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Air Suspension Description and Operation
The purpose of the Air Suspension System is the following for the rear suspension under loaded and unloaded conditions:
- Keep the vehicle visually level
- Provide optimal headlight aiming
- Maintain optimal ride height
The Air Suspension System consists of the following items:
- Air Suspension Compressor assembly
- Air Suspension Sensors
- Rear Air Springs
The Air Suspension System must have a voltage supply of at least 12.6V to operate properly.
The Air Suspension System will maintain the rear (side) height within four mm (0.15 in) in all loading conditions and the leveling function will deactivate if the vehicle is overloaded. The side-to-side height variation is maintained within 8 mm (0.31 in). When the ignition is turned OFF, the Electronic Suspension Control Module (ESCM) remains active for between 30 minutes and 2 1/2 hours. The system will exhaust pressure within 30 minutes after the ignition is turned off, to lower the vehicle after unloading. In a temperature-controlled environment, the leakage of the complete load leveling system will not result in more than 1.four mm (0.05 in) drop of rear suspension height at GVWR during a 2four hour period. If the outdoor temperature drops from +20°C (+68°F) to −5°C (+23°F), the rear height may drop as much as 25 mm (1 in). However, the Air Suspension System should return to the specified height during the next ignition cycle.
Air Suspension Compressor Assembly
The Air Suspension Compressor Assembly houses the DC motor/air compressor, an Exhaust Valve Solenoid, and two Air Spring Isolation Valve Solenoids. The ESCM sends a voltage signal to each height sensor and measures the inductance of the return signal to determine if one or both air springs need to be inflated or deflated. Before the compressor starts, the exhaust valve is activated, providing compressor head pressure relief. Next, the Compressor Relay activates to start the compressor. When the trim height is achieved the relay is de-energized. To prevent the compressor from overheating, the compressor operates on a duty cycle. If the compressor is requested to operate continually, the compressor duty cycle is limited to 300 seconds at maximum output for the first cycle. After this time, the compressor automatically switches off for at least 92 seconds. After that, the compressor activates for 16 seconds and de-activates for 92 seconds. This 16/92 second duty cycle continues until the compressor can remain off for more than 92 seconds.
The Air Suspension Compressor Assembly has three different operating modes:
- Single Value Mode inflates and deflates the two air springs independently. This is the normal operating mode of the system. The system will enter Mean Value Mode when the sensor output indicates the difference between the height on each side is greater than 15.5 mm.
- In Mean Value Mode, the ESCM averages the signals from each height sensor to maintain an average trim height. If the measurement differential between the two (side) heights reaches approximately four mm, the system will enter Lock Mode and no leveling occurs. This compensates for uneven loading of the vehicle or if the vehicle is parked on an uneven surface. The Air Suspension Compressor Assembly has a wheel change (jack) control that prevents the air springs from deflating completely. If the ESCM detects that one or both of the rear wheels has a side height measurement of more than 235 mm, the system enters Lock Mode and suspends leveling functions until the suspension is moved toward the frame.
In some cases, the Air Suspension Compressor Assembly may set a flag that requires resetting the ESCM. The ESCM will reset on its own after the ignition has been turned off for more than 2 1/2 hours or if the air suspension fuse has been removed with the ignition turned OFF.
Air Suspension Sensors
The Air Suspension Sensor Arm attaches to an armature that rotates within a coil. The inductance of the coil, (not the resistance) changes depending on the position of the armature in the coil. The Air Suspension Module determines the angle of the sensor arm by sending an inductive voltage through the coil and measures the response time. The sensors must be calibrated to the correct side-to-side (D) height.
Rear Air Springs
The air springs are mounted in the frame in the same location where the coil spring is mounted for a vehicle without air suspension. Support pieces are attached to the axle for the air springs.
Electronic Suspension Control Description and Operation
Electronic Suspension Control Description
The Electronic Suspension Control (ESC) system independently controls each of the four shock absorbers to control the vehicle ride characteristics. The ESC system is capable of making these changes within milliseconds. The ESC system consists of the following major components:
- The Electronic Suspension Control (ESC) module
- The front/rear Position Sensors
- The front/rear adjustable Shock Absorbers
- The shock absorber Electrical Actuators, which are integrated within the shock absorbers
- The rear Air Springs
The ESC controls the damping mode selection according to the following factors:
- Vehicle speed
- Steering wheel position
- Body-to-wheel displacement
The ESC module evaluates these inputs to separately control the shock absorbers, providing an enhanced ride and comfort level over the widest possible range of operating conditions.
Electronic Suspension Control Module
The Electronic Suspension Control (ESC) module provides electronic control logic and output drive for each shock absorber. The ESC module makes decisions due to road and driving conditions based on various inputs. The ESC module receives input information by sensors directly connected to the ESC module or by other systems through the serial data line and uses these inputs to independently control the shock absorbers at each corner.
Electronic Suspension Control Position Sensors
The Electronic Suspension Control (ESC) position sensors provide the ESC module with the body-to- wheel displacement input. The ESC module uses this and other inputs to control the stiffness of the shock absorbers. If any vehicle body or wheel motion is detected, the ESC module determines how soft or firm each shock absorber should be to provide the best ride. ESC position sensors mount at each corner of the vehicle between the control arm and the body/frame.
Electronic Suspension Control Shock Absorber or Strut
The Electronic Suspension Control (ESC) shock absorbers provide variable damping to resist suspension movement. The ESC shock absorber has the capability of providing multiple modes or values of damping forces, in both compression and rebound direction. The damping forces are modified utilizing electrical actuators located internally within the shock absorbers.
Electronic Suspension Control Operation
The Electronic Suspension Control (ESC) system uses the information from other systems in order to execute certain functions.
The ESC system does not have a malfunction indicator lamp, but instead uses the instrument panel cluster (IPC) for the display functions. When the ESC system detects a malfunction that sets a DTC, the ESC system sends a message on the serial data line directly or through the powertrain control module (PCM) to the IPC, which will display one of the following messages:
- SHOCKS INOPERATIVE
- SERVICE SUSPENSION SYSTEM or SERVICE RIDE CONTROL
- SPEED LIMITED
The SHOCKS INOPERATIVE
message will only be displayed if the ESC system detects a malfunction that sets a DTC and causes the ESC system to disable all four shock absorbers. The ESC system will send a message on the serial data line to the IPC to display this message.
The SERVICE SUSPENSION SYSTEM
or SERVICE RIDE CONTROL
messages is displayed if the ESC system detects any malfunction that sets a DTC. The ESC system will send a message on the serial data line to the IPC to display this message.
The SPEED LIMITED
message is only displayed if the ESC system detects a malfunction that sets a DTC and causes the ESC system to disable all four shock absorbers. The ESC system will send a message on the serial data line to the PCM indicating that all four shock absorbers were disabled. The PCM then sends a message to the IPC to display this message.
The ESC module has the ability to store DTCs as current or history codes. Most ESC system malfunctions will display a message in the IPC and set a DTC. The message will remain ON until the RESET
button is pressed on the Driver Information Center (DIC). As long as the DTC is current, the message is displayed after every ignition cycle and the RESET button must be pressed to bypass the message.
The ESC system uses an ignition cycling diagnostic approach in order to reduce the occurrence of false or intermittent DTCs that do not affect the functionality of the ESC system. This allows for the fail-soft actions to be taken whenever a malfunction condition is current, but requires the malfunction to be current for a certain number of ignition cycles before the corresponding malfunction code and message will be stored or displayed.
If the ESC detects a malfunction, the ESC system defaults with a fail-soft action. A fail-soft action refers to any specific action the ESC system takes in order to compensate for a detected malfunction. A typical ESC fail-soft action would be if the ESC system detects a malfunction with a shock absorber.
Automatic Level Control
The Automatic Level Control (ALC) system maintains a desired rear suspension position under all types of loading conditions.
When the vehicle is unloaded, the rear suspension is at a desired position, which is monitored by the ESCM using the position sensor signal voltage inputs. As weight is added to the rear of the vehicle, the position sensor's signal voltage inputs change. When the ESCM detects a steady substantial change in the position sensor signal voltages for at least 10 seconds, it responds by commanding the compressor relay ON, which activates the compressor causing the air pressure in the air spring to inflate. This inflation raises the rear of the vehicle to regain the desired suspension position. When weight is removed from the vehicle, the ESCM responds by commanding the exhaust valve ON causing the air pressure in the air spring to deflate. This deflation lowers the vehicle to regain the desired suspension position.
The compressor is a positive displacement piston air pump driven by a 12-volt DC permanent magnet motor. The compressor draws filtered air through an intake line attached to an under body rail. The compressed air is sent through a dryer containing a moisture-absorbing chemical that dries the compressed air before it is sent to the air springs. Each time the compressor activates, the ESCM commands the exhaust valve ON for 1.5 seconds to release air from the compression chamber in the compressor’s cylinder head ensuring low motor current draw upon compressor activation. The ESCM also limits the amount of compressor run time to 180 seconds to protect the compressor components from thermal damage.
The ESCM has the ability to detect malfunctions within the ALC system. Any malfunctions detected will cause the DIC to display the SERVICE LEVELING SYSTEM message.
Front Suspension General Description (GNB)
Hi-Per Strut Front Suspension:
The two primary purposes of the front suspension are to:
Isolate the driver from irregularities on the road surface.
Define the ride and handling characteristics of the vehicle.
The front suspension absorbs impacts of the tires travelling over irregular road surfaces and dissipates the impact energy throughout the suspension system. This isolates the occupants of the vehicle from the road surface. The rate at which the suspension dissipates the impact energy and the amount of energy that is absorbed defines the vehicle’s ride characteristics. Ride characteristics are controlled by the Electronic Suspension Control system. The Electronic Suspension Control system individually controls the damping force of each of the four shock absorbers. The suspension system must allow for the vertical movement of the tire and wheel assembly as the vehicle travels over irregular road surfaces while maintaining the tire's horizontal relationship to the road.
This requires that the steering knuckle be suspended between a lower control arm via the yoke assembly to the strut assembly. The lower control arm outermost point is attached to the yoke assembly and then through the king pin bushing to the steering knuckle. The innermost end of the control arm attached at 2 points to the vehicle frame through semi-rigid bushings. The upper portion of the steering knuckle attaches to the yoke assembly though a ball stud. The yoke assembly connects to the strut assembly and vehicle body though an upper bushing. The steering knuckle travels up and down independent of the vehicle body structure and frame.
The up-and-down motion of the steering knuckle as the vehicle travels over bumps is absorbed predominantly by the coil spring. This spring is retained under tension over the strut assembly. A strut is used in conjunction with this system in order to dampen out the oscillations of the coil spring. The strut is filled with oil and has a moveable shaft that connects to a piston inside the strut. Valves inside the shock absorber offer resistance to oil flow to prevent rapid movement of the piston and shaft. Each end of the shock absorber connects in such a way to use the recoil action of the spring. Each end of the strut is designed as the connection point of the suspension system to the vehicle and acts as the coil spring seat. This allows the strut to use the dampening action to reduce the recoil of just a spring. The Lower Control Arm is pivots at the vehicle frame vertically. The ball joint allows the steering knuckle to maintain the perpendicular relationship to the road surface.
Front suspensions systems utilize a stabilizer bar. The stabilizer bar connects between the left and right strut assemblies through the stabilizer link and stabilizer bar insulators. This bar controls the amount of independent movement of the suspension when the vehicle turns. Limiting the independent movement defines the vehicles handling characteristics on turns.
The advantage of the HiPerStrut front suspension (GNB) over the McPherson-type wheel suspension is less spindle length, which leads to:
Reduced smooth road shake
Less torque steer
Increased cornering power, no camber loss during cornering
Robustness against wheel imbalance
The HiPerStrut design provides improved ride and handling performance with a premium steering feel.
The design enables larger diameter wheels.
The design allows camber adjustment with screws on the steering knuckle upper ball stud.
Rear Suspension Description and Operation
The XTS Rear Suspension System is the independent link type. Rear suspension adjustment is achieved through adjustable toe links and lower control arms. The rear air springs are fixed between the body and Lower Control Arm. The rear suspension consists of two shock absorbers attached to the knuckle and the reinforced body areas.
The Rear Suspension System performs the following functions:
Maintains the relationship of the rear axle to the body
Controls the torque reaction on acceleration and braking
The suspension system consists of the following components:
Stabilizer shaft, insulators and stabilizer links
Upper control arms
Lower control arms