "in-trim zone," it means that the vehicle bodyis too low and the sensor energizes the compressor relay circuit. This fires up the compressor and adds air to the shocks. Conversely, if the control sensor arm is below the "in-trim zone," it means the vehicle body is riding too high and the sensor energizes the exhaust solenoid circuit to release some pressure. This system usually works well, but GM says a minimum of 8 to 14 psi has to be present at all times in the air shock bladder for the shocks to maintain their pliability and reliability. The air drier actually contains hardware that retains that pressure.
As for diagnosis, everything on this GM system is symptom driven, because there is no super smart module to speak of.
All these systems use air dryers for obvious reasons. Oh, and the Montana has a nifty little panel on the left side of the cargo area that will allow the owner to use the compressor to air up bicycle tires and stuff.
Higher end GM cars with air suspension mostly use the same pump but will have different timing strategies and may be able to trim the height of right and left sides separately.
The ride height sensor has four wires, all connected to the module. There’s a power, a ground, and two inputs, one for low trim and the other for high trim – the module operates the compressor to keep that sensor between those two trim inputs. The Air Suspension module won’t make automatic corrections with a door open, so remember that fact if you’re troubleshooting a problem with one of these. The no-change-with-door-open deal is a precautionary feature to prevent the car doors from being caught on curbs if the suspension needs a downward correction.
On the rear-only system, there are three solenoids that control the air. There are “fill” solenoids at each spring (these kind of look like big fuel injectors) and there is a vent solenoid in the compressor
Town Cars, Crown Vickies, and most SUV platforms (Explorer, Expedition, etc.) had rear-only suspension. The Ford car air suspension has an air suspension kill switch in the trunk (it simply closes a circuit between pins 2 and 22 at the module). On the Expedition that switch is above the passenger’s right foot. The need to flip the kill switch is easy to overlook because many, if not most of these vehicles don’t have air suspension, and if you raise one of these on the lift, the module will think the vehicle is riding high and will deflate the air springs so that when you let the vehicle back down it squats down on the lift.
In that case, I always open the door, turn on the key, and close the door, then wait for the system to correct. This doesn’t usually damage the springs, but if you find a folded or twisted air spring that has
been riding that way, you’re supposed to replace it, and when you do, you’re supposed to make sure it’s inflated before you lower the vehicle. You can do that with your nifty homemade tool connected to the Self Test connector if you know how to trigger the process (see inset).
I worked on the earliest version of the four wire systems on the Lincoln Mark VII platforms when I was at Lincoln Mercury in the early 1980s, and the air suspension self-test procedure would blink the air bag light at 1.8 hertz while it was deflating and reinflating the springs one at time, and as I remember, you had to open and close the driver’s door as you progressed through the stages of the test to trigger the various phases of the diagnosis. During the self-test cycle, the module was watching the ride height sensors for feedback on how things were going, and I was watching the action from the outside to see if anything was askance during these maneuvers.
The newer system has similar self test functions, and can communicate through the OBDII connector if you have the right scan tool, but they also still have the old self test connector in the trunk (it looks
As you see, we could have spent a lot more time on this than we have, but it really isn’t that complicated. You can find leaky air springs with soapy water – or by removing them, inflating them, and submerging them like you would a tire. It all comes down to a common sense understanding of the way each system works.
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