Seat Belt Tensioner: Description and Operation

The seat belt side buckle pretensioner assembly consists of the following components:
^ An end release buckle
^ A buckle sleeve
^ A routing cable with piston
^ A piston tube
^ A mounting bracket
^ A gas generator

The front seat belt system features belt pyrotechnic pretensioners and web locks, which improve the effectiveness of the belts and air bags by keeping the occupants firmly in place during the first critical milliseconds of a collision. The seat belt pretensioners compensate for any slack in the seat belts by pulling the belts snug the instant a crash occurs. The same sensors that activate the vehicle's air bags also fire a tiny pyrotechnic charge in each pretensioner. Gas from the charge pushes a piston forward. This motion can remove over six inches of slack from the belt. A yellow flag pops up indicating that the pretensioner was activated. Once activated, the pretensioner must be replaced.

When the vehicle is in a frontal crash of sufficient force, the inflatable restraint Sensing and Diagnostic and Seat Belt Pretensioner Module (SDM) causes current to flow through the frontal air bag deployment loops and the pretensioner deployment loops. Current passing through the initiator ignites the material in the pretensioner gas generator. The gas produced from this reaction rapidly shortens the seat belt buckle height. The seat belt side buckle pretensioners will deploy in the event of a mild frontal collision (pretensioner-only deployment). They will deploy immediately before the frontal air bags deploy in the event of a mild-to-severe frontal collision (air bag and pretensioner deployment).