Just like most other Fords & Lincolns, the air suspension module(brain) on the Mark VIII will test everything real quick when the ignition is switched to the on position. If it sees something it can't deal with(something out of range), it will turn the check suspension message on and shut down the system. NOTE: While the check suspension message is on, the system WILL NOT make any adjustments(up or down).
To keep from making any unnecessary adjustments, the module(brain) will only "look" at the sensors every couple of minutes or so. When the module "looks" at a sensor, it will make an adjustment either up or down if needed.
It will make a raising adjustment by first signaling the compressor relay, which powers up the compressor and pressurizes all the lines. It will then open up the solenoid(s) on whatever corner or corners it needs to make the adjustment to and allow air to flow into the air struts/springs.
When the desired ride height is reached, the solenoid(s) will close and the compressor will turn off.
It will make a lowering adjustment by opening the vent solenoid in the compressor as well as whatever solenoid on whatever corner or corners it needs to make the adjustment to and allow air to flow out of the air strut/springs, through the lines, through the dryer and finally through the vent solenoid on the compressor. When the desired ride height is reached, all the solenoid(s) will close.
NOTE: The Mark VIII has what is known as a 3 channel system. This means it only has 3 sensors. One in the left front, one in the right front and one in the rear. It can adjust the front individually, but because it only has 1 sensor in the rear, it adjusts the rear as an average.
The biggest difference between the '93-'98 Mark VIII air suspension system is that it has 2 different heights....and the correct height depends on the speed at which the vehicle is traveling.
When the ignition is first turned on or after the vehicle drops below approx. 40 mph, it will raise itself to "city height"....which is good for clearing potholes, manhole covers & such that is frequently encountered at lower speeds.
For '93 and early '94 Mk 8's, the vehicle automatically lowers to "curb height"....which is almost an inch at hwy. speeds(approx. 60-65 mph). For Mk 8's mid year '94 through '98, the vehicle lowers approx. half an inch at hwy. speeds.
When the vehicle is parked, ignition off and door(s) are opened then closed, the vehicle will vent once again to "curb height".
The module is the "brains" of the system. It makes any height changes necessary based on the data from all 3 height sensors and the vehicle speed sensor. (Doesn't go bad that often) This module also handles the EVO (Electronic Variable Orifice) for the steering. This module is located above the RF kickpanel and can be seen by lowering the glovebox. The air suspension/evo module is the one that has 2 connectors going to it. One is black and the other is gray.
AIR SPRING SOLENOID
Each air spring/strut has an electrically operated solenoid that the module uses to isolate each air strut/spring from each other. The module can control the left front, right front or the rearend independently. The solenoids act as a "gate" for air. No air goes in or out unless the solenoid is opened or "signaled" by the module. All solenoids have a 2 pin connection. Its a simple circuit of "power" and "ground". The part of the solenoid that holds the connector looks like a "D" if you look at it from a top view. To open the solenoid, power should be applied to the top of the "D" and the negative should be applied to the bottom of the "D".
The air line is held in the solenoids by way of a collet. The air line can be disconnected from the solenoid by pushing in on the orange plastic ring and pull out on line.
If the module determines the front or rear of the car should be raised, the air compressor will come on, the solenoids will be opened up and air is allowed from the compressor/dryer, through the air lines and into whichever air struts/spring until the desired height has been reached. When this height has been reached, the solenoids will close and the compressor will shut off.
If the module determines the vehicle should be lowered, the solenoids on whichever air strut/spring will be opened up, as well as the vent solenoid in the air compressor. The vented air will pass out of the strut/spring, through the air lines, through the compressor/dryer, then to the atmosphere. Once the vehicle has reached the desired height, the solenoids on the air springs and compressor will close.
The module is programmed to only try to raise the vehicle for "up to" 90 seconds. If the vehicle has not reached the desired height in this timeframe, the module will "time out". Which means it will turn on the check suspension message and shut down the system. The vehicle will not try to make any adjustments until after the ignition has been turned off, then back on again. Once the ignition is turned back on, it will then try again for 90 seconds.
The module is also programmed to only try to vent the vehicle for "up to" 45 seconds. If the vehicle has not reached the desired height by this time, the module will "time out". Which means it will turn on the check suspension message and shut down the system. The vehicle will not try to make any adjustments until after the ignition has been turned off, then back on again. It will then try again for 45 seconds.
While these solenoids don't go bad that often, they do need to be resealed when removed. Click here for instructions how to reseal your solenoids.
The sensors are the eyes so to speak for the module. The sensors let the module know the height of the car. These sensors are attached to the suspension by way of a ball stud top and bottom. The sensors can be disconnected by pushing down on the little metal tab and pulling off the ball stud.
The Mark VIII has what is known as a 3 channel system. In other words, there are 2 sensors in front and only one in the back. The one in the back is for both sides as an average. The rear solenoids are wired together, so when one gets signaled to open, the other will too. This is why the Mark VIII may sometimes get out of "kilter" when the car is parked in an unlevel area.
The compressor supplies air to the air springs/struts. The compressor on the Mark VIII is the biggest and most powerful in the Lincoln line. It is mounted in the RF fenderwell. It has 4 air lines coming out of the dryer which go to each solenoid on each air strut & air spring. It does not matter which line goes into each hole in the dryer. You may find that the lines are molded and one line may fit better when its inserted in a certain hole, but it won't make any difference when adding or removing air from the system.
The dryer is like a common manifold. In other words, the compressor/dryer assembly doesn't know what corner(s) the adjustment is being made on, it only knows to come on when told to do so, or to vent when told to do so. Again, the module controls that by opening whichever solenoid(s).
Any air compressor produces moisture, so a dryer is needed to trap and absorb the moisture BEFORE it gets to the struts/springs. The dryer consists of moisture absorbing silica gel beads and 2 metal plates. These plates can rust very badly and actually turn to powder. This powder can then clog up the dryer first and then make its way to the vent solenoid and clog it up also. So now what started out to be a dryer problem, has turned into a compressor and dryer problem.
The dryer is the only part of the compressor dryer assembly that is sold separately from the dealer.
The compressor is fused by way of a 50 amp fuse in the engine compartment. Although possible,it is VERY RARE to see this fuse blown. This compressor is so powerful, it will actually break the piston rod when it gets worn! NOTE: When these compressors start going bad, they will pull allot of amps. This is extremely hard on the relay! While a relay may get you going again, the compressor is most likely what took out the relay to begin with. Replacing one without the other is usually not a wise move.
COMPRESSOR VENT SOLENOID
The vent solenoid on the compressor is used as a "vent to atmosphere" solenoid. When the module gives the command to vent the car down, this vent solenoid and whichever solenoid on the strut/spring that the module needs to vent, opens up and vents air out of the system. This vent solenoid is usually one of the first things to go bad on the Mark VIII compressor. This is due to excessive moisture in the system that allows the piston in the solenoid to rust and corrode. The main reason this solenoid has such a hard time with moisture, is because the compressor is mounted on its side. If theres any moisture in the general area of the compressor, the moisture will find its way to the vent solenoid just from gravity. NOTE: The piston inside the solenoid is metal, so it can't deal with moisture very long before rust starts setting in.
Since the compressor draws allot of amps, it is powered by a separate relay. On the earlier Mark VIII's, the relay can be found on the same black metal frame the compressor is mounted in. This relay is a solid state relay and is very tricky to test.
NOTE: On the earlier mark VIII's, you can test the old relay by swapping the anti-lock relay. IT IS THE EXACT SAME RELAY. If the compressor goes bad and in turn draws allot of amps, this relay will usually go bad.
WHAT USUALLY HAPPENS?
LEAKING AIR STRUT/SPRINGS
With age, the rubber air bladders on the air struts & springs will dry rott and eventually leak air. The leak will almost always be "on the fold of the rubber". This is where the rubber folds over itself and that area changes with vehicle height. Upon inspection, you may not see any visible cracks until the height has been achieved where the car has sat most of its life. Once this area has been found, cracks will magically open up on the fold.
Early in the stages of a leak, the air springs & struts will only leak while being driven. Remember that the system is all automatic, so if there is a leak, you probably won't realize theres even a problem until one of two things happen:
#1 The leak gets so bad that its leaking more air out than the compressor can put back in.
#2 The compressor is damaged and can't keep up with the load.
As was stated before, in the early stages of a leak, it will only leak while being driven. As time goes on, you will probably notice the car going down one or more nights in a weeks time. The number of days the car is down will increase as time goes on.
Even driving the car, the leak will get progressively worse and eventually to the point where you can't drive the car.
ANY air compressor produces moisture. When a system has a leak and the compressor has to run 2-10 times more than it would normally, the compressor will produce 2-10 times more moisture than it would normally.
In time, the moisture absorbing gel beads in the dryer lose their ability to remove the moisture from the incoming air. Moisture is then able to make its way back to the rest of the system. Turning every low lying area into a moisture reservoir.
The dryer also turns into a reservoir for water. In other words, when the compressor vents, moisture will be blown back through the compressor. This is very hard on a part that was designed to be operated dry. These problems include, but are not limited too:
Swelling of the piston rings, which makes the electric motor have to work that much harder to do the same job and increases the heat made by the compressor, which is hard on any sealing o-rings in the high heat area.
Rusted and corroded compressor vent solenoid
Rusted and corroded end plates inside the dryer
Rusted and corroded intake & exhaust valves in the compressor
Damaged armature & brushes
WHAT CAN BE DONE?
Because so many problems start with a leak, I recommend doing a leak test once a month on any car over 5 years old. This way, you will find the leak that is only a maximum of one month old BEFORE it has had a chance to do any more damage.
Click here for the recommended leak test
In my opinion, about 80% of all Mark VIII's will encounter an excessive moisture problem at one time or another. This is mainly because of the mounting point of the compressor, as it is the lowest point on the system. Just by gravity alone, moisture will find it's way to the compressor.....much less being pushed by high pressure! If you find out you have an excessive moisture problem, you'll need to replace the dryer every 2 months for 3 or 4 times. In time, it should "sop-up" the accumulated moisture.
REMEMBER, The faster you find and repair a leak, the smaller your parts & repair bill will be!