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Servicing Air Suspensions
Copyright AA1Car Adapted from an article written by Larry Carley for Brake & Front End magazine
When electronic suspensions with air springs were first introduced 20 years ago on the Lincoln Continental and Mark VII, few people would have imagined that someday this new technology would create a booming business for servicing air ride suspensions. But it has, and it's a market that shows no signs of deflation anytime soon. Here's why:
Air springs and electronic ride controls provide a cushy, boulevard ride, but the ride doesn't last forever. All air suspensions share a common vulnerability: air leaks. And when a system can no longer hold air, it goes flat.
The undercar environment is a harsh one that's exposed to road splash, salt and debris. Rubber loses elasticity as it ages, becomes hard and eventually cracks. After seven to 10 years of service, many of these older systems start to develop leaks that allow air to escape from the system. The same thing can happen to plastic air lines. Wiring connectors, solenoids, compressors and height sensors are also vulnerable to corrosion and vibration, which over time may lead to failures that disrupt the normal operation of the air ride system.
Now comes the good part (or the bad part if you're the vehicle owner). When an air ride suspension system goes flat, it can be very expensive to fix. OEM parts are sky high and may not even be available for some of the older applications. Remanufactured OEM and new aftermarket electronic air struts and compressors can provide a more cost-effective alternative for those who want to retain the full functionality of their air ride suspension. The other option is to replace the original air springs and/or electronic struts or shocks with a conversion kit that includes conventional coil steel springs with ordinary struts or shocks.
AIR SUSPENSION CONVERSION KITS
Aftermarket conversion kits for the older cars with air ride suspensions have become a hot item in recent years because the kits provide a repair solution for vehicles that might otherwise be too expensive to fix. For example, a set of four new OEM air struts and a compressor for a 10-year-old 1994 Lincoln Continental retail for around $3,500. Add in the installation labor and it adds up to a lot of money to spend on a car that is worth maybe $2,500. The same car could be converted to a regular coil spring suspension for around $500 to $600 in parts, and it would probably be a lifetime repair (no future air leaks, compressor failures or electronic glitches to worry about).
On a Lexus, the numbers are even higher. The OEM air struts list for $1,100 per wheel! If the compressor also needs to be replaced, the parts bill alone is around $5,800. By comparison, a conversion kit for this vehicle typically sells for less than $800 and includes struts for all four wheels.
The only tradeoff of installing a conversion kit is that a vehicle will no longer have a cushy air ride suspension or be self-leveling. For many owners, that's no big deal because many people say air ride suspensions are overly soft and allow too much wallow when cornering. They would actually prefer a firmer handling suspension. Others, though, might be reluctant to trade their air ride suspension for a conventional suspension. Even so, most of these people will probably realize it makes more sense to convert the suspension than to sink a pile of money into a car that has already depreciated so much in value.
The ironic part about all of this is that some people will gladly spend thousands of dollars to transform their vehicle into a pavement scraping "low rider" while others moan and complain about having to spend so much to fix their unintentional low rider because the air suspension has gone flat.
One other change that is usually required when converting an air ride suspension is to disable or "fool" the system module. Procedures vary depending on the application, but may require removing a fuse, cutting a wire or splicing a resistor into a strut wiring harness to trick the module into thinking a solenoid or height sensor is still there when it is not. Make sure to review the conversion kit's instructions or check other sources like manufacturer TSBs. Not following the proper procedures can effect other systems controlled by the body control module.
AIR RIDE SUSPENSION OPTIONS
This brings up another potential service opportunity: converting conventional suspensions to air ride suspensions. It's not a traditional kind of repair job, but it is one that can be profitable, especially if you're searching for a niche market to expand your business. A whole new generation of younger vehicle owners are spending serious money these days customizing sport compact cars. The hot nameplates are Honda, Accord and Mitsubishi, but you'll also see Chevy Cavaliers, Ford Focuses and other nameplates among their ranks.
The upgrades are done to improve ride and handling as much as cosmetics. The owners of sport compact cars will buy larger diameter 17- to 19-inch alloy wheels, sticky ultra low profile street performance tires, beefier sway bars, high pressure monotube gas struts and shocks, drilled and vented brake rotors, wings, spoilers, low restriction air intakes and mufflers, and lots of other bolt-on accessories.
Coil-over kits that replace the stock struts are a popular upgrade for making suspension adjustments and lowering ride height, and now we're seeing aftermarket coil-overs that use an air spring instead of a steel spring for a fully adjustable suspension on the fly. One such system that has just been released uses air strut suspension with adjustable height, rate and dampening. The kit includes air spring struts, compressor, dryer, lines and driver-adjustable control module. It's available now for the 2002 and up Subaru WRX, and will also be available for a number of popular sport compact cars.
Another upgrade possibility is installing air shocks or air helper springs on vehicles that are used for towing or hauling. Here, air provides extra load carrying capacity to keep the rear end from sagging when a vehicle is heavily loaded. Installation is easy and requires no wiring or other changes.
If performance modifications are not your thing, there's still plenty of repair work to be done on aging vehicles with electronic air suspensions. Current applications include Lincoln Continental, Town Car, Mark VII and VIII, Ford Crown Vic, Mercury Grand Marquis, fullsize Chevy, Buick and Oldsmobiles, Cadillac Deville, Seville and Eldorado, various Chrysler models, Dodge Dynasty, older Jaguar models, Land Rover, Range Rover, Lexus, Lincoln Navigator, Ford Expedition and Ford F250 pickups, and Mazda MPV.
AIR RIDE BASICS
With an air ride suspension, hollow inflatable rubber bags (air springs) are used in place of ordinary coil steel springs or leaf springs. The rubber bags are flexible and have a certain amount of give, so they help absorb and dampen bumps to better isolate the suspension from the vehicle's occupants. Air springs also provide a variable spring rate and offer increased stiffness the more they are compressed.
On some vehicles, an air bladder is incorporated into a strut or shock to provide additional dampening and/or ride height control. Air shocks are one such example, and are often used on the rear for automatic load leveling. On some applications, a strut may have both a conventional steel spring with an additional air bladder on top to vary ride height and/or spring stiffness.
Most air suspensions and automatic load leveling systems use some type of height sensor to monitor ride height. When the suspension is lower than its normal ride height, the system module energizes a relay to turn on a compressor and pump air into the air springs, shocks or struts through solenoid valves. When the desired ride height is reached, the module turns the compressor turns off and remains on standby until further corrections are needed. If ride height is too high because a heavily loaded vehicle has just been unloaded, the system module will open solenoids to vent air from the springs.
Maintaining a consistent ride height is important because ride height affects wheel alignment, tire wear, handling, traction and the aiming of the headlights. It can even affect fuel economy at highway speeds. Some late-model SUVs with air ride suspensions automatically lower themselves an inch or two at highways speeds to improve their aerodynamics and reduce wind resistance. The driver may also have a ride height switch that allows him to increase ride height and ground clearance when driving off-road.
On many applications, the air ride system may remain active for a certain period of time after the engine has been turned off. The system continues to monitor ride height and may make corrections by adding or venting air as needed. There may be a built-in delay so the corrections do not occur immediately when the occupants exit the vehicle or unload the trunk.
Understanding the operating logic of an air ride suspension system is important when trying to make a diagnosis because all of these systems are different. If you don't understand the logic, you can't always tell if the system is functioning correctly or not.
LINCOLN AIR SUSPENSION
On the older Lincoln systems, the control module monitors ride height for up to an hour after the ignition is turned off, venting pressure as needed to maintain correct ride height. During this time it will lower, but not raise the vehicle. At the end of the hour, the compressor will kick in for up to 30 seconds to add air if ride height is low. After that, it shuts down and makes no further corrections until the ignition is turned back on.
When the ignition is turned on, the system will raise ride height if necessary, but not lower it. After 45 seconds, it will then make lowering corrections as needed, but only if all the doors are closed. It will not make any corrections if the brake pedal is depressed. Once the system is in its "driving" mode, it averages input from the height sensors before making any adjustments in attitude. This prevents the suspension from trying to compensate for every bump in the road. Only when a continuous high or low indication is present for more than 45 seconds will it react and make a change.
If a change in ride height isn't accomplished within three minutes after the need is indicated, the system warning lamp comes on and stays on as long as the ignition remains on. When the key is turned off, the warning lamp will go out and remain out unless the same problem is indicated the next time the key is on.
AIR RIDE PROBLEMS
Air suspension problems fall into one of three categories:
GENERAL SERVICE PRECAUTIONS
The first thing to keep in mind when servicing vehicles with electronic air suspensions is to watch out for those that are self-leveling - especially if the vehicle is going to be raised on a lift to change the tires or work on the brakes.
If you're working on a Lincoln or a Ford Crown Victoria with an electronic air suspension, there's a switch in the trunk to deactivate the system. If the system is not turned off, it will remain active after the ignition is switched off and try to level itself when the wheels are raised off the ground by venting air from the air springs. When the vehicle is then lowered back down, the suspension may go flat because there's no air in the bags. Later systems are supposed to maintain a minimum pressure in the springs so this doesn't happen, but it's no guarantee the vehicle will have enough ride height to clear the lift.
You also want to deactivate a self-leveling suspension when doing brake or suspension work. Why? Because you don't want the suspension moving because a height sensor is telling the system module the vehicle is riding high.
Disconnecting a battery cable will also deactivate an electronic self-leveling suspension, but you'll also cut power to all the other onboard electronics. So to save yourself the embarrassment of erasing all the preset channels on the radio as well as the PCM's memory, use the deactivation switch (if one is provided) or find and pull the fuse that powers the self-leveling control module or air compressor. Don't forget to reactive the system after the vehicle has been serviced or it may cause the warning light to come on then next time the vehicle is driven.
Another thing to keep in mind on Lincolns, Fords and other vehicles with full air spring suspensions is that the air springs support the weight of the chassis and are under pressure (25 psi minimum - unless the springs have gone flat). Do not vent an inflated spring or remove a spring solenoid unless the vehicle is supported on a frame contact hoist, or the chassis is supported by safety stands when the wheels are off the ground. Why? Because the chassis will drop when air is let out of the springs.
- Electrical - Problems with the height sensors, air spring solenoids, module control circuit, compressor relay, compressor or vent solenoid. These can be isolated by using the system's self-diagnostic procedures (if available) and various volt/ohm checks. Service consists of replacing the faulty component or wiring, or readjusting the height sensors.
Electronic problems will usually set a fault code and turn on a warning light. On the older vehicles, there is usually a self-diagnostic procedure for reading the codes and doing self-tests. On newer vehicles, codes can usually be accessed with a scan tool. On the more sophisticated systems, you may need a bidirectional scan tool with the appropriate software to do system tests and to recalibrate height sensors.
On late-model Cadillacs with the CVRSS electronic suspension, you can get codes by pressing the WARM and OFF buttons on the climate control system simultaneously. Refer to a manual for the code retrieval and clearing procedure when using the panel buttons.
- Air supply - Leaky or obstructed air lines, faulty compressor or spring solenoids. Diagnosis is made via the self-test and visual inspection. A bad compressor or solenoid must be replaced, but damaged air lines can be repaired by splicing.
NOTE: One of the leading causes of compressor failure is air leaks (usually in the air springs or lines). If the compressor is constantly running, it's going to work itself to an early death.
If you're replacing a compressor, the dryer should also be changed to protect the new unit from moisture. If the dryer is saturated with moisture, it can create backpressure that can overload and burn out a compressor.
- Air springs - Leaking, damaged, deflated or unfolded springs. Damaged springs must be replaced as an individual assembly (replacement in pairs is not necessary with air springs). If unfolded, the spring must be carefully inspected for cuts or cracks before it is reinflated.
Sources for Air Suspension Conversion Kits:
American Air Suspension
More Shock Absorber & Strut Articles:
Shock & Strut Diagnosis
How To Inspect Your Car's Suspension
Floor Jacks & Jack Stands (How to Safely Raise Your Vehicle)
Checking Chassis Ride Height
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