Front Seat Belt Pretensioners; Front Seat Belt Reminder Sensors; Airbag System Warning Lights - Alfa Romeo 159 Training Material

ELECTRICAL
SYSTEM

9.7.4 Front seat belt pretensioners

Features
The pretensioners fitted on the front seat belt buckles are pyrotechnic devices, electrically activated by a signal from
the ECU. The same logic that controls the airbags also activates the seat belt pretensioners. The pretensioners
recover belt slackness and hold the occupant against the backrest from the first instants of the collision to reduce
movements inside the passenger compartment.
The belts are also equipped with two-threshold progressive load limiters to decrease the force transmitter to the
occupant's chest: the load limiter force levels considerably reduces the risk of fractures to shoulder bones and ribs
also for the most delicate bones (e.g. the elderly).
The pretensioner mechanism is shown in the following figure.
1. Gas generator
2. Piston
3. Cylinder
4. Metallic wire
Operation
When a sufficiently high deceleration is detected by the system accelerometers, the electronic sensor in the ECU
sends a signal to fire the pyrotechnic charges of the gas generators. The combustion of the propellant develops a
chemical reaction and forms inert gas whose pressure generates a force that pushes a piston. The piston is
connected to the metallic wire which via a rotation pin pulls the buckle downwards.
NOTE: After deploying the pretensioners, the belts remain blocked and must be replaced.

9.7.5 Front seat belt reminder sensors

When the seat belts are not worn, the risk of injury for occupants is higher than that of seat belt wearers also in very
less severe crashes. Specific signals from the seat belt reminder sensors in the front seat belt buckles and from a
pressure sensor embedded in the seat cushion foam are used by the ECU to deploy the airbags but not the
pretensioners if a passenger is sitting in the seat. The pretensioners are not deployed in this case which would only
determine additional cost for repairing the vehicle.

9.7.6 Airbag system warning lights

1. Airbag system failure warning light on instrument panel
2. Passenger airbag deactivate warning light on front ceiling light
3. Seat belt reminder warning light on front ceiling light
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