a lot of info here:
Principles of Operation
The 4-wheel air suspension (4WAS) module commands changes in vehicle height that are necessary for both the load leveling and the vertical height adjustment features.
The 4WAS load leveling feature automatically makes adjustments in vehicle height so that the vehicle is always at trim height, and that constant front-to-rear vehicle attitudes are maintained over the load range of the vehicle. Adjustments in height that are necessary to correct height differences between the vehicle left and right sides for the 4WAS system are restricted to what can be reliably achieved with three height sensors.
The height adjustment feature has three vehicle heights within a two-inch span:
Kneel height — improves the ease of entering and exiting by lowering the vehicle one inch below the trim height in the front and rear when the vehicle ignition is in the OFF or LOCK positions and the vehicle is stationary.
Trim height — normal vehicle ride position; vehicle moves to trim position when the ignition is ON, when the transmission is initially shifted to DRIVE or REVERSE mode, when all doors are closed or when speed of more than 24 km/h (15 mph) is detected.
Off-road height — improves ground clearance by raising the vehicle one inch above trim height in the front and rear when the vehicle is at 4x4 low modes and the vehicle speed is less than 40 km/h (25 mph).
The 4WAS module uses the universal bus protocol, (UBP), module communications network for sending and receiving various signals with the exception of speed, information from the three height sensors, and the air suspension service switch. For additional information about UBP, refer to Section 418-00 .
The 4WAS system holds vehicle height when any door or rear hatch is opened. The system stores front and rear vehicle height the moment any door is detected open. The system then maintains this height regardless of the addition or removal of a load. The system returns to its commanded height when all doors are closed or vehicle speed exceeds 24 km/h (15 mph).
Vehicle Height Modes
Ignition State
Transfer Case Mode
Speed or Speed Range
Resulting Vehicle Height
OFF
4x2, A4WD, 4x4 HI
N/A
Kneel (TRIM - 25 mm)
ON and RUN
4x2, A4WD, 4x4 HI
All speeds
TRIM
ON and RUN
4x4 Low
Below 40 km/h (25 mph)
Off-Road (TRIM + 25 mm)
ON and RUN
4x4 Low
Above 40 km/h (25 mph)
TRIM
OFF
4x4 Low
N/A
Stays at current position (prior to ignition transition)
Air Suspension Service Switch
The air suspension service switch signal is needed to wake up the 4WAS module. Without the module receiving this signal, the 4WAS system is inoperative at speeds below 24 km/h (15 mph) and will not react when the vehicle is raised or lowered. At speeds above 24 km/h (15 mph), the system will be active as if the air suspension service switch is in the ON position.
Vehicles without message center
When the air suspension service switch is in the OFF position and the ignition switch is in the RUN position, a "CHECK SUSP" will appear in the RH corner of the instrument cluster.
Vehicles with message center
When the air suspension service switch is in the OFF position and the ignition switch is in the RUN position, the message center will display "SUSPENSION SWITCHED OFF"
Air Compressor
The air compressor:
- consists of the compressor and vent solenoid; neither are replaceable as individual items.
- is a single cylinder electric motor driven unit that provides pressurized air as required.
- is powered by a solid state relay, which is controlled by the 4WAS module.
- passes pressurized air through the compressor air drier that contains silica gel (a drying agent). Moisture is then removed from the compressor air drier when vented air passes out of the system during vent operation.
- has a single port drier that can be replaced separately.
The vent solenoid:
- allows air to escape from the system during venting actions.
- is part of the air compressor cylinder head.
- shares a common electrical connector with the air compressor motor.
- is enclosed in the cylinder head casting, which forms an integral valve housing that allows the valve tip to enter the pressurized side of the system.
- has an O-ring seal that prevents air leakage past the valve tip.
- opens when the 4WAS module determines that lowering is required.
- provides an escape route for pressurized air when system pressures exceed safe operating levels.
- has a 1034 kPa (150 psi) internal relief valve.
- is installed with the air compressor as a unit.
Air Spring
The 4WAS system has front and rear air springs. The air springs provide a varying spring rate proportional to the system's air pressure and volume. The air suspension system regulates the air pressure in each air spring by compressing and venting the system's air. Increasing air pressure (compressing) raises the vehicle; decreasing air pressure (venting) lowers the vehicle. Vehicle height is maintained by the addition and removal of air in each air spring through an air spring solenoid installed in the upper spring cap and energized through the 4WAS module command.
Air Lines and Fittings
NOTE: Apply a soapy water solution to the air lines to verify the location of air leaks. If the air lines are cut, ruptured or leaking, a service repair kit is available.
Nylon air lines with quick connect fittings connect to the air compressor and air spring solenoids. The compressor assembly output line is divided by the use of a fitting to supply the front and rear air springs. Fittings are also used to divide the air supply to the RH and LH air spring solenoids.
Air Suspension Height Sensors
There are two sensors used for the front suspension and one sensor used for the rear suspension. The sensors send a voltage signal to the 4WAS module. The output ranges from approximately 4.50 volts at minimum height (when the vehicle is low or in full jounce) to 0.50 volt at maximum height (when the vehicle is high or in full rebound). The sensors have a usable range of ± 45° compared to total suspension travel of 196 mm (7.84 in) at the front wheels and 240 mm (9.6 in) at the rear wheels. Therefore, the sensors are mounted to the suspension at a point where full suspension travel at the wheel is relative to the travel at the height sensor. The front height sensors are mounted to the frame and the links are attached to the upper arm ball studs. The rear height sensor is mounted to the frame and the link is attached to the rear lower arm ball stud.
When the height sensor indicates that the vehicle is lower than trim under normal driving conditions, the air compressor will turn on and pump compressed air to the system. When the sensor indicates that the vehicle is raised above trim under normal driving conditions, this will cause the air to be vented from the system to lower the vehicle to its trim height level.
Compressor Relay
A solid state relay is used in the air suspension system for compressor control. The relay incorporates a custom power metal oxide semi-conductor field effect transistor (MOSFET) and ceramic hybrid circuitry. The relay switches high current loads in response to low power signals and is controlled by the logic of the 4WAS module.
The compressor solid state relay is energized by the 4WAS module to have high current flow from the battery to the compressor motor.
4-Wheel Air Suspension (4WAS) Module
NOTE: The 4WAS module must receive a ride height adjustment calibration and must pass a pneumatic test when it is installed new or swapped.
The 4-wheel air suspension (4WAS) module controls the air compressor motor (through a solid state relay), all air spring solenoids, and provides power to front and rear height sensors. The 4WAS module also controls vehicle height adjustments by monitoring the three height sensors and other universal bus protocol (UBP) signals. The 4WAS module conducts all fail-safe and diagnostic strategies, and contains self-test and communication software for testing the vehicle and related components.
WARNING: Never rotate an air spring solenoid valve to the release slot in the end cap fitting until all pressurized air has escaped from the spring to prevent damage or injury. 
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