Audi A8 2019 Service Training page 67

Automatic belt retractor
Front and rear automatic seat belt retractors
The rear outer automatic belt retractors are equipped with
pyrotechnic belt tensioners and additionally with reversible
belt tensioners with electric motors.
The four reversible belt tensioners:
› Left Front Seat Belt Tensioner Control Module J854.
› Right Front Seat Belt Tensioner Control Module J855.
› Left Rear Seat Belt Tensioner Control Module J1097.
› Right Rear Seat Belt Tensioner Control Module J1098.
They are connected to Airbag Control Module J234 by a
sub-bus system. The seat belt tensioner control modules
are actuators and are activated by J234.
Belt park function (convenience function)
The reversible seat belt tensioners are also equipped with a
"belt park function". With the belt park function, the seat
belt is retracted by the electric motors of the reversible
seat belt tensioners.
The belt park function is activated when the buckle on a
fastened seat belt is opened, and the seat belt is slowly
retracted by the automatic retractor. The seat belt may
retract slowly at low temperatures or when the retractor
mechanism has lost tension.
The seat belt retracts about five seconds after seat belt
buckle opens or if the seat belt has retracted about 30%
relative to the fastened belt buckle. The belt park function
can be activated when the ignition is switched on or off. If
the vehicle is already in sleep mode, the reversible seat belt
tensioners are active for about five seconds after the seat
belt is fastened and monitor the seat belt.
When Audi pre sense basic detects specific driving situa-
tions, corresponding signals are sent to the data bus. J234
evaluates the signals and, if necessary, instructs the seat
belt tensioner control module to partially or fully tension
the seat belts via an electric motor.
If an automatic seat belt retractor with reversible seat belt
tensioners is replaced, the basic setting procedure must be
performed using the "Guided Fault Finding" function. In
this process defined information is written to the airbag
control module. If the basic setting procedure has not been
carried out, a DTC is logged by J234.
If movement of the seat belt is detected (occupant still
fastened in the vehicle), this time can be extended to a
maximum of 20 minutes. If a reversible seat belt tensioner
detects the seat belt retracting in sleep mode, it wakes up
the airbag control module. The airbag control module can
then initiate the retraction of the seat belt by the corre-
sponding belt tensioner control module.
As mentioned previously, the seat belt is only retracted by
the electric motors when the seat belt buckle has been
opened and the seat belt is retracting slowly.
The reversible seat belt tensioners have a Hall-effect sensor
which measures the extension of the seat belt based on the
rotation of the belt retractor shaft. The seat belt retraction
time is determined by the airbag control module.
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