1999 AUDI A6 ALLROAD ESP

33
The PDC damper
In order to maintain the degree of damping
and thereby the handling characteristics at a
constant level between partially and fully
laden, the Audi A6 self-levelling air suspen-
sion and the Audi allroad quattro 4-level air
suspension both have a continuously variable
load recognition system Þtted to the rear axle.
Along with the constant natural frequency of
the bodywork, the vehicle bodywork
maintains virtually constant vibration
characteristics irrespective of the load thanks
to the air springs.
When the vehicle is partially-laden, good
driving comfort is achieved and body
movements are damped sufÞciently Þrmly at
full load.
The PDC damper (Pneumatic Damping
Control) is responsible for this. The damping
force can be varied according to the air spring
pressure.
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PDC valve
Hoses
Air springs
1 1,2 1,4 1,6 1,82,0Body weight ratio
Degree of damping D
PDC damper
Conventional dampers242_057
Coaxial arrangement of air springs/PDC damper

39
¥ Environmentally friendly, uses air
¥ Good operating safety due to great
stability.
¥ Electronic control system with
comprehensive self-diagnosis functions
¥ Maintenance-free Along with the principle advantages of self-
levelling (see Principles), the system realised
in the A6 has the following advantages:
¥ Virtually load-independent suspension
and vibration behaviour.
¥ Little space requirement due to compact
design, especially in the axle area.
¥ Self-levelling even available when engine
is off.
¥ Rapid raising and lowering times
¥ Low energy requirement
Air springs with PDC damperSelf-levelling suspension, A6 quattro
drive
Air supply unit242_041

40
The air springs
The installation of the air springs on the front-
wheel drive and the quattro drive is the same
as in the steel spring version. This allowed the
use of the axle design from the production
running gear with few modiÞcations.
In the front wheel drive version the piston is
conical in shape to allow sufÞcient clearance
for the spring movement between the
bellows and the piston.
In the quattro drive the air springs are
combined coaxially with the dampers to
act as a suspension strut.
Self-levelling suspension, A6
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quattro drive
Coaxial arrangement of air springs/PDC damper
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Front-wheel drive
Separate arrangement of air springs/PDC damper
Air springs may not be moved while
at atmospheric pressure since the
U-bellows cannot uncoil on the piston
and would be damaged.
In a vehicle with depressurised air
springs, the corresponding air springs
must be Þlled with the aid of the
diagnostic tester (see Workshop
Manual) before raising or lowering
the vehicle.

55
Self-levelling suspension
warning lamp K134
The warning lamp É
... is constantly illuminated in the case of
corresponding system errors or when the
system is switched off.
... ...ßashes in the case of extremely low or
high levels. <Ð55 mm/>+30 mm.
... ... ßashes during Þnal control diagnosis.
... ßashes when the control system is
switched off (only possible with diagnostic
tester).
After switching off the ignition, K134
illuminates for a function check and
extinguishes after the control system has
performed internal control unit test sequence
(unless an error is present).
Warning lamp K134
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The vehicle should not be driven as
long as the warning lamp is ßashing, as
low vehicle parts may be damaged due
to inadequate ground clearance.
If the warning lamp remains on
continuously to indicate a system error,
the system is switched off.
The driver is requested to contact the
nearest Audi service centre.

58
Self-levelling suspension, A6
The control concept
Stationary mode
The stationary mode is recognised at a speed
of <5 km/h.
In stationary mode, vehicle level deviations
due to e.g. passengers entering or leaving the
vehicle, or due to loading or unloading the
luggage compartment, are readjusted within
a short response time, in order to restore the
reference level as far as possible even before
the journey has begun.
The response time is 1 or 5 seconds,
depending on the level deviation. If the
deviation is great (extremely low position) the
response time is 1 second, at lesser level
deviations (normal deviation) it is 5 seconds. Driving mode
The driving mode is recognised at a speed of
>10 km/h.
In driving mode, level changes in the air
springs caused by fuel consumption or
temperature-related volumetric changes
(changing ambient temperatures) are
adjusted.
So that acceleration or braking does not
affect the control system, long response
times are set in the driving mode.
The response times are between 50 seconds
and 15 minutes depending on the control
thresholds.
Driving mode
Stationary mode
Run-on mode/run-up modeSleep mode
Control process

59
Sleep mode
To minimise electricity consumption the
control system switches to Òsystem idleÓ
(sleep mode) after 15 minutes.
There is no level adjustment in sleep mode.
ÒWake-upÓ is primarily triggered by the door
switch signal.
If the door switch signal fails, the system is
activated when the ignition is switched ÒONÓ
or by the driving speed signal.
The system can switch between sleep mode
and run-up mode, triggered by the door
switch signal, a maximum of 5 times. After
this, the system can only be activated via
terminal 15 and/or the driving speed signal. Run-on mode/run-up mode
After ÒIgnition OFFÓ, the control system is in
the so-called run-on/run-up mode. The
control unit remains active for a maximum of
15 minutes (via terminal 30) until it goes into
sleep mode.
The run-on/run-up mode is used to adjust
level deviations after parking the vehicle or
prior to starting on a journey.
The limit value in the rebound direction is
increased by 25 mm in the run-on/run-up
mode so that when the driver and/or the
passenger re-enters the vehicle, it does not
sink lower than the reference level or in order
to minimise any necessary vehicle raising
period.
The same response times apply as those
indicated for the stationary mode.

60
Self-levelling suspension, A6
Other features of the
control concept
Lifting platform mode
System behaviour:
If the vehicle is raised on a lifting platform the
system will react in the same manner as for a
level increase, by discharging the air spring
pressure.
The vehicle body would normally lower at this
point. Lowering ceases once the reference
level is reached.
As, however, the reference level is not
reached when a vehicle is raised on a lifting
platform, the air spring pressure would
continue to discharge down to the residual
supporting pressure. To prevent this
happening, the control system incorporates a
lifting platform mode.
This evaluates the level signal during the
discharge process and recognises a lifting
platform (no lowering, despite discharge),
whereby the system switches to lifting
platform mode.
When it recognises a lifting platform,
discharge stops and lowering ceases.
The system exits lifting platform mode by
means of the evaluation of further input
signals.It is normal for the rear axle to sink
after placing the vehicle onto the lifting
platform as a certain amount of time
will elapse before the system switches
to lifting platform mode and will
therefore release some pressure.
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