The Phaeton
Air Suspension
Design and Function
Self-Study Program Course Number 899303 Rebound Bump Vibrating Mass Position of Rest Spring T ravel P Amplitude Amplitude cvr-outside-front-ssp275 6/30/03 1:59 PM Page 1
Volkswagen of America, Inc. Service Training
Printed in U.S.A. Printed 08/2003
Course Number 899303
©2003 Volkswagen of America, Inc.
All rights reserved. All information contained in this manual is based on the latest information available at the time of printing and is subject to the copyright and other intellectual property rights of Volkswagen of America, Inc., its affiliated companies and its licensors. All rights are reserved to make changes at any time without notice. No part of this document may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, nor may these materials be modified or reposted to other sites without the prior expressed written permission of the publisher.
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Trademarks: All brand names and product names used in this manual are trade names, service marks, trademarks, or registered trademarks; and are the property of their respective owners.
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Table of Contents
The Self-Study Program provides you with information regarding designs and functions.
The Self-Study Program is not a Repair Manual.
For maintenance and repair work, always refer to the current technical literature.
Important/Note! New!
Introduction ... 1 Suspension and Damping Basics ... 2
Vehicle Suspension, Types of Vibration Acting on a Vehicle,
Vibrations, Definitions, Suspension System, Spring Characteristics, Spring Travel
Air Suspension Basics ... 9
Air Suspension, Characteristics of Air Springs, Spring Characteristics, Air Spring Design
Damping System Basics ... 15
Shock Absorbers, Twin-Tube Gas Pressure Shock Absorber, Shock Absorber Tuning
System Description ... 19
Air Suspension with Continuously Controlled Damping, Component Locations, Operation and Display, Self-Leveling Suspension, Damper Tuning, Control Strategy of the Self-Leveling Suspension, Manual Deactivation and Activation, Actions of Air Suspension After “Ignition Off,” Shock Absorber Control, Diagram of Air Suspension System with Continuously Controlled Damping, Schematic Diagram of the Overall System, System Overview
Design and Function ... 34
Level Control System Control Module J197, Air Spring Struts, Air Supply Module, Compressor Unit, Pressure Accumulator, Air Supply Strategy, Self-Leveling Suspension Pneumatic Diagram, Solenoid Valves, Level Control Pump Temperature Sensor G290, Level Control Pressure Sensor G291, Vehicle Level Control System Sensors, Body Acceleration Sensors, Wheel Acceleration Sensors, Interfaces, Functional Diagram – Air Suspension with Continuously Controlled Damping, Additional Interfaces, Emergency Running Mode
Service ... 66
Self-Diagnosis
Introduction
1
Vehicle contact with the road is controlled by the suspension system components. Conflicting requirements for maximum comfort, optimum driving safety, and minimum noise transmission from the road surface to the vehicle interior place heavy demands on suspension system designers. Vehicles like the Phaeton, which are
expected to meet high standards of comfort, represent a special challenge that necessitates a compromise between the various requirements on the
suspension system.
One solution is a controlled suspension system including:
• Full load-bearing self-leveling suspension system.
• Continuously controlled damping. Control is based on the “skyhook control strategy.”
In a perfect world, damping would be controlled as if the vehicle body were suspended by a hook from the sky, floating above the road without interference from the surface of the road.
The purpose or this Self-Study Program is to introduce and describe this new system.
Suspension and Damping Basics
2
Vehicle Suspension
When a vehicle is moving, the
external forces that act upon it produce movements in the three possible directions of motion – the transverse, longitudinal, and vertical axes.
The aim of good suspension and damping system design is to minimize the effect of these forces on driving comfort, driving safety, and operating safety by striking a balance between the suspension system and the vibration damping system.
A basic distinction can be drawn between the suspension system and the vibration damping system. The task of both systems
is to absorb and reduce the forces produced and, if possible, to keep them away from the vehicle body and its occupants:
Driving safety – They help maintain continuous tire contact with the road, which is important for steering and braking. Driving comfort – Harmful or unpleasant vibrations are kept away from the
passengers and the cargo remains intact. Operational safety – The vehicle body and assemblies are protected against high impact and vibration loads.
Suspension and Damping Basics
3
Types of Vibration
Acting on a Vehicle
In addition to the upward and downward movement of a moving vehicle, vibrations occur about and in the direction of the longitudinal, transverse, and vertical axes. The following terms are generally used to describe the vibrations that can occur in a motor vehicle.
Longitudinal axis vibration terminology: • Twitching – Vibration in the direction
of the longitudinal axis (shudder, back-and-forth motion).
• Rolling – Torsional vibration (rotation) around the longitudinal axis (snaking, rolling, and tilting motion).
Transverse axis vibration terminology: • Drifting – Vibration in the direction
of the transverse axis (side slip, side-to-side motion).
• Pitching – Torsional vibration (rotation) around the transverse axis (up-and-down motion of the front of the vehicle relative and opposite to the motion of the rear). Vertical axis vibration terminology:
• Bouncing – Vibration in the direction of the vertical axis (road surface impacts, vertical vibration, up-and-down motion). • Yawing – Torsional vibration (rotation)
around the vertical axis (side-to-side motion of the front of the vehicle relative and opposite to the motion of the rear).
SSP275/009 Longitudinal Axis • Twitching • Rolling Transverse Axis • Drifting • Pitching Vertical Axis • Bouncing • Yawing
Suspension and Damping Basics
4
Vibrations
The tires, suspension elements, body, and vehicle seat form a system which is capable of vibration, i.e. when an external force such as a bump on the road acts on this system, it oscillates back and forth about its position of rest. These vibrations repeat themselves until they die away. The vibrations are defined by their amplitude and frequency.
Intrinsic body frequency is a major factor in vehicle occupant comfort and safety. • An intrinsic body frequency of less than
1 Hz can cause nausea, depending on one’s predisposition.
• Frequencies above 1.5 Hz are detrimental to driving comfort.
• Frequencies higher than 5 Hz are perceived as shocks.
Intrinsic body frequency is essentially determined by the spring rate and the size of the sprung mass.
Definitions
Sprung mass – The weight of the car supported by its suspension including the engine, transmission and body.
Vibration – Oscillating movement of a mass (e.g. bump and rebound of the vehicle body).
Amplitude – Maximum distance of an oscillating mass from its position of rest (vibration displacement, spring travel). Period – Duration of a single, complete vibration cycle.
Frequency – Number of vibration cycles (periods) per unit of time. The customary designation for cycles per second is Hertz, abbreviated Hz.
One complete vibration cycle per second = 1 Hz
Intrinsic Frequency – Natural vibration frequency (free vibrations) of a given mass. Resonance – The physical characteristics of a mass that allow it to vibrate easily at its intrinsic frequency with very little excitation force, like a lightly plucked guitar string. Shock absorption – Describes the decay of vibrations.
Suspension and Damping Basics
5
Larger masses or softer springs produce a lower intrinsic body frequency with increasing spring travel (amplitudes).
The intrinsic frequency of
unsprung masses like the wheels and tires with attached brake, hub, and axle components is approximately 10 to 16 Hz. Smaller masses or firmer springs produce higher intrinsic body frequencies with decreasing spring travel.
SSP275/005 Time Period Spring T ravel SSP275/004 Time Period Spring T ravel SSP275/003 Damped Vibration Time Period Amplitude Position of Rest Vibrating Mass Rebound Bump Spring T ravel
6
Suspension and Damping Basics
Suspension System
The tires, the springs, and the seats with their cushioning effect collectively form the vehicle suspension system.
As key components of this system, the spring and shock absorber elements are the link between the wheel suspension and the vehicle body.
Suspension elements include: • Steel springs (leaf springs, helical
springs, torsion bar springs)
• Air springs (flexible air bladder springs and toroidal bellows springs)
• Hydro-pneumatic springs (piston and diaphragm type hydraulic accumulators) • Rubber springs
• Anti-roll bars
• Combinations of these elements
On the vehicle, a distinction is made between unsprung masses and sprung masses.
Unsprung masses include items like wheels, brakes, final drive shafts, wheel bearings, and wheel bearing housings. Sprung masses include the vehicle body with suspension and drivetrain parts. The general aim of vehicle tuning is to keep unsprung masses to a minimum. This minimizes interference with the vibration characteristics of the vehicle body and improves suspension response (and as a result, driving comfort).
The following components contribute to reducing unsprung masses:
• Light-alloy suspension parts • Light-alloy brake calipers
• Light-alloy hollow-spoke wheels • Weight optimized tires
SSP275/002
Seat Springs Spring and
Shock Absorber Element Sprung Mass Spring and Shock Absorber Element Unsprung Mass
Progressive Linear Firm Linear Soft Travel Force
7
Suspension and Damping Basics
Spring Characteristics
The characteristics of a spring are obtained by applying a force of increasing magnitude to a spring in a spring press and plotting the change in spring travel against the force applied.
The spring rate is calculated from the ratio of change in force and change in travel.
Spring rate = force : distance [pounds/inch (N/cm)]
A “firm” spring has a steeper spring rate characteristic than a “soft” spring.
If the spring rate is constant over the full distance traveled by the spring, then the spring has a linear characteristic.
If the spring rate increases over the distance traveled by the spring, then the spring has a “progressive” characteristic.
The characteristics of a coil spring can be influenced as follows:
• Spring diameter • Spring wire diameter
• Number of windings in the spring Features of springs with progressive characteristics include:
• Uneven winding pitch (1). • Conical winding shape (2). • Conical wire diameter (3).
• Combinations of these elements (4).
Examples of Spring Characteristics
SSP275/006
SSP0275/007
1 2
4 3
Suspension and Damping Basics
8
Spring Travel
The necessary total spring travel (stot) of a
vehicle without self-leveling suspension comprises the static bump (sstat) and the
dynamic spring travel (sdyn) resulting from
vehicle vibration when the vehicle is fully laden and when it is unladen.
stot = (sstat (fully laden) – sstat (unladen)) + sdyn
The static spring travel (sstat) is the distance
which the spring is compressed when stationary depending on payload. This is the difference between the static compression of the fully laden vehicle (sstat (fully laden)) and
the static compression of the unladen vehicle (sstat (unladen)).
sstat = sstat (fully laden) – sstat (unladen)
Where a spring characteristic performance curve is flat (soft spring), the difference, and so the static compression, between
the unladen and fully laden vehicle is large. Where the spring characteristic curve is steep (firm spring), the static compression is small.
Definitions
The unladen position is the compression of the spring when the serviceable vehicle (including a full tank of fuel, tool kit, and spare wheel, but without a driver or passengers) is standing on its wheels. The design position is the position which the serviceable vehicle adopts when loaded with three persons, each weighing 150 pounds (68 kg).
The controlled position is the position in which the vehicle is held by the
self-leveling elements of the air suspension system, regardless of payload.
SSP275/008
Payload
Spring Travel (s)
sstat Soft Spring
sstat Firm Spring Unladen
Position Fully Laden
Position Soft Spring
Performance Curve Firm Spring
Performance Curve
Air Suspension Basics
9
Air Suspension
Air suspension is a variable-height vehicle suspension system and can be combined with continuously controlled damping. The self-leveling suspension keeps the vehicle body at constant pre-determined ground clearance level. The controlled position is equal to a constant distance between the center of the wheel and the lower edge of the fender wheel opening. The vehicle level is controlled by adjusting the pressure acting on the air springs and the associated change of air volume in the air springs.
Static compression is always set to sstat = 0, regardless of payload.
The advantages of a self-leveling suspension are:
• The vehicle can be sprung comfortably. • The static level of the vehicle is constant,
regardless of payload. • Reduced tire wear.
• No payload-dependent change of drag coefficient (CD).
• Maximum rebound and bump travel are maintained in all load states.
• Full ground clearance is maintained, even at maximum payload.
• No changes of toe and camber due to changes in payload.
Controlled Position
Load-Bearing Force
10
Air Suspension Basics
In addition to the basic advantages of a self-leveling, full load-bearing air suspension system as described above, the system can also be used to set different vehicle heights. Three level settings are possible on
the Phaeton:
• The normal suspension level.
• A raised suspension level for poor road surfaces or rough terrain.
• A low suspension level that is set automatically while travelling at fast highway speeds.
Full load-bearing air suspension means that only air springs are used as load-bearing spring elements at all wheels.
Combined suspension systems are described as partially load-bearing because they consist of a combination of hydraulically or pneumatically controlled steel and gas struts.
1124 lbs (5000 N) 899 lbs (4000 N) 674 lbs (3000 N) 450 lbs (2000 N) 225 lbs (1000 N) SSP275/063b +3.15 in (+80 mm) Spring Travel Dynamic Bump Dynamic Rebound -3.15 in (-80 mm) -1.57 in (-40 mm) Constant +1.57 in (+40 mm) 0 sstat SSP275/063a Spring Characteristics Fully Laden Design Position Unladen
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Air Suspension Basics
Characteristics of Air Springs
Spring Force and Spring Rate
The spring force or load-bearing force of an air spring is defined by its geometric dimensions (the effective circular area) and the excess pressure acting on the air spring. The effective circular area is defined by the effective circle diameter.
With the configuration of a piston in a cylinder, the piston diameter corresponds to the effective circle diameter.
Load-Bearing Force
SSP275/011
Compressed Diameter Rebounded Diameter
SSP275/012
The effective diameter of the air spring is defined by the diameter at the lowest point of the air spring (rebounded and compressed). Minor changes in this effective diameter lead to relatively large changes in the area of the circle, and so the load-bearing force of the air spring.
Excess Pressure
Circle Diameter
12
The load-bearing force of the spring can be adapted to the load situation simply by changing the effective internal pressure in the air spring.
The different pressures – depending on payload – result in different spring characteristics or spring rates.
The spring rates do not change in direct proportion to total body weight.
The intrinsic body frequency, a key factor in handling performance, remains
almost constant.
Spring compression changes the effective air spring diameter because it rolls back on the roll piston.
The illustrations show the effect of roll piston contour on effective diameter.
Air Suspension Basics
Rebounded Compressed SSP275/014a Rebounded Diameter Air Spring Roll Piston Outer Guide Bump Stop SSP275/014 Compressed Diameter
13
Spring Characteristics
In principle, the spring characteristic of an air spring with a cylindrical piston is progressive.
The spring characteristic curve (steep or flat) is governed by the air spring volume. The existing air volume is compressed by dynamic compression. Assuming that bump travel is constant, the pressures in a low volume system rise more rapidly than in a system with a large air spring volume. • A flat spring characteristic curve
(soft spring) is produced by a large air spring volume.
• A steep curve (firm spring), on the other hand, is produced by a small air spring volume.
The characteristic curve can be influenced by the roll piston contour. Modifying the contour of the roll piston changes the effective diameter, and so the load-bearing force (spring force), of the air spring. An air spring can be tuned for the required application by adjusting the following parameters:
• Size of effective circular area.
• Amount of air spring volume (air volume). • Outer contour of the roll piston.
Air Suspension Basics
SSP275/015
Load-Bearing Force of Spring
Spring Travel s 87 psi (600 kPa) 102 psi (700 kPa) 116 psi (800 kPa) 131 psi (900 kPa) Flat Curve (Soft Spring) Large Air Spring Volume Steep Curve (Firm Spring) Small Air Spring Volume -s 0 +s
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Air Suspension Basics
Air Spring Design
There are two different types of air springs: • Partial load-bearing
• Full load-bearing
In the partial load-bearing type, a combination of steel and gas struts generates the load bearing force of the air spring.
In the full load-bearing type like those used on the Phaeton, the air springs function as the load-bearing spring elements.
The full load-bearing air spring assembly includes:
• Upper housing with outer guide • Air spring (flexible air bladder) • Roll piston (lower guide)
• Auxiliary accumulator (where required) • Integrated shock absorber
Flexible Air Bladder
The air spring is comprised of a special flexible air bladder made of a high-quality multi-layer elastomer material with reinforcing fabric inlays of nylon cord. The reinforcing fabric absorbs the forces arising in the air spring. The inner layer is specially designed to be airtight.
The combination of individual layers in the flexible air bladder provides for good rolling characteristics of the air spring and precise response to the movement of the suspension.
The materials are resistant to all external influences in a temperature range
from -31°F to +194°F (-35°C to +90°C). The outer guide is a metallic sleeve used to absorb some of the internal pressure in the air spring and control the shape of the flexible air bladder as it moves over the roll piston. This is an externally guided air spring.
Air springs configured without the outer guide are designated “unguided.”
Typical Externally Guided Full Load-Bearing Air Spring
Outer Guide
Roll Piston
Oil Reservoir Twin-Tube
Shock Absorber Air Spring (Flexible Air Bladder) Gas Cushion Upper Housing Air Volume SSP275/027
15
Damping System Basics
Shock Absorbers
The task of the shock absorbers is to reduce the vibration energy of structural and wheel vibrations as rapidly as possible by converting vibration energy to heat. Without shock absorbers, the vibrations introduced into the vehicle would build up to the extent that the wheels could lose contact with the road surface. As a result, the vehicle would no longer be steerable. There are several different types of shock absorbers.
Single-Tube Gas Pressure Shock Absorber
In a single-tube gas pressure shock absorber, the working and oil reservoirs are located in a single cylinder. Changes in oil volume, due to temperature effects and movement of the piston rod and piston when the springs are compressed, are equalized by the gas in the pressurized gas cushion – approximately 363 to 465 psi (2500 to 3000 kPa).
The damping valves for the compression and tension stages are integrated into the piston. SSP275/081 Separating Piston Pressure Valve Gas Cushion Tension Valve Piston with Valves Oil Reservoir Piston Rod Typical Single-Tube
16
Damping System Basics
Twin-Tube Gas Pressure
Shock Absorber
The twin-tube gas pressure shock absorber has established itself as the automotive industry standard for vibration damping. As the name indicates, this type of shock absorber consists of two tubes, one inside the other.
The inner tube acts as the working cylinder. It is completely filled with hydraulic fluid. The piston together with the piston valves and the piston rod move up and down inside the working cylinder.
The base of the working cylinder comprises the base plate and bottom valves.
The outer tube surrounds the oil reservoir. It is only partially filled with oil. There is a gas cushion above the oil.
The oil reservoir equalizes changes in the volume of oil in the working cylinder. Vibrations are damped by the two damping valve units located on the piston and at the base of the working cylinder. They comprise a system of spring washers, coil springs, and valve bodies with restrictor bores. During the compression stage, damping is defined by the bottom damper valve and partly by the flow resistance of the piston. During the rebound stage, vibrations are damped by the piston damper valve alone. This valve produces a defined resistance to the oil as it flows through it to the bottom of the working cylinder.
Piston Rod Working Cylinder Oil Reservoir Gas Cushion Compression Stage Bottom Non-Return Valve Bottom Damper Valve Piston Damper Valve Typical Twin-Tube
Gas Pressure Shock Absorber (Compression Stage) SSP275/082 Piston Non-Return Valve Piston Inner Tube Outer Tube
17
Damping System Basics
Shock Absorber Tuning
In vibration damping, there is a distinction to be made between the compression and rebound stages.
The damping force during the compression stage is less than the damping force during the rebound stage. As a result, jolts caused by rough road surfaces are transmitted to a lesser degree to the vehicle body.
Shock absorber tuning affects both driving comfort and safety.
Shock absorbers with adjustable damper tuning and continuous control are installed on luxury vehicles. The control module determines within a matter of milliseconds the degree of damping that is required for each wheel.
The degree of damping indicates how quickly the vibrations must be reduced. It is dependent on the damping force of the shock absorber and the amount of the sprung mass.
Given the same shock absorber tuning, increasing the sprung mass reduces the degree of damping and the vibrations are reduced more slowly. Conversely, a
reduction in the sprung mass increases the degree of damping and the vibrations are reduced more quickly.
SSP275/017
High Degree of Damping
Time
Spring T
ravel
SSP275/018
Low Degree of Damping
Time
Spring T
18
Damping System Basics
Damping Force
The damping force of a shock absorber can be determined using a testing device that can generate different compression and rebound rates while maintaining a constant stroke.
The values determined in this way can be represented in force-velocity diagrams. These diagrams clearly show the characteristics of the shock absorbers. A distinction is made between
progressive, digressive, and linear action shock absorbers.
Progressive Action Damping Characteristic Force-Velocity Diagrams Showing Shock Absorber Characteristic Curves -0.85 (-0.26) 1.17 (0.52) 0.85 (0.26) -1.71 (-0.52) 0 Velocity ft/s (m/s) Linear Action Damping Characteristic T ractive Force Compressive Force -0.85 (-0.26) 1.17 (0.52) 0.85 (0.26) -1.71 (-0.52) 0 Velocity ft/s (m/s) T ractive Force Compressive Force SSP275/019c SSP275/019a Digressive Action Damping Characteristic T ractive Force Compressive Force -0.85 (-0.26) 1.17 (0.52) 0.85 (0.26) -1.71 (-0.52) 0 Velocity ft/s (m/s) SSP275/019b
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System Description
Air Suspension with
Continuously Controlled Damping
The full load-bearing air suspension with continuously controlled damping used on the Phaeton keeps the vehicle at a constant level above the road, regardless of payload. A constant static ground clearance adapted to driver input or vehicle road speed is maintained between the road and the vehicle floor pan.
The system includes:
• Level Control System Control Module J197.
• Air spring and a vehicle level control system sensor at each corner.
• Adjustable shock absorber integrated into the air spring strut at each corner.
• Compressor with air drier and temperature sensor.
• Solenoid valve block with four strut valves, a drain valve, a pressure accumulator valve, and an integrated pressure sensor.
• Pressure accumulator.
• Air lines from the compressor to the individual air spring struts and to the pressure accumulator.
• Wheel acceleration sensor on each air spring strut (measurement range ± 13 g). • Three body acceleration sensors
(measurement range ± 1.3 g).
The Phaeton has three different suspension height settings, the first two of which are driver selectable:
• “Normal” suspension level.
• “High” suspension level, 1 inch (25 mm) above “Normal”; intended for driving on bad roads.
• “Low” suspension level, 0.6 inch (15 mm) below “Normal”; intended for driving at high speed on a highway. Low ride height is selected and deselected automatically depending on vehicle road speed.
The system also switches automatically to other levels depending on the
driving situation by means of a special control strategy.
Ride height adjustments are made in the background and are not normally noticed by the driver.
When driving at high speeds, the ground clearance is automatically lowered from the “High” suspension level to the dynamically more stable “Normal” suspension level. At even higher speeds, the ground clearance is automatically reduced to the “Low” suspension level which is not selectable by the driver.
The “Low” suspension level is automatically deselected when the vehicle speed drops below a pre-determined speed.
The suspension damper tuning “Comfort” setting is automatically adjusted to firmer settings at higher speeds in order to ensure safe handling and track stability.
- -4 3 2 1 5 6 7 100 140 180 220 70 50 30 40 20 10 60 12 10 16 1/2 1/43/4 60 30 90 80 50 20 110 80 120 160 200 240 14 260 120 MAP VEHICLE CLIMATE RESET FM AM CD SCAN BAL/FAD NAV NAV SET AUDIO TRIP DATA SETTINGS MANUAL ON/DARK HELP
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System Description
Component Locations
Control Module with Indicator Unit in Instrument Panel Insert J285
Air Spring Strut, Front Axle (2) Residual Pressure
Maintaining Valve Left Front Level Control System Sensor G78
Right Front Level Control Sensor G289 Left Front Wheel Acceleration Sensor G337 Right Front Wheel Acceleration Sensor G338
Left Front Body Acceleration Sensor G341 Right Front Body Acceleration Sensor G342
Front Information Display Control Head J523 with Menu Selection Rotary Knob E460, Dampening Adjustment Button E387, and Level Control Button E388
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System Description
Rear Body Acceleration Sensor G343 (Luggage Compartment)
SSP275/020
Left Rear Wheel Acceleration Sensor G339 Right Rear Wheel Acceleration Sensor G340
Left Rear Level Control System Sensor G76 Right Rear Level Control System Sensor G77
Air Spring Strut, Rear Axle (2)
Residual Pressure Maintaining Valve Pressure
Accumulator
Level Control System Control Module J197 Level Control Pump Temperature Sensor G290 Air Supply Module with Compressor, Air Drier, and Solenoid Valve Block with Integrated Level Control Pressure Sensor G291
System Description
22
Operation and Display
The system features a full load-bearing air spring with self-leveling suspension at each wheel on both the front and rear axles. Each air spring strut assembly combines an air spring integrated with a continuously adjustable damping shock absorber. The system is controlled by the Level Control System Control Module J197. The system is operated by Dampening Adjustment Button E387 and Level Control Button E388 as described in the operating manual.
These buttons are located on the center console behind the gear selector lever.
Display
Pressing the appropriate button opens a pop-up menu in the Infotainment display. Selections can then be made using Menu Selection Rotary Knob E460 and the display function keys.
The driver can then select between two of the three suspension levels:
• “Normal” suspension level (preset) • “High” suspension level
Four suspension damper tuning settings are also driver selectable:
• “Comfort” • “Basic” (preset) • “Sport 1” • “Sport 2” - -NAV SET NAV TRIP DATA AUDIO RESET FM AM CD SCAN CLIMATE VEHICLE MAP MANUAL BAL/FAD SETTINGS HELP ON/DARK SSP275/038
Gear Selector Lever
Level Control Button E388 Dampening Adjustment Button E387 Menu Selection Rotary Knob E460
23
System Description
Self-Leveling Suspension
The Level Control Button E388 must be pressed to set the level. By turning the Menu Selection Rotary Knob E460, the driver can select between “High”
suspension level or “Normal” suspension level. The screen corresponding to the level selected is displayed.
When the “High” suspension level is set, Level Control Button E388 is lit.
The driver can exit the menu by pressing the Menu Selection Rotary Knob E460.
“Normal” Suspension Level Display
AUTO AUTO
NAV AUDIO SETTINGS ON
MAP CLIMATE
AUTO AUTO
NAV AUDIO SETTINGS ON
MAP CLIMATE
SSP275/035 “High” Suspension Level Display
24
Damper Tuning
The driver can select damper tuning by pressing Dampening Adjustment Button E387.
By turning the Menu Selection Rotary Knob E460, the driver can select one of the following four damper tuning settings: • “Comfort”
• “Basic” (preset) • “Sport 1”
• “Sport 2”
The corresponding screen is displayed. The driver can exit the menu by pressing Menu Selection Rotary Knob E460. In the “Comfort,” “Sport 1,” and “Sport 2” settings, the Dampening Adjustment Button E387 is lit.
The damper tuning always resets from the “Sport 2” position to the “Basic” position when the ignition is turned off.
System Description
“Comfort” Damper Tuning Display
AUTO AUTO
NAV AUDIO SETTINGS ON
MAP CLIMATE
AUTO AUTO
NAV AUDIO SETTINGS ON
MAP CLIMATE
SSP275/037 SSP275/036
25
System Description
Control Strategy of the
Self-Leveling Suspension
The relative positions of the vehicle body and the wheels are measured by four level control system sensors located between the axle carriers and the lower wishbones: • Left Front Wheel Acceleration
Sensor G337
• Right Front Wheel Acceleration Sensor G338
• Left Rear Level Control System Sensor G76
• Right Rear Level Control System Sensor G77
The reading from each sensor is compared with a default that is stored in the Level Control System Control Module J197 for the corresponding wheel. The Level Control System Control Module J197 must learn these defaults.
The air required for self-leveling is normally provided by the compressor up to a
maximum pressure of 232 psi (1600 kPa). At road speeds above 22 mph (35 km/h), adjustments are controlled by the
compressor. The pressure accumulator is also replenished as required.
At road speeds below 22 mph (35 km/h), adjustments are controlled by the pressure accumulator, which has a capacity of 1.32 gallons (5 liters). A sufficient pressure difference of approximately 44 psi (300 kPa) between the pressure accumulator and the air spring is required for this purpose.
Effect of Changes in Vehicle Payload If the height of the vehicle in relation to the road surface changes due to changes in vehicle payload, the Level Control
System Control Module J197 readjusts the suspension to the preset default level. Level adjustments are made through several valves integrated into the solenoid valve block.
To increase the height, air is channeled to the four air springs through the four suspension strut solenoid valves:
• Left Front Suspension Strut Valve N148 • Right Front Suspension Strut Valve N149 • Left Rear Suspension Strut Valve N150 • Right Rear Suspension Strut Valve N151 To decrease the height, air is discharged to the atmosphere through the Solenoid for Level Control System N111, which is essentially a drain valve.
Activation of the compressor to replenish air pressure in the pressure accumulator is independent of the
26
Automatic Level Adjustments Changes in vehicle speed will trigger automatic adjustments in the self-leveling suspension system.
Automatic decrease in suspension level occurs from:
• “High” to “Normal” at speeds of 75 mph (120 km/h) or greater. • “Normal” to “Low”: — After 30 seconds at 87 mph (140 km/h) or greater. — Immediately at 112 mph (180 km/h) or greater.
System Description
SSP275/021Automatic increase in suspension level occurs from:
• “Low” to “Normal”:
— After 60 seconds at speeds of 62 mph (100 km/h) or less. — Immediately at 50 mph (80 km/h) or less. 0 50 62 75 87 99 112 124 mph (0) (80) (100) (120) (140) (160) (180) (200) (km/h) 60 seconds 30 seconds “High” Level Manual Adjustment Automatic Adjustment - 0.6 inch (-15 mm) 0 inch (0 mm) Level +1 inch (+25 mm) “Low” Level “Normal” Level
27
System Description
Manual Deactivation and Activation
In special situations, it may be necessary to deactivate the self-leveling suspension system. To change a wheel or to carry out work on the vehicle while it is raised on a hoist for example.
When the vehicle is raised on a hoist or other lifting equipment at the lift points, air is allowed to escape from all four air springs until the Level Control System Control Module J197 determines that the vehicle has been lifted.
The self-leveling suspension initially detects when the vehicle body is too high in relation to the wheels and adjusts the vehicle height accordingly by allowing air to escape from the springs.
As a result, the vehicle may have a very low ground clearance when the lifting equipment is lowered.
To avoid this discharge of air, the self-leveling suspension must be deactivated before lifting the vehicle.
The self-leveling suspension is automatically raised to the “Normal” level again after the engine, and therefore the compressor, has been running for a short period of time.
Deactivating the System
The self-leveling suspension can be deactivated by pressing both Dampening Adjustment Button E387 and Level Control Button E388 simultaneously for about five seconds.
A message indicating that the self-leveling suspension has been deactivated will appear on the instrument panel display. Activating the System
The suspension system can be reactivated manually by pressing both Dampening Adjustment Button E387 and Level Control Button E388 simultaneously for about five seconds.
The system will automatically reactivate when the Level Control System Control Module J197 determines that the vehicle is travelling at a speed of 6.2 mph (10 km/h) or greater.
28
Actions of Air Suspension
After “Ignition Off”
System Response to Immediate Level Changes
After “ignition off,” the Level Control System Control Module J197 remains active for approximately one minute. During this time it can execute suspension adjustments to compensate for changes in payload, provided that sufficient pressure is available in the pressure accumulator. The Level Control System Control Module J197 always remains active for one minute until no further door hood, or luggage compartment lid operations are detected.
System Description
System Response to Gradual Level Changes Gradual level changes can occur when the air in the air springs heats up during vehicle operation and cools down again at the end of the trip. To compensate for gradual level changes, three adjustments are made to achieve optimum ground clearance after “ignition off.”
These adjustments are made after approximately two, five, and ten hours, provided that sufficient pressure is available in the pressure accumulator.
29
System Description
Shock Absorber Control
The damping control system registers the condition of the road surface and the
movements of the vehicle using four wheel acceleration sensors and three body
acceleration sensors.
The characteristics of the individual shock absorbers are adjusted according to the calculated damping requirements. In this case, the shock absorbers function as active components during compression and rebound cycles.
Continuous damping control is based on shock absorbers whose characteristics are electrically adjustable. These shock absorbers are integrated in the
air spring struts.
Damping force can be set depending on the characteristic map via the proportional dampening adjustment valves built into the shock absorbers. As a result, they can adapt the damping force to the
driving situation and road conditions within milliseconds.
Shock absorber damping is adjusted
depending on the vertical acceleration rates of the wheels and the vehicle body.
The Level Control System Control Module J197 always attempts to set the damping force according to the “skyhook control strategy.” Ideally, this strategy would
control damping as if the vehicle body were suspended by a hook in the sky and
hovering above the road without
interference from the surface of the road. Maximum driving comfort is achieved by attempting to reach this ideal.
Firm damping is achieved by low control rates. Soft damping is achieved by high control rates.
78.74 59.06 39.37 19.69 0 19.69 39.37 59.06 78.74 in/s
(2000) (1500) (1000) (500) (0) (500) (1000) (1500) (2000) (mm/s)
SSP275/022 Characteristic Map of Damper Force
in Phaeton Front Axle
Damper Speed in Inches per Second (mm/s)
Damper Force in Pounds (dN)
Compression Stage Rebound Stage
lbs (dN) 13.49 (600) 11.24 (500) 8.99 (400) 6.74 (300) 4.50 (200) 2.25 (100) 0 (0) 50 mA 600 mA 1200 mA 1800 mA 50 mA 600 mA 1200 mA 1800 mA
30
Diagram of Air Suspension
System with Continuously
Controlled Damping
This system diagram highlights the relationships with other vehicle systems as well as display and operating elements.
System Description
Air Spring Strut with Electrically Adjustable Shock Absorber
Wheel Acceleration Sensors:
Left Front Wheel Acceleration Sensor G337 Right Front Wheel Acceleration Sensor G338 Left Rear Wheel Acceleration Sensor G339 Right Rear Wheel Acceleration Sensor G340 Body Acceleration Sensors:
Left Front Body Acceleration Sensor G341 Right Front Body Acceleration Sensor G342 Rear Body Acceleration Sensor G343
Connection via CAN Data Bus
Connection via On-Board Power Supply
SSP275/025
Front Information Display Control Head J523 with Menu Selection Rotary Knob E460
Control Module with Indicator Unit in Instrument Panel Insert J285
Level Control System Control Module J197 Inputs via CAN Data Bus: • ABS Control Module
with EDL/ASR/ESP J104 • Engine Control Module • Control Module with
Indicator Unit in Instrument Panel Insert J285
• Vehicle Electrical System Control Module J519 • Infotainment System Pressure Accumulator Compressor Dampening Adjustment Button E387 Level Control Button E388
31
System Description
Schematic Diagram
of the Overall System
SSP275/023
Door, Hood, and Luggage Compartment Lid Signals
Infotainment
Right Rear Dampening Adjustment Valve N339 Level Control System
Control Module J197 Drivetrain CAN Data Bus
Diagnosis via K Wire Diagnosis via CAN Data Bus
Convenience CAN Data Bus
Battery Management
Engine Control Module
Relay for Compressor Level Control System J403
Headlight Range Control
Left Front Dampening Adjustment Valve N336
Right Front Dampening Adjustment Valve N337 Left Rear Dampening Adjustment Valve N338 Level Control Pressure Sensor G291 Steering Angle Sensor G85 Electronic Stabilization Program Terminal 30 and Terminal 15 Status Signals Dampening Adjustment Button E387 and Level Control Button E388 Level Control Pump Temperature Sensor G290
Solenoid for Level Control System N111 (Drain Valve)
Level Control Accumulator Valve N311
Right Rear Suspension Strut Valve N151 Left Rear Suspension Strut Valve N150 Right Front Suspension Strut Valve N149 Left Front Suspension Strut Valve N148
Left Rear Level Control System Sensor G76 Right Rear Level Control System Sensor G77
Left Front Level Control System Sensor G78 Right Front Level Control Sensor G289
Left Front Wheel Acceleration Sensor G337
Right Front Wheel Acceleration Sensor G338
Left Rear Wheel Acceleration Sensor G339
Right Rear Wheel Acceleration Sensor G340
Left Front Body Acceleration Sensor G341
Right Front Body Acceleration Sensor G342
Rear Body Acceleration Sensor G343
Control Module with Indicator Unit in Instrument Panel Insert J285 with Data Bus On-Board Diagnostic Interface J533
-MAP FM AM CD SCAN BAL/FAD CLIMATE RESETVEHICLE NAV NAV SET AUDIOSETTINGS MANUAL ON/DARK HELP
32
System Overview
SensorsSystem Description
Additional Signals:Door, Hood, and Luggage Compartment Lid Contact Signals
Left Front Body Acceleration Sensor G341 Right Front Body Acceleration Sensor G342 Rear Body Acceleration Sensor G343 Left Front Wheel Acceleration Sensor G337 Right Front Wheel Acceleration Sensor G338 Left Rear Wheel Acceleration Sensor G339 Right Rear Wheel Acceleration Sensor G340 Level Control Pressure Sensor G291
(Integrated in Solenoid Valve Block)
Level Control Pump Temperature Sensor G290 Left Rear Level Control System Sensor G76 Right Rear Level Control System Sensor G77 Left Front Level Control System Sensor G78 Right Front Level Control Sensor G289 Dampening Adjustment Button E387 Level Control Button E388
4 3 2 1 5 6 7 100 140 180 220 70 50 30 40 20 10 60 12 10 16 1/2 1/43/4 60 3090 80 50 20110 80 120 160 200 240 14 260 120
33
System Description
Control Module with Indicator Unit in
Instrument Panel Insert J285 with Data Bus On-Board Diagnostic Interface J533
Actuators
SSP275/026
Left Headlight Range Control Module J567 Right Headlight Range Control Module J568 (Integrated in the Headlights)
Relay for Compressor Level Control System J403 Left Front Dampening Adjustment Valve N336 Right Front Dampening Adjustment Valve N337 Left Rear Dampening Adjustment Valve N338 Right Rear Dampening Adjustment Valve N339 (Integrated in Air Spring Struts)
Level Control Accumulator Valve N311 (Integrated in Solenoid Valve Block) Left Front Suspension Strut Valve N148 Right Front Suspension Strut Valve N149 Left Rear Suspension Strut Valve N150 Right Rear Suspension Strut Valve N151 (Integrated in Solenoid Valve Block) Solenoid for Level Control System N111 (Drain Valve Integrated in Solenoid Valve Block) Level Control System
Control Module J197
16-Pin Connector T16 (Diagnostic Connection)
Design and Function
34
Level Control System
Control Module J197
The Level Control System Control Module J197 is located in the luggage compartment on the left-hand side behind the side trim. It is bolted behind the backup fuse and electronics boxes.
The Level Control System Control Module J197 has the following tasks:
• Control air suspension and the shock absorbers.
• Monitor the overall system. • Diagnose the overall system. • Communicate via the drivetrain
CAN data bus.
SSP275/083
The Level Control System Control Module J197 has a redundant processor design (dual processors); the air spring algorithm runs primarily on the first processor and damping control runs primarily on the second processor.
Design and Function
35
Air Spring Struts
Air spring struts with externally guided, two-layer air springs are used on the front and rear axles of the Phaeton.
The air spring is a flexible air bladder arranged concentrically around the twin-tube gas-filled shock absorber. The thin wall of the air spring provides excellent suspension response. The desired spring rate is achieved by combining the effects of the roll piston contour, the outer guide, and an auxiliary accumulator directly attached to the strut.
Dampening Adjustment Valve Connecting Wires
SSP275/027a
Air Spring Cover
Air Spring Roll Piston Protective Bellows Auxiliary Accumulator Shock Absorber Outer Guide
Shock Absorber Piston Rod Auxiliary Spring (Bump Stop) Compression/Rebound Bearing Strut Support Bearing
Front Axle Air Spring Strut
Different auxiliary accumulators are used at the front and rear axles.
• The cylindrical accumulators on the front axle struts have a capacity of 24.4 cubic inches (0.4 liter).
• The ball accumulators on the rear axle struts have a capacity of 73.2 cubic inches (1.2 liters).
36
The air spring struts are designed to
minimize the effect of transverse forces on the shock absorbers. The special design of the strut support bearing on the front axle air spring struts and the controlled flexing at the fluid-cushioned top ends of the rear struts help reduce the effects of transverse forces on the shock absorbers.
Design and Function
A residual pressure maintaining valve is mounted directly on the air connection of each air spring strut. It maintains a residual pressure of about 51 psi (350 kPa) in the air spring strut. This permits easy assembly and mounting of the components.
The outer guide protects the air spring against dirt and damage besides its function of guiding the flexible air bladder and the protective bellows.
SSP275/028 Rear Axle Air Spring Strut
Dampening Adjustment Valve Connecting Wires
Shock Absorber Air Spring Cover
Auxiliary Accumulator
Air Spring
Roll Piston
Protective Bellows
Outer Guide
Shock Absorber Piston Rod Auxiliary Spring (Bump Stop) Compression/Rebound Bearing
37
Design and Function
Dampening Adjustment Valves
The continuous damping control twin-tube gas-filled shock absorbers are adjustable over a wide range of damping forces via electrically controlled valves integrated in their pistons:
• Left Front Dampening Adjustment Valve N336
• Right Front Dampening Adjustment Valve N337
• Left Rear Dampening Adjustment Valve N338
• Right Rear Dampening Adjustment Valve N339
The oil flow through these valves, and hence the damping force, can be adapted to momentary demands within a few milliseconds by varying the electric current flowing through the adjustment valve solenoids.
The wheel acceleration sensors mounted on each shock absorber generate signals which, together with the signals supplied by the three body acceleration sensors, are used to calculate the required damper setting.
Since the system can rapidly detect and control rebound and compression stages, it permits adjustment of the damping force required for the momentary driving situation.
The driving situation dependent control maps for system reaction to various driving situations are stored in the Level Control System Control Module J197.
In certain driving dynamic states – e.g. longitudinal or transverse dynamics – the “skyhook control” is deactivated and the shock absorbers are controlled by other dynamic models.
Example of a Dampening Adjustment Valve SSP275/093 Hollow Piston Rod Dampening Adjustment Valve Connecting Wires Housing Container Tube Cylinder Tube Auxiliary Valve Oil Flow Main Damping Valve Valve Spring Armature Solenoid
38
Air Spring Strut, Front Axle
Air Spring Components (Blue Highlighted Area)
Design and Function
Shock Absorber Components (Green Highlighted Area)
SSP275/084 Air Spring SSP275/086 Auxiliary Accumulator Flexible Air Bladder
Connection for Dampening Adjustment Valve
Left Front Dampening Adjustment Valve N336 or Right Front Dampening Adjustment Valve N337 Shock Absorber Piston Rod
39
Design and Function
Air Spring Strut, Rear Axle
Air Spring Components (Blue Highlighted Area)
Shock Absorber Components (Green Highlighted Area)
Left Rear Dampening Adjustment Valve N338 or Right Rear Dampening Adjustment Valve N339 Connection for Dampening Adjustment Valve SSP275/085 Shock Absorber Piston Rod SSP275/087 Air Spring Flexible Air Bladder Auxiliary Accumulator
40
Air Supply Module
The air supply module is a compact unit. It is mounted to the underbody on an anti-vibration mount in the spare-wheel well adjacent to the activated charcoal filter (EVAP canister).
A plastic cover with vents provides protection from dirt.
The compressor is supplied with air from the luggage compartment. Air is drawn into the compressor through the silencer with air filter, cleaned, and pumped into the system.
The Level Control Pump Temperature Sensor G290 protects the compressor against overheating and ensures availability of the air supply for the air suspension in all climatic and driving conditions.
The air supply module includes a compressor unit with:
• Electric motor
• Dry-running compressor • Air drier
• Residual pressure maintaining unit • Maximum pressure limiter
• Solenoid for Level Control System N111 (drain valve)
• Silencer with air filter
• Level Control Pump Temperature Sensor G290 (temperature sensor for
overheating protection)
• Pneumatic drain valve with pressure relief valve
The air supply module also includes a solenoid valve block with:
• Control valves for each air spring strut: — Left Front Suspension Strut
Valve N148
— Right Front Suspension Strut Valve N149
— Left Rear Suspension Strut Valve N150
— Right Rear Suspension Strut Valve N151
• Control valve for the for the pressure accumulator:
— Level Control Accumulator Valve N311
• Monitor for the pressure accumulator: — Integrated Level Control Pressure
Sensor G291
41
Design and Function
SSP275/031
Solenoid Valve Block
Drain Line Pneumatic Drain Valve Air Drier Level Control Pump Temperature Sensor G290 Compressor T-Connection for Intake Circuit and Drain Circuit Intake/Drain Line Silencer/Filter
Electric Motor Vibration Isolator
42
Compressor Unit
Compressed air is produced by a single-stage piston compressor with integrated air drier.
To prevent internal contamination of the air springs and the air drier cartridge, compressor design is of the dry-running type.
Lifetime-lubricated bearings and a piston ring made of polytetrafluorethylene ensure a long service life.
Design and Function
The Solenoid for Level Control System N111 (drain valve), a pneumatic drain valve with pressure limiting valve, and three non-return valves are integrated in the air drier housing.
To protect the compressor against overheating, it is turned off if excess temperature occurs.
SSP275/032
Pneumatic Drain Valve with Pressure Limiting Valve Solenoid for
Level Control System N111 (Drain Valve) Pressure Connection Drain Connection Non-Return Valve 2 Electric Motor Intake Fitting Lifting Piston Piston Ring Diaphragm Valve “Closed” Cylinder Non-Return Valve 1 Air Drier Non-Return Valve 3
43
Intake/Compression Cycles
During the upward movement of the piston, air is drawn into the compressor crankcase through the intake fitting via the silencer/filter. Air in the cylinder is
compressed above the piston and flows into the air drier via non-return valve 1.
Design and Function
The compressed and dried air flows via non-return valve 2 and the pressure connection to the valves and the
pressure accumulator. SSP275/039 Air Drier Pressure Connection Non-Return Valve 2 Compressor Crankcase Intake Fitting Piston Moves Upward Non-Return Valve 1
44
Bypass Air Flow
During the downward movement of the piston, air drawn into the crankcase bypasses the diaphragm valve and flows into the cylinder.
Design and Function
SSP275/040
Fill/Lift Cycles
To fill the air springs and thus raise the vehicle, the Level Control System Control Module J197 activates the compressor relay (Relay for Compressor Level Control System J403) and the four air spring suspension strut valves at the same time.
Piston Moves Downward Bypass Air Flow Diaphragm Valve “Open”
45
Drain/Lowering Cycles
(Front Axle Components Shown) Left Front Suspension Strut Valve N148, Right Front Suspension Strut Valve N149, and Solenoid for Level Control System N111 (drain valve) are activated (open)
Design and Function
during the drain cycle. The air spring pressure flows toward the pneumatic drain valve and from there via the air drier, the pressure limiting valve, and the
silencer/filter into the spare-wheel well in the luggage compartment.
Drain Cycle Pneumatic Diagram (Front Axle Components Shown)
SSP275/042 SSP275/041
Pressure Limiting Valve and Solenoid for Level Control System N111 (Drain Valve) “Open”
To Silencer/Filter
Air Drier Pneumatic Drain Valve “Open”
M
Signals from Level Control System Control Module J197
Signal from Level Control System Control Module J197 Signal from Relay for Compressor
Level Control System J403
Right Front Suspension Strut Valve N149
Left Front Suspension Strut Valve N148 Non-Return Valve 2 Non-Return Valve 3 Drain Restrictor Air Drier Non-Return Valve 1 Silencer/Filter Solenoid for Level Control System N111 (Drain Valve) Pneumatic Drain Valve
Design and Function
Pneumatic Drain Valve
The pneumatic drain valve performs two functions:
• Residual pressure maintenance • Pressure limitation
To prevent damage to the air springs (flexible air bladders), a specific minimum pressure of at least 51 psi (350 kPa) must be maintained. This is referred to as the air spring residual pressure.
The maintenance residual pressure ensures that pressure in the air spring system does not drop below 51 psi (350 kPa) during pressure relief during normal operation.
Residual pressure cannot be maintained if there is a leak in the system upstream of the pneumatic drain valve.
46
When an air spring pressure of more than 51 psi (350 kPa) is applied, the valve body lifts against the spring force of the two valve springs and opens valve seats 1 and 2.
The air spring pressure is then admitted into the air drier via the flow restrictor and non-return valve 3.
After passing through the air drier, the air passes the open valve seat of the pressure limiting valve and through the silencer/filter in the spare-wheel well in the luggage compartment.
A sharp decrease in pressure downstream of the flow restrictor leads to a reduction in relative atmospheric humidity, thereby increasing the amount of moisture that will be absorbed by the air that is discharged into the luggage compartment.
SSP275/043 To Silencer/Filter Valve Seat 2 Valve Seat 1 Air Drier Non-Return Valve 3 Flow Restrictor Valve Body Pressure Limiting Valve “Open” Pneumatic Drain Valve
Solenoid for Level Control System N111 (Drain Valve) “Open”
M
Design and Function
47
Pressure Limiting Valve
The pressure limiting valve protects the system against excessively high pressures, as for example if the compressor fails to cut out due to a defective contact in the Relay for Compressor Level Control System J403 or a defective Level Control System Control Module J197.
In such a case, the pressure limiting valve will open against the spring force when the pressure exceeds approximately 290 psi (2000 kPa), and air conveyed by the compressor will escape through the silencer/filter.
SSP275/044
Pressure Limiting Valve Actuation Pneumatic Diagram
Intake Fitting Lifting Piston
Pressure Limiting Valve “Open”
To Silencer/Filter
No “Drain Valve Open” Signal from
Level Control System Control Module J197 No “Cut Out” Signal from Relay for
Compressor Level Control System J403
Silencer/Filter
Pneumatic Drain Valve with Pressure Limiting Valve
Compressor
48
Air Drier
The air in the self-leveling air suspension system must be dehumidified to avoid problems caused by the condensation of water in the system:
• Corrosion • Freezing
An air drier is used to dehumidify the air. The air drier uses a regenerative process to remove moisture from the air.
The air compressed in the self-leveling air suspension system is routed through a silicate granulate and dried in the process. This granulate is able to absorb
atmospheric humidity amounting to over 20% of its natural weight, depending on temperature.
When the air in the system is released to lower the air springs, it flows back through the silicate granulate and is discharged to the atmosphere.
During this phase, the dry air extracts the moisture from the granulate that was absorbed during the intake cycle.
Design and Function
As a result of this regenerative process, the air drier requires no maintenance. It is not subject to a replacement interval.
Since the air drier is only regenerated by discharged air, the compressor must not be used to fill other vessels with compressed air.
Moisture in the system signifies that the air drier has failed or the system is
otherwise malfunctioning.
SSP275/045
Air Drier with Silicate Granulate Filling
49
Design and Function
Pressure Accumulator
Extraction of compressed air from the pressure accumulator allows the vehicle level to be raised quickly with a minimum of noise. The pressure accumulator is only filled while the vehicle is moving. As a result, compressor operation is barely audible.
Provided that sufficient pressure is available in the pressure accumulator, the vehicle level can be raised even if the compressor is not running. Pressure is sufficient when the pressure difference between the pressure accumulator and the air springs is at least 44 psi (300 kPa) before increasing the level.
The pressure accumulator is made of
aluminum and has a capacity of 1.32 gallons (5 liters). The maximum operating pressure is about 232 psi (1600 kPa).
Air Supply Strategy
At road speeds below 22 mph (35 km/h), air is primarily supplied by the pressure accumulator, provided that sufficient pressure is available.
The pressure accumulator is only filled when the vehicle is travelling at speeds above 22 mph (35 km/h).
At road speeds above 22 mph (35 km/h), air is primarily supplied by the compressor. This supply strategy ensures that the system operates silently and conserves vehicle battery capacity.
The compressor starts
running when compressed air is extracted from the pressure accumulator even if the
driver has not adjusted the vehicle’s level.
Pressure Accumulator
p
M
50
Self-Leveling Suspension
Pneumatic Diagram
Design and Function
Signal from Level Control System Control Module J197 Signal from Relay for
Compressor Level Control System J403 Right Rear Suspension Strut Valve N151 Left Rear Suspension Strut Valve N150 Level Control Accumulator Valve N311 Level Control Pressure Sensor G291 Non-Return Valve 2 Non-Return Valve 3 Drain Restrictor Air Drier Non-Return Valve 1 Motor for Compressor – Level Control System V66 Silencer/Filter
Solenoid for Level Control System N111 (Drain Valve) Pneumatic
Drain Valve
SSP275/065
Right Front Air Spring Strut Left Front
Air Spring Strut Right Rear
Air Spring Strut Left Rear
Air Spring Strut Pressure Accumulator Right Front Suspension Strut Valve N149 Left Front Suspension Strut Valve N148
51
Design and Function
Solenoid Valves
The air spring self-leveling suspension system has six solenoid valves.
The Solenoid for Level Control System N111 (drain valve) together with the pneumatic drain valve form a functional unit that is integrated in the air drier housing. The Solenoid for Level Control System N111 is a 3/2-way drain valve and is normally closed when de-energized. The pneumatic drain valve has two tasks: to limit total system pressure and to maintain residual pressure.
The following valves are combined in the solenoid valve block:
• Level Control Accumulator Valve N311 • The four air spring suspension
strut valves:
— Left Front Suspension Strut Valve N148
— Right Front Suspension Strut Valve N149
— Left Rear Suspension Strut Valve N150
— Right Rear Suspension Strut Valve N151
The valves in the solenoid valve block are designed as 2/2-way valves and are normally closed when de-energized. The system pressure on the air spring and accumulator side acts in the closing direction.
To avoid confusion when connecting them, the pressure lines and their matching
connections on the solenoid valve block are color coded.
SSP275/066
Green – Right Front Suspension Strut Valve N149
Black – Left Rear Suspension Strut Valve N150
Blue – Right Rear Suspension Strut Valve N151 Solenoid Valve Block Electrical Connection Lilac – Level Control
Accumulator Valve N311 Red – Left Front Suspension Strut Valve N148
Compressor Connection
52
Level Control Pump
Temperature Sensor G290
To protect the compressor from overheating and thus ensure system availability, Level Control Pump Temperature Sensor G290 is attached to the compressor cylinder head.
Design and Function
The Level Control System Control Module J197 shuts the compressor down and inhibits starting when a maximum permissible compressor temperature is exceeded.
Level Control Pump Temperature Sensor G290
p
53
Design and Function
Level Control Pressure
Sensor G291
The Level Control Pressure Sensor G291 is integrated in the solenoid valve block and monitors the pressure in the pressure accumulator and the air springs. Level Control Pressure Sensor G291 generates a voltage signal proportional to the pressure.
Information on accumulator pressure is required to make plausibility checks on the up-control functions and perform self-diagnosis.
The individual pressures of the air springs and the pressure accumulator can be determined by activating the solenoid valves accordingly.
The individual pressures are measured while the air springs or the pressure accumulator are evacuating or filling. The pressures determined in this way are stored and updated by the Level Control System Control Module J197.
During vehicle operation the accumulator pressure is determined and updated by the Level Control System Control Module J197 every six minutes.
SSP275/068 Level Control Pressure Sensor G291 Pressure Accumulator Solenoid Valve Block Left Rear Suspension Strut Valve N150
Level Control Accumulator Valve N311
54
Vehicle Level Control
System Sensors
The four vehicle level control system sensors are wheel angle sensors: • Left Rear Level Control System
Sensor G76
• Right Rear Level Control System Sensor G77
• Left Front Level Control System Sensor G78
• Right Front Level Control Sensor G289 Changes in the level of the vehicle body are registered and converted to angular changes using coupling rod kinematics. The wheel angle sensors used operate according to the induction principle. The signal output provides an
angle-proportional pulse-width-modulated signal for the self-leveling suspension.
Design and Function
SSP275/076
Coupling Rods
SSP275/075 Front Axle Vehicle Level
Control System Sensor
Rear Axle Vehicle Level Control System Sensor
The four sensors are identical; only the mounts and the coupling rod kinematics are specific for each side and axle.
Deflection of the sender crank, and hence the output signal, is opposed on the left and right. As a result, during suspension compression for example, the output signal rises on one side and drops on the other side.
