Seat belt pretensioners

SEAT BELT PRETENSIONERS

The seat belt pretensioner is a pyrotechnic device integrated in the buckle that hooks onto the belt which, in the event of a head-on collision takes up the inevitable slack in the belt caused by the action of the weight of the body and keeps it as close to the seat as possible.

In fact, if the belt is kept in close contact with the body, it can gradually absorb the kynetic energy generated during a collision.

There are many reasons why a seat belt cannot keep a body fully against the seat back without the adoption of this device; the main ones are:

• time delay in the action of the inertial locking mechanism (locking of the reel);
• stretching of the belt fibres;
• "over-extension" of the belt from the reel;
• garments of a certain thickness which create a space between the belt and chest.

From the above, it can be deduced that in reality the belt would effectively begin to restrain its wearer only after a certain amount of movement forward of the body: this is why it is necessary to tension the belt at the time of the crash and thus prevent it from extending.

Operation

When an impact is detected, the control unit suitably commands the firing circuit which supplies the gas generator (1), in a similar manner as for the Air Bag module.

The pressure of the gas that develops is applied on the surface of the piston (2), and creates a force that pushes the piston into the cylinder (3).

A steel cable (4) is shrunk onto the piston (2) and the end of the cable is connected to the belt reel. The movement of piston (2), therefore, shortens the belt by a few centimetres (appr. 7-8 cm), taking up the slack mentioned previously.

NOTE: the pretensioner operating thresholds are lower than those of Air Bags, as mentioned previously, because even a slight impact can cause extension of the belts.

AIR BAG CONTROL UNIT

REMOVAL/REFITTING

Before doing any work on the system carefully adhere to to the SAFETY INSTRUCTIONS given in "GROUP 55 - ELECTRIC SYSTEM DIAGNOSIS" Section "Air Bag and Pre-tensioners".

In particular disconnect both battery terminals, isolate them and wait for 10 minutes before doing any work.

• (only for SPIDER). Open the boot lid.
• (only for GTV). Remove the rear seat (see Group 70).

1. Slacken the two fastening screws and remove the control unit cover.