Audi A8 2019 Service Training page 76

System characteristics of the reversible seat belt tensioners
› After reducing seat belt slack, the seat belt is released
again.
› If a seat belt is not buckled, the slack is not reduced for
this seat belt.
› The four reversible seat 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,
are connected to the Airbag Control Module J234 via a
sub-bus system. J234 instructs the seat belt tension-
ers to reduce the belt slack.
Longitudinal dynamics function
Hazard braking
If the brake pressure reaches a defined value during a
hazard braking maneuver, Audi pre sense basic initiates
electrical partial tensioning of the reversible seat belt
tensioners.
A message indicating that Audi pre sense is active appears
in the instrument cluster. No audible signal is given.
Emergency braking
During an emergency braking maneuver, the reversible seat
belt tensioners are fully tensioned. A distinction is made
between the following three criteria for emergency braking.
› An emergency braking situation exists if J104 detects
that brake pressure had reached a specific value within a
defined period of time. If the conditions are met,
Audi pre sense basic initiates electrical full tensioning of
the reversible seat belt tensioners. A message indicating
that Audi pre sense is active appears in the instrument
cluster. No audible signal is given.
› An emergency braking situation exists if the given condi-
tions are not met, but if J104 has detected an emergency
braking maneuver based on the pedal rates. If the condi-
tions are met, Audi pre sense basic initiates electrical
full tensioning of the reversible seat belt tensioners. A
message indicating that Audi pre sense is active appears
in the instrument cluster. No audible signal is given.
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Tensioning of the reversible seat belt tensioners
In the case of Audi pre sense basic, tensioning of the
reversible seat belt tensioners can be carried out at a speed
of higher than 18.6 mph (30 km/h) when the longitudinal
and transverse dynamics functions are active. To activate
longitudinal dynamics, the vehicle must be moving for-
wards. An exception here is the crash function in the low
relative speed range (refer to
› An emergency braking maneuver also exists if the elec-
tromechanical parking brake is applied when traveling
forwards at a speed of approximately 18.6 mph (30
km/h) or higher. Audi pre sense basic then initiates the
full tensioning of the reversible seat belt tensioners. In
this case, no Audi pre sense message appears in the
instrument cluster. The seat belts are released again
under the following conditions:
› When the electromechanical parking brake button is
released.
› When the vehicle comes to a standstill.
› When the driver presses his foot down heavily on the
accelerator and thus overrides the emergency braking
of the electromechanical parking brake.
page 75).