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

12
Control strategies
The vehicle rises automatically to high level 1
at the following speeds and time thresholds:
If the system has lowered the vehicle to low
level (motorway mode), it will automatically
raise it to high level 1 when its speed drops
below 60 km/h for longer than 30 seconds.Vehicle speed Time
<60 km/h >30 seconds
<30 km/h immediately
Control unit strategies
4Z7 907 553B
Variations to the description of control unit
4Z7 907 553A are outlined below:
•No parking level control
•Automatic raising to high level 1
Preconditions for automatic raising to high
level 1:
•The system must not be switched to
manual mode.
•Between ignition ON and OFF, the driver
must select high level 1 or high level 2
once.
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Automatic lowering
Lowering processes
after 30 seconds
Automatic raising to HL1 after
30 seconds
Raising processes
Immediate raising Speed km/h
Speed km/h LL NL HL1HL2
LL NL HL1HL2

34
However, all level sensors (left and right) are
required in the air suspension system run-up
and run-on mode (ignition OFF).
To allow the left-hand level sensors to deliver
measured values in the case of the 4-level air
suspension in the allroad quattro, power is
supplied to control unit J431 (HRC) from
control unit J197).
This ensures that voltage is supplied to all
level sensors when control unit J197 is active.
Self-levelling suspension
control unit J197
The central element of the system is the
control unit which, in addition to its control
functions, enables the monitoring and
diagnosis of the entire system.
The control unit detects the signal from the
level sensors and uses it to determine the
current vehicle level. This is compared with
the reference level and corrected if necessary,
depending upon further input variables
(interfaces) and its internal control
parameters (reaction times and level
deviations).
It differentiates between various control
situations and controls them via the relevant
control concepts (see Control concept).
Comprehensive self-diagnosis facilitates
inspection and service of the system (see
Workshop Manual).
There are two control units currently in use,
depending on the country.
Control units with the part numbers
4Z7 907 553A and 4Z7 907 553B have different
control strategies (see page 10 onwards).
A common control strategy for all countries
(as for Index “B”) is planned for the future.
Control concepts
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The system can be tested via the self-
diagnosis or test adapter 1598/35.
For further information, see “Service”
chapter.
Address word 34
Power supply to the headlamp range control
system
As previously described in the “Level sensors”
section, voltage is supplied to the left-hand
level sensors by the headlamp range control
unit J431)
Headlamp range control requires neither run-
up nor run-on times, so the voltage is
normally supplied to control unit J431 via
terminal 15 (ignition ON) (see function
diagram, page 32).

35
Level change process:
Raising - rear axle first, then front axle
Lowering - front axle first, then rear axle
Modes
Height mode/driving mode
Reaction times upon level deviation
Control characteristics during level change
Level change is basically performed axle by
axle, whereby level differences between the
left and right sides are compensated (e.g. if
loaded on one side).Driving speed Reaction time
<5 km/h
Height modeapprox. 5 seconds
approx. 1 second at
extremely low level
>10 km/h
Driving modeapprox. 50 seconds or
15 minutes
depending on the level
deviation
Run-on mode/run-up mode
The run-on mode enables the compensation
of level deviations after the vehicle has been
parked (e.g. caused by passengers leaving the
vehicle or unloading the vehicle) and before
driving off (e.g. caused by intense cooling,
leakage or loading).
In this mode, delay times before commencing
a journey are kept to a minimum.After “Ignition OFF”, the control unit is in the
so-called run-on mode. The control unit
remains active for a maximum of 15 minutes
(via terminal 30) until it goes into sleep mode.
Due to the limited energy available when the
engine is switched off, control limits are
extended and controls are limited in both
number and duration.

36
Control concepts
The system can switch between sleep mode
and run-on/run-up mode, triggered via the
door contact signal, a maximum of 15 times.
For the 15 subsequent wake-up procedures,
the system switches to sleep mode after only
1 minute.
The system can then only be activated via
terminal 15 and/or the speed signal. Sleep mode
To minimise electricity consumption the
control unit switches to “system idle” (sleep
mode) after 15 minutes.
There is no level adjustment in sleep mode.
“Wake-up” is primarily triggered by the door
contact signal. If the door contact signal fails,
the system is activated when the ignition is
switched “ON” or by the driving speed signal.
The vehicle should be raised as quickly
as possible in order for the control unit
to recognise the lifting-platform mode.
It is often advisable to switch off the
system during repair work (e.g. during
axle measurement or if the pressure
lines have been detached, in order to
prevent the compressor running
unnecessarily).
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Lifting platform mode
The control unit evaluates the level signals
while a stationary vehicle is being lowered
and thereby initiates lifting platform
operating mode.
The aim of the lifting platform mode is to
prevent excessive discharge of the air springs
when the vehicle is completely raised.