292 292 Service adaptive air suspension in the Audi A8 Home study program 292 All rights reserved, including the right to make technical changes Copyright* 2002 AUDI AG, Ingolstadt Department I/VK-35 D-85045 Ingolstadt Fax +49 (0)841/89-36367 000.2811.12.
The development of the running gear is subject to conflicting objectives. For now, besides "classic" aims such as function, driving safety, strength and durability, requirements such as weight reduction, driving comfort and acoustics are increasingly gaining in importance. At first glance, many requirements appear to be in mutual opposition. A car designed to be very comfortable will lose out in terms of driving safety when driven at the limit.
Contents Page Introduction Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 New technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Operation and display Vehicle levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Operation and display system . . . . . . . . . . . . . . . . . . . . . . . .
Introduction Basics The basics for understanding air suspension systems are contained in home study programs 242 and 243 and are of course also valid for the system to be introduced in the A8 from model year 2003. New technology The new A8 heralds a new system in terms of technical content and range of functions. It differs from the known system of the allroad quattro in the following features: 292_001 CDC instead of PDC damping control: The control takes account of the current driving status.
Encased pneumatic springs: The air bellows are encased in an aluminium cylinder. The result is a considerable improvement in the response characteristic. (See description "Pneumatic springs" in the "System components" section.) 292_003 Operation: Integration in the MMI means that operation is user-friendly, logical and easy to learn. (See description in the "Operation and display" section.
Operation and display Vehicle levels The A8 comes either with a standard running gear (adaptive air suspension) or a sporty running gear (adaptive air suspension-sport). Standard running gear: The following programs can be selected either manually or automatically: "automatic" mode: Basic vehicle level, comfort-oriented suspension with appropriately adapted damper map. The vehicle is lowered by 25 mm after 30 seconds at speeds of 75 mph (120 km/h) or more ("motorway lowering").
"dynamic" mode: Vehicle level is 20 mm lower than in "automatic" mode. The damper map is automatically set to sporty. After 30 seconds at speeds of 75 mph (120 km/h) or more, the car is lowered by another 5 mm ("motorway lowering"). - 20 mm "dynamic" mode: - 20 mm 292_004 "lift" mode: Vehicle level is 25 mm higher than in "automatic" mode, comfort-oriented suspension like "automatic" mode.
Operation and display Sporty running gear: "automatic" mode: Basic vehicle level corresponds to "dynamic" mode in the standard running gear, sporty suspension with appropriately adapted damper map (more comfortable than "dynamic" mode). The vehicle is lowered by another 5 mm after 30 seconds at speeds of 75 mph (120 km/h) or more ("motorway lowering"). "dynamic" mode: Level as for "automatic" mode of sporty running gear, hard, sporty suspension with appropriately adapted damper map.
Operation and display system The process of switching from one mode to another and the display/monitoring of the system status all form part of the MMI operating system. The adaptive air suspension menu is opened directly in the MMI display in the centre console when the "CAR" button is pressed. This ensures that adaptive air suspension has first priority. This means that any other functions already in the display are blanked out in favour of the adaptive air suspension operating/status display.
System components Vehicle overview Dash panel insert Front operator/display unit (MMI) Adaptive air suspension control unit Body acceleration sender Pneumatic struts, FA Vehicle level sender, FA Solenoid valve block with pressure sender Air supply unit 10
Body acceleration sender Pneumatic struts, RA Accumulator Vehicle level sender, RA 292_012 11
System components Control unit J197 The control unit is the central element of the system. It is installed in the vehicle in front of the glove box. It processes the relevant messages from the other bus users, and the discreet input signals (see function diagram and CAN information exchange). The principal result of this processing work are the signals to actuate the compressor, the solenoid valves and the shock absorbers.
Suspension/shock absorber strut All four suspension/shock absorber struts are constructed in the same way. Pneumatic spring Construction: The pneumatic spring is encased in an aluminium cylinder. In order to prevent dirt from getting between the cylinder and the air bellows, the area between the piston and the cylinder is sealed by a sleeve. The sleeve can be replaced during servicing, but the air bellows cannot be replaced separately.
System components Shock absorber Construction: A twin-tube gas-filled shock absorber with continuous electrical control is used (ccontinuous damping control =CDC shock absorber). The main damping valve 3 in the piston 1 is mechanically pre-tensioned by a spring 4. A solenoid 5 is situated above the valve, and the connecting cable is routed to the outside through the hollow piston rod. Function: For general information on the function of a twin-tube gas-filled shock absorber, see SSP 242.
Air supply unit The air supply unit is installed at the front left of the engine compartment. This prevents any impairment of the acoustics in the passenger compartment. Furthermore, more effective cooling can be achieved. This increases the amount of time the compressor can be switched on, and thus the quality of control. Design: The air supply unit is the same as the one used in the allroad quattro (see SSP 243). 10 Function: It functions in an identical manner to the unit used in the allroad quattro.
System components Solenoid valve block Solenoid valves The solenoid valve block contains the pressure sender and the valves for actuating the pneumatic springs and the accumulator. It is installed in the wheel housing between the wheel housing liner and the left-hand A-pillar. Construction/function: The construction and function of the solenoid valves are largely the same as in the allroad quattro (see SSP 243).
Notes 17
System components Pneumatic diagram Air supply unit Solenoid valve block 292_020 1 Compressor V66 2 Air drier 3a, 3b Non-return valves 4 Exhaust throttle 5 Electrical exhaust solenoid valve N111 6 Pneumatic exhaust solenoid valve 7 Additional silencer 8 Air filter 9a Valve for strut FL N148 9b Valve for strut FR N149 18 9c 9d 10 11 12 13a 13b 13c 13d Valve for strut RL N150 Valve for strut RR N151 Valve for accumulator N311 Pressure sender G291 Accumulator Pneumatic spring FL Pneumatic spring FR Pneum
Pressure build-up The valves 9a, 9b and 9c, 9d are electrically actuated in pairs (front axle and rear axle). The compressor takes in the air through the air filter 8 and the additional silencer 7. The compressed air flows via the air drier 2, the non-return valve 3a and the valves 9 to the pneumatic springs. When the pneumatic springs are filled by the accumulator, the valve 10 and the valves 9 for the appropriate axle open.
System components Senders (sensors) Compressor temperature sender G290 Construction: An NTC resistor is housed in a small glass case. Function: The sender records the temperature of the cylinder head compressor. Its resistance decreases sharply as the temperature rises (NTC: negative temperature coefficient). This change in resistance is analysed by the control unit. The current temperature calculated in each case determines the maximum compressor running time.
Acceleration sender In order to achieve optimum damping for every driving condition, knowledge of the body movement (sprung mass) and axle components (unsprung mass) characteristic over time is required. The acceleration of the body is measured by three senders. Two of these are situated on the front axle MacPherson strut towers, the third in the rear right wheel housing. The acceleration of the axle components (unsprung masses) is determined by evaluating the signals from the vehicle level senders.
System components Rest condition: The seismic mass is situated exactly in the middle between the counter-electrodes. The two capacitors C1 and C2 have the same capacitance. * * Metal-plated electrodes Seismic mass Silicon spring Capacitor C1 Capacitor C2 292_026 Accelerated condition: Mass inertia causes the seismic mass to be deflected from its central position. The distance between the electrodes changes. As the distance is reduced, the capacitance increases.
Vehicle level senders G76, G77, G78, G289 Construction: The sender construction and the PIN designation are the same as those of the allroad quattro (description in SSP no. 243). The four senders are interchangeable, but the brackets and coupling rods must be fitted to the correct side and axle. 292_028 Function: The senders record the distance between the links and the body, and thus the level of the vehicle. The sensing function is now effected with 800 Hz (200 Hz in the allroad).
System functions General control concept Changes in level are effected by axle, with correction of differences in level between the left and right-hand sides (e.g. caused by one-sided loading of the vehicle) At vehicle speeds of 22 mph (35 km/h) or less, the accumulator is the preferred energy source. A prerequisite for this is a pressure difference of at least 3 bar between the accumulator and the pneumatic springs.
Notes 25
System functions Control concept for standard running gear "automatic" mode (basic level) The suspension is oriented towards a more comfortable ride. Automatic motorway lowering of 25 mm takes place after 30 seconds at speeds of 75 mph (120 km/h) or more. The vehicle is automatically lifted back up to basic level once the speed drops below 44 mph (70 km/h) for 120 seconds or if the speed drops below 22 mph (35 km/h).
"lift" "automatic" "comfort" "dynamic" 292_053 Acceptance level for selecting "lift" mode 50 mph (80 km/h) System automatically leaves "lift" mode at v > 63 mph (100 km/h), vehicle is not lifted back up automatically Automatic lift to revert to sporty/basic level (dependent on speed/time) Automatic motorway lowering after > 30 s at v > 75 mph (120 km/h) 27
System functions Control concept for sporty running gear Differences from the standard running gear: – Different, sporty suspension and damping – Same levels but different damping maps for "dynamic", "automatic" and "comfort" modes at speeds of less than 75 mph (120 km/h) – Basic vehicle level 20 mm lower than with standard running gear 292_052 "lift" "automatic" "comfort" "dynamic" Basic level of standard running gear Acceptance level for selecting "lift" mode 50 mph (80 km/h) System automatically
Control concept for special operating conditions Cornering Suspension adaptation is interrupted during cornering manoeuvres and continued afterwards. The system recognises that cornering is taking place by means of the signals from the steering angle and lateral acceleration senders. The damping forces are adapted to suit the current driving situation. This effectively eliminates undesirable body movements (e.g. rolling) caused by the driving dynamics.
System functions Braking manoeuvres Damping control is employed, particularly during ABS/ESP braking manoeuvres. Damping is regulated as a function of the current braking pressure. This keeps nose diving, squatting and rolling of the vehicle body to a minimum. Longitudinal axis Transverse axis Squatting/ nose diving Rolling 292_033 Starting manoeuvres During starting manoeuvres, the mass inertia of the body leads above all to squatting.
Pre-travel and after-run mode Any difference from the specified height before driving commences or before ignition on is corrected. When the door, boot lid or terminal 15 is actuated, the system is woken from sleep mode and goes into pre-travel mode (see section on Interfaces). Any difference in height caused, for example, by climbing out of or unloading the vehicle after switching off the ignition, is corrected in after-run mode.
System functions Using a jack (service mode) There is no automatic recognition. Adaptive air suspension must be deactivated if a jack is to be used. This is done by actuating the MMI control knob in the CAR -> SETUP menu. This mode is deactivated either by resetting in MMI or by driving at a speed of >9 mph (15 km/h).
Extreme low level Extreme low level (65 mm or more below normal level) is shown by the low level indicator lamp and the warning lamp flashing. Extreme low level can occur after the vehicle has been at a standstill for a very long period. 292_045 Extreme high level Extreme high level (50 mm or more above normal level) is shown by the warning lamp flashing. Extreme high level may occur briefly when heavy objects are unloaded.
Interfaces System overview of components with bus link (CAN, MOST) J197 Adaptive air suspension control unit Diagnosis J533 Gateway J285 Control unit with display unit in dash panel insert J345 Trailer recognition control unit J220 Motronic control unit J518 Access and start authorisation control unit CAN drive CAN multi-function MOST bus J104 ESP control unit CAN comfort Diagnostic CAN J523 Control unit for front operating and display unit J431 Control unit for headlight range control J527 Control
System overview of components without bus link N111 Exhaust solenoid valve for adaptive air suspension (integrated in air supply unit) CAR button SETUP button G76, G77, G78, G289 FA and RA vehicle level senders G290 Compressor temperature sender G291 Adaptive air suspension pressure sender (integrated in solenoid valve block) N148, N149, N150, N151 Strut valves (integrated in solenoid valve block) N311 Accumulator valve (integrated in solenoid valve block) N336, N337, N338, N339 Shock absorber adjus
Interfaces CAN information exchange J197 Adaptive air suspension control unit – – – – – System status (all) Actuate warning lamp (5) Actuate low level indicator (5) Advance warning of level adjustment (4) Advance warning of compressor startup (4) – Lifting/lowering (4) – Height of FR, FL, RR, RL (4) J533 Gateway – Reversing light On/Off – Current status of mileage, time, trailer recognition signals J285 Control unit with display unit in dash panel insert (5) – Mileage – Date and time J345 Trailer recogn
J220 Motronic control unit (1) – – – – Engine speed Driver’s desired torque Current position of brake light switch Current position of brake test switch CAN drive CAN multi-function CAN comfort MOST bus J104 ESP control unit (2) – – – – – Information which is transmitted by the adaptive air suspension control unit ABS braking activated ESP override activated Current vehicle speed Lateral acceleration Braking pressure Information which is received and evaluated by the adaptive air suspension control un
Interfaces Function diagram G76 G77 G78 G289 G290 G291 J393 G341 G342 G343 Vehicle level sender, rear left Vehicle level sender, rear right Vehicle level sender, front left Vehicle level sender, front right Temperature sender for adaptive air suspension compressor Adaptive air suspension pressure sender Central control unit for comfort system (for door signal) Body acceleration sender, front left Body acceleration sender, front right Body acceleration sender, rear J197 Adaptive air suspension control unit
8 292_051 Ancillary signals 1 CAN High 2 CAN Low 39
Interfaces Other interfaces The wake-up signal to wake the adaptive air suspension control unit from sleep mode is transmitted by the comfort system central control unit J393. It uses a pulse-width-modulated signal. The duration of the signal pulse varies depending on whether the doors and/or boot lid are opened or closed. J393 transmits a signal even if terminal 15 is detected but the doors and/or boot lid are not opened or closed.
The signal for headlight range control The adaptive air suspension control unit sends the headlight range control unit a CAN message about the momentary body level on all four wheels. The headlight range control unit uses this information to calculate the required headlight setting in each case. The terminal X signal Loads that consume a lot of power are briefly switched off during the starting process. This applies to the compressor of the adaptive air suspension system.
Service Control unit code The code for both the standard and the sporty running gear is 15500. System initialisation The system initialisation process includes calibration of the vehicle level senders. This is required whenever a sender or the control unit is replaced. System initialisation is performed using diagnostic tester VAS 5051(address word: 34 – adaptive air suspension). The height of each wheel is measured from the centre of the wheel to the lower edge of the wheel housing.
Final control diagnosis Final control diagnosis checks the function of the compressor, the solenoid valves and the struts/shock absorbers. Diagnosis is executed automatically in three steps. Final control diagnosis is implemented when the vehicle is stationary and the ignition is on. The engine may be running. During final control diagnosis, the yellow warning lamp in the dash panel insert flashes. 1. Test of each individual strut by lowering it 20 mm below its current level for a period of 30 seconds 2.
Notes 44
292 292 Service adaptive air suspension in the Audi A8 Home study program 292 All rights reserved, including the right to make technical changes Copyright* 2002 AUDI AG, Ingolstadt Department I/VK-35 D-85045 Ingolstadt Fax +49 (0)841/89-36367 000.2811.12.
