Suspension Control ( Automatic - Electronic ): Description and Operation
The compressor relay, compressor vent solenoid and all air spring solenoids incorporate internal diodes for electrical noise suppression and are polarity sensitive. Care must be taken when servicing these components not to switch the battery feed and ground circuits or components damage will result. The electrical power supply to the air suspension system must be shut off prior to hoisting, jacking or towing vehicle. This can be accomplished by disconnecting the battery or turning off the power switch located in the luggage compartment. Failure to do so may result in unexpected inflation or deflation of the air springs which may result in shifting of the vehicle during service procedures. Do not attempt to install or inflate any air spring that has become unfolded. Any spring which has unfolded must be refolded prior to being installed in a vehicle. The air spring refolding procedure should only be used to service an air spring which has never supported the vehicle's weight while in the improperly folded position. Do not attempt to inflate any spring which has been collapsed while uninflated from the rebound (hanging) position to the jounce stop. When installing a new air spring, care must be taken not to apply a load to the suspension until springs have been inflated using air spring fill procedure. When front air springs are replaced, the height sensor must be checked and replaced if damaged. After inflating an air spring in hanging position, it must be inspected for proper shape. Failure to follow the above information may result in a sudden failure of the air spring or suspension system.
Fig. 1 Air Suspension System:
Fig. 2 Exploded View Of Front Suspension:
Fig. 3 Exploded View Of Rear Suspension:
Used on Mark VII models, the Air Suspension System, Fig. 1, is an air operated, microprocessor controlled suspension which replaces conventional coil springs with air springs, providing automatic front and rear load leveling.
The front air springs, Fig. 2, are mounted to the upper spring pocket in the crossmember and on the lower suspension arms as in conventional suspension systems. The rear springs, Fig. 3, are mounted ahead of the rear axle, outboard of the body side members and on the lower suspension arm.
A piston type electrically operated air compressor, attached to the left fender apron, supplies the air pressure necessary for system operation. All air passing through the system is filtered through a regenerative type dryer, located on the compressor manifold. A vent solenoid, also located on the manifold, controls exhaust air.
Air flow through the entire system is controlled by the interaction of the air compressor, solenoids, height sensors and the control module.
OPERATION
Fig. 1 Air Suspension System:
System operation is maintained by the addition or removal of air to or from the air springs, resulting in a predetermined front and rear suspension height. This predetermined height is known as the vehicle trim height. The trim height is controlled by three height sensors, two of which are located at the front wheels and a third at the rear suspension, Fig. 1. The height sensors are attached to the body and suspension arms and will lengthen or shorten, depending on the amount of suspension travel. As weight is added to the vehicle, the body settles, shortening the height sensors. The height sensors signal the control module, which then activates the air compressor through a relay, and signals the air spring solenoids to open. As the body rises, the height sensors lengthen. When the predetermined trim height is reached, the air compressor and solenoid valves are de-activated by the control module. As weight is removed, the body rises, lengthening the height sensors, and the height sensors signal the control module. The control module then opens the air compressor vent solenoid and the air spring solenoid valves. As the body lowers, the height sensors shorten. When the predetermined trim height is reached, the air compressor vent valve and air spring solenoid valves are closed by the control module.
The air required for leveling the vehicle is distributed from the air compressor to each spring by four nylon air lines which start at the dryer and end at the individual springs. Each air line is color coded to identify the spring to which they belong. The dryer is used to dry the air before it is delivered to each spring. The air required for compression and the vent air enter and exit through a common port on the compressor head. Vented air is controlled by a solenoid valve in the compressor head.
Electrical power to operate the system is distributed by the main body harness. The control module controls the air compressor relay, vent solenoid and the four air spring solenoids to provide the air requirements of the springs. The module also provides the power and ground circuits to the height sensors, while monitoring the input from the sensors and the Ignition Run/Brake and On/Door Open circuits. These inputs are used by the module in determining vehicle leveling requirements, which are then carried out by the air system components controlled by the module. The control module also provides for system self diagnosis, a routine for filling the air springs and operation of the system warning lamp.
CONTROL LOGIC
Ignition Off
When the ignition switch is turned off, the system will continue to operate for approximately one hour. During this time, the system will service requests to lower the vehicle as required, provided no sensor was reading high at the time the ignition switch was turned off. Vent time is limited to 10 seconds for the rear springs and 3 seconds for the front. Approximately 1 hour after the ignition switch is turned off, the system will correct for a low vehicle height by activating the air compressor. Compressor run time is limited to 15 seconds for the rear springs and 30 seconds for the front.
Ignition In Run
When the ignition switch is first turned to the RUN position, the system will raise the vehicle as necessary. No down requests will be serviced for approximately 45 seconds. After the 45 second period, up and down requests will be serviced provided no door is open. If any door is open, no down requests will be serviced until the door is closed. However, if the brakes are applied with the doors closed, neither up nor down requests will be serviced except for a rear up request already in progress.
GENERAL OPERATING CONDITIONS
1. Requests are serviced in the following Order: Rear Up, Front Up, Rear Down, Front Down.
2. With ignition in RUN, failure to service any request within 3 minutes will result in the activation of the warning lamp. The lamp will stay on during that complete ignition cycle. However, only the request that was being serviced will be affected. The control module will continue to service all other requests as usual.
3. The rear spring solenoids will always be operated in tandem, while the front solenoids may operate independently.
4. Front and rear requests are never serviced at the same time.
5. Turning the ignition from RUN to OFF will clear all memory in the control module, and the warning lamp may not indicate failure when the ignition is returned to the ON position. When charging the battery, ensure ignition switch is off, as damage to the compressor or compressor relay may result.