4-Wheel Vehicle Dynamic Suspension
Air Suspension Pneumatic Diagram
The air suspension control module commands changes in vehicle height that are necessary for both the load leveling and vertical height adjustment features.
The 4-wheel air suspension (4WAS) system load leveling feature automatically makes adjustments in vehicle height so that the vehicle is always at trim height and 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 two 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, the transmission is initially shifted to DRIVE or REVERSE mode, all doors are closed or speed of more than 16 km/h (10 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 system uses two height sensors, a steering sensor, GEM and transfer case inputs and other vehicle sensors to measure the driver and road inputs. The system changes vehicle height on the front and rear axle separately using an air compressor, five air solenoids, various air lines, an air spring integrated inside the front shocks and a rear air spring with a solenoid.
The air suspension 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 16 km/h (10 mph).
Vehicle Height Modes
Air Suspension Switch
The air suspension switch supplies power to the air suspension control module. Without the air suspension control module receiving this power, the 4-wheel air suspension system is inoperative and will not react when the vehicle is raised or lowered.
If the air suspension is disabled by turning off the air suspension switch, a "CHECK SUSP" will appear in the RH corner of the cluster with the ignition in the RUN position.
Air Compressor
The air compressor:
^ Is not interchangeable with the rear air suspension (RAS) 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 air suspension control 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.
^ The drier may be replaced separately.
^ The drier has dual ports and is not interchangeable with the RAS drier.
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 air suspension control module determines lowering is required.
^ Provides an escape route for pressurized air when system pressures exceed safe operating levels.
^ Has a 1792 kPa (260 psi) internal relief valve.
^ Is installed with the air compressor as a unit.
Air Spring
The 4WAS vehicles use air springs in the rear. 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 air suspension control module command.
The air springs replace the conventional coil springs.
Air Shocks
Adjustable air shock absorbers are used for the 4WAS system vehicles. Enclosed in each shock absorber is an air spring. Replace a faulty shock absorber as a unit.
The air spring integrated into each air shock is capable of independently raising and lowering each corner of the vehicle based upon the pressure and volume of air supplied. The air suspension system regulates the pressure in each air shock by compressing and venting system air. Increasing air pressure (compressing) raises the vehicle and increases the total spring rate (spring effect of the air shock plus the front torsion bar) of the wheel being modified. Conversely, decreasing air pressure (venting) lowers the vehicle and effective spring rate. Vehicle height is then maintained by the addition and removal of air in each air shock.
Front Fill Solenoid
The front fill solenoid connects the output of the compressor assembly to the two front air shocks. When energized along with the front gate solenoid, air pressure to the front suspension can be modified, affecting its height relative to the body.
Rear Fill Solenoid
The rear fill solenoid connects the output of the compressor assembly to the two rear air springs. When energized along with the two rear solenoids, air pressure to the rear suspension can be modified, affecting its height relative to the body.
Front Gate Solenoid
The front gate solenoid provides pneumatic isolation of the left and right sides of the vehicle. The separation is necessary to eliminate the transfer of air from the left to the right front air shocks during roll conditions. When the front fill solenoid and front gate solenoid are off (closed), the left and right air shocks are separated, allowing a pressure differential to be generated. In a turn, the increased pressure in the two outermost air shocks raises the two outer wheel spring rates, decreasing vehicle roll.
Conversely, opening (energizing) the gate solenoid on smooth driving surfaces connects the left and right air shocks. This forces left and right side air pressures to be the same. Also, allowing the transfer of air reduces the resistance to vertical wheel movement, resulting in a softer ride.
Air Lines
There are six nylon air lines with quick connect air fittings in the vehicle that connect the air compressor, solenoids, air shocks and air springs. One line connects the output of the compressor assembly to the front fill solenoid and the other two lines connect the output of the compressor assembly to the rear fill solenoid. Another line connects the output of the rear fill solenoid to both rear air springs. The fifth air line in the front connects the output of the front fill solenoid to both the right front air shock and the front gate solenoid. The last air line connects the output of the gate solenoid to the left air shock. The air lines cannot be spliced or repaired if the air lines have been cut or ruptured.
Air Suspension Height Sensor(s)
Two height sensors are mounted on the vehicle. The sensors send a voltage signal to the air suspension control module. The output ranges from approximately 4.75 volts at minimum height (when the vehicle is low or in full jounce) to 0.25 volts at maximum height (when the vehicle is high or in full rebound). The sensors have a usable range of 80 mm (3 inch) compared to total suspension travel of 200 - 250 mm (8 - 10 inch) at the wheel. Therefore, the sensors are mounted to the suspension at a point where full suspension travel at the wheel is relative to 80 mm (3 inch) of travel at the height sensor. The front height sensor upper socket is attached to the frame bracket at the upper control arm mounting area and the lower height sensor socket is attached to the upper control arm bracket. The rear height sensor is attached between the No. 5 frame crossmember (upper socket) and the panhard rod (lower socket). Replace a sensor as a unit.
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 air suspension control module.
The compressor solid state relay is energized by the air suspension control module to have high current flow from the battery to the compressor motor.
Control Module
NOTE: The 4WAS control module is also used for the RAS system. The internal processor recognizes external circuitry to determine if it is installed in a 4WAS or a RAS system.
NOTE: The air suspension control module is calibrated with information from the air suspension height sensors. A new or swapped air suspension control module requires the ride height adjustment calibration process to be performed.
A microcontroller-based electronic air suspension control module controls the air compressor motor (through a solid state relay) and all system solenoids. The air suspension control module also provides power to front and rear height sensors. The air suspension control module controls vehicle height adjustments by monitoring the two height sensors, vehicle speed, a steering sensor, acceleration input, the door ajar signal, transfer case signals and the brake pedal position (BPP) switch. The air suspension control module also conducts all fail-safe and diagnostic strategies and contains self-test and communication software for testing the vehicle and related components.
The air suspension control module is interchangeable between the RAS and 4WAS system.
The air suspension control module monitors and controls the system through a 32-pin two-way connector. The air suspension control module is keyed so that it cannot be plugged into an incorrect harness. There are two sides of the harness connection to the air suspension control module. Each is uniquely colored and keyed to prevent reversing the connections.
Solenoid, Air Spring
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.
The air spring solenoid:
^ Allows air to enter and exit the air spring during leveling operations.
^ Is electrically operated and controlled by the air suspension control module.
^ Is only replaced as a unit.
Steering Sensor
The steering sensor provides the steering rate and position to the air suspension control module through two signals.
Pressure Relief Valve
The pressure relief valve (PRV) protects the rear air suspension components by venting the system to a specific pressure in the case of a system malfunction. The system will decrease the pressure to an acceptable level to maintain vehicle height. In the normal operation mode with normal system pressure, the PRV is constantly closed and does not have any effect on the system performance or function. The PRV is replaceable without any air lines.