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

41
Air suspension strut design
In the case of the quattro suspension strut,
the connection/seal between the air spring
(piston) and the damper is made via a double-
sealed bayonet connector.
The bayonet connector must be absolutely
clean and is greased before assembly with a
special lubricant (see Workshop Manual).
It is assembled by pushing on and rotating
the air spring.
Always check for leaks on the O-ring
seals at the marked positions. The
sealing surfaces must be clean, free
from corrosion and pitting (aluminium
parts) and greased as required (see
Workshop Manual).
242_009 O-ring
2 O rings
Bayonet connectorLocking cap

42
Self-levelling suspension, A6
Specially adapted rubber bushes prevent any
signiÞcant vibration transfer to the bodywork.
Ensure that rubber bushes are installed
correctly!
Both halves of the metal box housing are
Þtted with a seal. This seal is essentially for
soundprooÞng. As the compressor sucks and
vents the air out of the metal box, it is
designed to allow a certain degree of leakage.
The air supply unit
The following components are contained in a
metal box inside the air supply unit:
Ð the V66 compressor with integrated air
dryer and discharge valve N111,
Ð transverse check valves N150 and N151,
Ð control unit J197
Ð and the relay for compressor J403
The components listed above are housed in a
special polyurethane foam (PUR foam)
insulation mat to ensure vibration and
acoustic damping. The insulation mat is
designed to Þx the positions of the individual
components within the metal box.
242_038Plastic box with control system J197
Discharge valve N11
Transverse check valves N150 and N151 Compressor V66
Rubber bush

43
Diagram of pneumatic
system
1 Suction Þlter
2 Compressor with motor V66
3 Non-return valve 1
4 Air dryer
5 Non-return valve 2
6 Non-return valve 3
7 Throttle
8 Discharge Þlter
9 Pneumatic discharge valve
123
465
7
9
11
10 Discharge valve N11
11 Valve for suspension strut HL N150
12 Valve for suspension strut HL N151
13 Rear left air spring
14 Rear right air spring
from the relay for
compressor J403from control unit
J197
10
1213242_034
from control unit J197
14
8

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

45
Suction/compression
When the piston moves
upwards, air is sucked into the
crankcase via the sinter Þlter. The
air is compressed above the
piston and enters the air dryer
via non-return valve 1.
The compressed and dried air
passes via non-return valve 2 to
the pressure connector which
leads to the transverse check
valves N150 and N151.
Overßow
When the piston moves back, the
air which has been sucked into
the crankcase passes via the
diaphragm valve into the
cylinder.
Filling/lifting
The relay for the compressor
and the air spring valves are
controlled simultaneously by the
control unit for Þlling (see
Suction/Compression).
242_015 Non-return valve 1
Air dryer
Pressure connectorNon-return valve 2
242_014 Diaphragm valve
Suction/compression
Overßow

46
Self-levelling suspension, A6
Discharge/lowering
The air spring valves N150 and N151 and
discharge valve N111 open during
compression. The air spring pressure ßows to
the pneumatic discharge valve and out of the
system from there via the air dryer and the
pressure limiting valve (see description of
pneumatic discharge valve).
242_056 Pneumatic discharge valve
with pressure limiting valve
Diagram of pneumatic system, discharge
11
12
107
64
9
8
4 Air dryer
6 Non-return valve 3
7 Throttle
8 Discharge Þlter
9 Pneumatic
discharge valve
10 N111
11 N150
12 N151
from control unit J197from control unit J197
242_059

47
The air dryer
The air must be dehumidiÞed in order to
prevent condensate and the associated
problems of corrosion and freezing. The
system used here is a so-called regenerative
air dryer system. A synthetically
manufactured silicate granulate is used as the
drying agent. This granulate can, depending
on the temperature, store up to 20% of its
own weight in water.
As the air dryer operates regeneratively, and
is only operated with oil-free, Þltered air, it is
not subject to replacement intervals and is
therefore maintenance-free.Because the air dryer is regenerated
only with waste air, the compressor
cannot be used to Þll any other
components. As this compressed air is
not fed back via the air dryer, no
regeneration can take place. For this
reason, the manufacturers do not Þt a
pressure connection for external
components.
Water/moisture in the system indicates
a fault in the air dryer or the system.
242_056 Granulate Þlling

48
Self-levelling suspension, A6
Regeneration
As outlined above, the compressed air is
initially fed through the air dryer and dried.
The moisture is temporarily stored in the air
dryer and the dried compressed air passes
into the system.
Discharge valve N111
The discharge valve N111 is a 3/2 way valve
(three connections and two switching
positions) and is closed without current. The
N111 is used only for discharge purposes
(lowering).
For lowering, the discharge valve is controlled
by the control unit J197 together with valves
N150 and 151.
(see description of the Pneumatic discharge
valve and under Discharge)
242_097
The air dryer is regenerated during discharge
(lowering). During discharge, the dried
compressed air (Òwaste airÓ) is fed back into
the air dryer where it re-absorbs the moisture
stored there and discharges it into the
ambient air.
Air dryer
242_016

49
At an air spring pressure of >3.5 bar, the valve
body lifts against the resilience of both valve
springs and opens valve seats 1 and 2. The air
spring pressure then passes to the air dryer
via the throttle and non-return valve 3. Once it
has passed through the air dryer, the air ßows
through the valve seat of the pressure
limiting valve and the discharge Þlter into the
ambient air.
The signiÞcant drop in pressure downstream
from the throttle results in the uptake of the
relative air humidity whereby the moisture
uptake of the Òwaste airÓ is increased. Pneumatic discharge valve
The pneumatic discharge valve performs two
tasks:
¥ It is a residual pressure retaining device
¥ and a pressure limitation device
A predeÞned minimum pressure (>3.5 bar) is
necessary to prevent damage to the air
springs (U-bellows).
The residual pressure retaining device
ensures that the pressure in the air
suspension system does not fall below
3.5 bar during de-pressurisation (except in
the case of leaks upstream from the
pneumatic discharge valve).
242_037 Discharge valve N11Pneumatic discharge valve (cover)
Valve bodyPressure limiting valve
Valve seat 1
Valve seat 2
Discharge Þlter Vent pipeNon-return valve 3
Air dryer Throttle

50
Self-levelling suspension, A6
The pressure limiting function protects the
system against inadmissible high pressure
e.g. when the compressor fails to switch off
due to a defective relay contact or defective
control unit.
242_035
242_017
Diagram of pneumatic system, pressure limiting function
12
9
8
1 Suction Þlter
2 Compressor
8 Discharge Þlter
9 Pneumatic discharge
valve
from relay J 403
In such cases, the pressure limiting valve
opens against the resilience from approx.
13.5 bar upwards, and the pressure is
discharged via the discharge Þlter.
Pressure limiting valve
Discharge Þlter
from control unit J197
from control unit J197