Belt tensioners (1) installed on both front seats and firmly connected to the belt buckle. In a frontal collision, they keep the seat belt snug against the body and prevent the front passenger from slipping under the belt. In a severe frontal impact, the belt tensioner will deploy together with the airbag. Both tensioners are activated at the same time even if there is no passenger in the front seat.
When the tensioner is activated, the charge in the capsule is ignited electrically. The released gases push the piston forward in the tube. A cable is attached to the piston, which pulls the seat belt buckle down through the swivel element. The belt is tensioned by 60 mm in about 0.005 seconds. Thanks to the locking mechanism in the housing, the piston is clamped in the forward position.
1. Seat belt buckle; 2. Piston; 3. Housing with a locking mechanism; 4. Capsule with a charge; 5. Ignition block; 6. Pivot/attachment point; 7. Tension cable.
The seat belt tensioner is out of service. After operation, both tensioners must be replaced as an assembly. You can identify a deployed pretensioner by a locked seat belt that no longer twists.
Precautions: assembly work on the front seat belts or tensioners must be carried out only in a workshop; Before disposing of a broken tensioner, it must be discharged in a workshop.