low oil pressure AUDI A6 ALLROAD 1999 C5 / 2.G Pneumatic Suspension System

19
Design of the air springs:
In passenger vehicles, air springs with
U-bellows are used as suspension elements.
These allow greater spring travel in restricted
spaces.
The air springs consist of:
¥ Upper housing closure
¥ U-bellows
¥ Piston (lower housing closure)
¥ Retaining rings
The construction of the U-bellows can be
seen in Þg. 242_032.
242_032
The outer and inner surfaces are made of an
elastomer material. The material is resistant
to all weather inßuences and is largely oil-
resistant. The inner surface Þnish is designed
to be particularly air-tight.
The stability supports absorb the forces
produced by the internal pressure in the air
springs.
Upper housing closure
Retaining ring
Internal surface coating
Woven insert 1
Woven insert 2
External surface coating
Piston
Coaxial arrangement of the air springs

27
Single pipe gas-pressure shock absorber
With the single pipe gas-pressure shock
absorber, the working chamber and the oil
reservoir are located in a single cylinder.
Volumetric changes caused by the piston rod
and the temperature changes in the oil are
compensated by another gas chamber which
is separated from the working cylinder by a
dividing piston. The level of pressure in the
gas chamber is approx. 25 - 30 bar and must
be able to sustain the damping forces during
compression.
The damping valves for compression and
rebound are integrated into the piston.
Comparison of single/dual pipe gas-pressure shock absorbers
Dual pipe gas-pressure shock
absorberSingle pipe gas-pressure shock
absorber
Valve function The tendency towards cavitation
is reduced by the gas pressure in
the oil reservoirMinimal tendency towards
cavitation thanks to high gas
pressure and separation of oil and
gas
Characteristic
curvesAny, due to separate valves for
compression and reboundDependant on the gas pressure
during compression
Short damping
strokesGood Better
Friction Low Higher due to seal under pressure
Design Greater diameter Longer due to gas chamber in the
cylinder
Installation
positionApproximately vertical Any
Weight Heavier Lighter
242_082
Piston with damping
valves
Dividing piston
Gas chamber
Damper valves

35
Design and function
The PDC valve inßuences the ßow
resistance of the working
chamber on the piston rod side
(working chamber 1).
Working chamber 1 is connected
to the PDC valve via bore holes.
The PDC valve has a low ßow
resistance when the air spring
pressure is low (no load or small
partial load). Part of the damping
oil bypasses the damping valve,
thereby reducing the damping
force.
The ßow resistance of the PDC
valve has a Þxed relation to the
control pressure (air spring
pressure).
The damping force is dependent
on the ßow resistance of the
relevant damping valve
(compression/rebound) plus that
of the PDC valve.
242_033 PDC valve Working chamber 1
Bottom valvePiston valve with
sealing collar
Working
chamber 2BoresGas Þlling
Throttle in air connectorRebound stop

36
Principles of air suspension
Function during rebound at low air
spring pressure
The piston is drawn upwards, part of
the oil ßows through the piston valve,
the remainder ßows through the bore
holes in working chamber 1 to the
PDC valve. As the control pressure (air
spring pressure) and consequently
the ßow resistance of the PDC valve is
low, the damping force is reduced.
242_052
242_051
Function during rebound at high air
spring pressure
The control pressure and
consequently the ßow resistance of
the PDC valve is high. Most of the oil
(depending on the control pressure)
is forced to ßow through the piston
valve, thereby increasing the
damping force.
Low air spring
pressure
High air spring
pressurePDC valve open
PDC valve closed

37
Function during compression at low
air spring pressure
The piston is pushed downwards and
damping is determined by the
bottom valve and to a certain extent
by the ßow resistance of the piston.
The oil displaced by the piston rod
ßows partly via the bottom valve into
the reservoir. The remainder ßows
through the bore holes in working
chamber 1 to the PDC valve. As the
control pressure (air spring pressure)
and consequently the low ßow
resistance of the PDC valve is low,
the damping force is reduced.
242_070
Function during compression at high
air spring pressure
The control pressure and
consequently the ßow resistance of
the PDC valve are high. Most of the oil
(in relation to the control pressure)
must ßow through the piston valve,
thereby increasing the damping
force.
Low air spring
pressure
242_069 High air spring
pressure
PDC valve closedPDC valve open

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
242_043
quattro drive
Coaxial arrangement of air springs/PDC damper
242_042
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.

44
Self-levelling suspension, A6
The compressor
The compressed air is generated by means of
a single stage piston compressor with
integrated air dryer. In order to avoid oil
contamination of the U-bellows and the dryer
cartridge, the compressor is a so-called dry
running compressor.
Permanently lubricated bearings and a PTFE
(polytetraßuoroethylene) piston ring ensure a
long service life.The discharge valve N111 and the pneumatic
discharge valve are integrated into the dryer
cartridge housing.
In order to protect the compressor from
overheating, it switches off at excess
temperatures (see Overheating protection
chapter, page 61).
Discharge valve N11Pneumatic discharge valve
with pressure limiting valve
Non-return valve 3Air dryer
Non-return valve 1
PTFE piston ring
Suction Þlter
Discharge Þlter
Pressure connector
242_013
Non-return valve 2