Front Strut


Rear Bag




Compressor Relay




Compressor/Dryer



Height sensor
 (disassembled)
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The Mark VIII has airbags at each corner. The front airbag is integrated into the design of the shock absorber and resembles a standard McPherson strut/coil spring combination. The rear airbag is a standalone unit separate from the rear shock. The air bag acts as the spring mechanism that works in conjunction with the shock absorber to provide a nice, smooth ride.

The system uses an onboard 12v air compressor to fill the bags when needed. Also included in the package and attached directly to the compressor is an air dryer. The function of the dryer is to capture the moisture that is released when the air is compressed and pumped into the air bags. Downstream from the dryer and mounted on each bag are solenoid operated valves. These valves allow air into and out of the bag, similar to a doorway. The valves open when voltage is applied to the solenoid. Additionally, the compressor has a solenoid operated valve that allows air to escape the system entirely.

Controlling everything behind the scenes is the suspension ECU. It's job is to monitor the system inputs and keep everything working in cadence. The ECU controls the air compressor through a solid state relay. When the time comes to raise the vehicle, the ECU grounds the compressor relay and the compressor begins to run. The ECU uses three height sensors to determine vehicle height, two up front and one out back mounted on the drivers side rear suspension arm. The ECU sends out a 5v reference voltage to each of the three height sensors. The voltage that comes back from the sensors tells the computer that:

A) The vehicle is too low
B) The vehicle is too high
C) The vehicle is at trim height

If the car is found to be too low, the computer engages the compressor, then one second later opens the solenoid valves, raising the car. As the car raises the voltage signal sent back to the computer changes. When the car reaches trim height the compressor shuts off and the solenoids close. In the case of scenario B, the computer will open the vent valve and the corresponding solenoid valve of the high corner, therefore lowering the car. If the height sensors check out fine, the computer does nothing but continue to monitor it's inputs, averaging the values and performing height corrections every 45 seconds if needed.



 Mark VIII Air-Suspension Diagram (NOTE: Height sensors not shown)


HEIGHT ADJUSTMENTS:

The suspension is programmed to do a number of things automatically. The vehicle has two different ride heights. The first is the parked height. This is the the height the car rests to when parked. To account for the weight of passengers and luggage, the car has a predetermined height that it will raise to when making height corrections. When you get in and drive the car, it raises 20mm above where it was sitting when parked. If you accelerate to 105km/hr (~63mph), the vehicle drops 20mm for a more stable high speed profile. As soon as your speed decreases to ~45mph, the vehicle raises 20mm. When you park the car and shut the last door, it drops 20mm down to parked height.

While driving, the computer uses a long term strategy. The computer is constantly checking it's height sensors and computing an average ride height. It will adjust vehicle height only when the average is not within predetermined limits. As mentioned, this is only necessary when speeds go above 63mph and when speed decreases back down to 45mph. There should not be any other height adjustments under normal circumstances. Occasionally, a corner may be high or low and the compressor will run or the car will drop, but this is not usually the case. If you hear the compressor running excessively, you probably have a leak. It is important that you fix a bad airsping quickly to avoid further damage to the compressor.

While the car is parked, the computer uses a different strategy, called the short term strategy. This means the computer will adjust immediately according to the input of its height sensors. This is why the car will vent when jacked up with the air suspension switch on. What happens is the sensors sense the car is too high so the computer tries to compensate by venting air out of the system. Since the the jack is under the car not allowing it to drop, the computer ends up venting for the maximum length of time (45 seconds). This should not hurt anything and the car should raise back up to trim height when started. This short term strategy has a time window of one hour. At the end of the hour period, the computer runs a check of it's sensors and will raise the car if necessary at that time.
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