Principles Of Operation
Vehicle Dynamic Suspension
Principles of Operation
WARNING: Shut off the electrical power to the air suspension system prior to hoisting or jacking an air suspension equipped vehicle. Failure to do so may result in unexpected inflation or deflation of the air springs, which may result in shifting of the vehicle during these operations. Failure to follow this instruction may result in serious personal injury.
WARNING: Vent all air pressure from the air suspension system prior to disconnecting or removing any air suspension components. It is dangerous to remove air suspension components while under pressure. Failure to follow this instruction may result in serious personal injury.
WARNING: Always inflate air springs before loading the suspension. If a load is applied to an uninflated air spring, the spring may be damaged and a new air spring will have to be installed. Damaged springs degrade vehicle handling, which could result in serious injury to vehicle occupant(s).
WARNING: Inspect the air spring for correct shape after it has been inflated with the wheels off the ground and the suspension unloaded. A deformed air spring could fail. Failure to follow this instruction may result in serious injury to the technician or vehicle occupant(s).
The air suspension system is designed to improve ride handling and general vehicle driving performance. A microprocessor controls the air suspension system. The microprocessor and the supporting hardware are contained in the Vehicle Dynamics Module (VDM). The VDM responds to signals from various sensors in the vehicle to maintain the programmed ride height while the vehicle is either moving or stopped. This is accomplished by opening and closing the solenoid valves to control the amount of air in the air spring(s). The VDM controls the air compressor by applying voltage through the compressor relay to inflate the air spring(s) and raise the vehicle. The VDM opens the vent solenoid to lower the vehicle by releasing air from the air spring(s) in response to signal inputs from the air suspension height sensor. The VDM has a preprogramming trim height.
The load leveling feature automatically makes adjustments in vehicle height so that the vehicle is always at target height and constant front-to-rear vehicle attitudes are maintained over the load range of the vehicle.
The system will remain active for 40 minutes after the ignition is turned off to make limited height adjustments as necessary.
Air Suspension Disable Feature
The air suspension system can be disabled through the message center. Refer to Air Suspension System Disable/Enable Air Suspension System Disable/Enable. When the driver elects to disable the air suspension system, the message center sends a message over the High Speed Controller Area Network (HS-CAN) bus to the VDM. Once the message is received, the VDM will take no further action to inflate or deflate the air springs, below 15 mph. The system will remain ON or OFF when the ignition key is cycled.
When the air suspension system has been disabled, the message center will display SUSPENSION SWITCHED OFF.
Air Compressor
The air compressor:
- consists of the compressor and vent solenoid; neither are serviced as individual items.
- is a single cylinder electric motor driven unit that provides pressurized air as required.
- is powered by an electromechanical relay, which is controlled by the VDM.
- 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 serviced 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 and forms an integral valve housing.
- has an O-ring seal that prevents air leakage past the dryer tip.
- opens when the VDM determines that lowering is required.
- provides an escape route for pressurized air when system pressures exceed safe operating levels.
- has a 1,034 kPa (150 psi) internal relief valve.
- is installed with the air compressor as a unit.
Air Spring
The air suspension system has 2 rear air springs. The air springs provide a varying spring rate proportional to the system air pressure and volume. The air suspension system regulates the air pressure in each air spring by compressing and venting the 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 VDM.
Air Tubes and Fittings
NOTE: Apply a soapy water solution to the air tubes to verify the location of air leaks. If the air tubes are cut, ruptured or leaking, a service repair kit is available.
Nylon air tubes with quick connect fittings connect to the air compressor and air spring solenoids. A Y-fitting is also used to divide the air supply to the RH and LH air spring solenoids.
Air Suspension Height Sensors
There are 2 suspension height sensors used for the rear suspension. The sensors send a voltage signal to the VDM. The output ranges from approximately 4.5 volts at minimum height (when the vehicle is low or in full jounce) to 0.5 volt at maximum height (when the vehicle is high or in full rebound).
The sensors have a usable range of± 45 degrees. 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 sensors are mounted to the frame and the link is attached to a ball stud on the lower suspension arm. The left and right sensors have opposite polarity and are not interchangeable.
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 the trim height level.
Compressor Relay
An electromechanical relay is used in the air suspension system for compressor control. The relay switches high current loads in response to low power signals and is controlled by the VDM.
The compressor relay is energized by the VDM to have high current flow from the battery to the compressor motor.
Vehicle Dynamics Module (VDM)
NOTICE: The Vehicle Dynamics Module (VDM) must receive a ride height adjustment calibration and pass a pneumatic test when it is installed new or swapped. Failure to follow these instructions will result in the air suspension system operating incorrectly.
The VDM controls the air compressor motor (through an electromechanical relay), all air spring solenoids and provides power to the rear height sensors. The VDM also controls vehicle height adjustments by monitoring the 2 height sensors and other HS-CAN messages. The VDM conducts all fail-safe and diagnostic strategies and contains self-test and communication software for testing the vehicle and related components.
The VDM monitors and controls the air suspension system through an 8-pin connector and a 16-pin connector. The connectors and the VDM are keyed so that the connectors cannot be plugged in incorrectly. The VDM uses the HS-CAN communications network for sending and receiving various information.
Air Spring Solenoid
WARNING: Vent all air pressure from the air suspension system prior to disconnecting or removing any air suspension components. It is dangerous to remove air suspension components while under pressure. Failure to follow this instruction may result in serious personal injury.
The air spring solenoid:
- allows air to enter and exit the air spring during leveling operations.
- is electrically operated and controlled by the VDM.
- is serviced separately from the air spring.