A seatbelt pre-tensioner is a device installed in late-model vehicles to reduce slack that may exist in a seatbelt system at the moment of a frontal collision. Having the seatbelt system snug allows the supplemental restraint airbag system to work most effectively. A loose seatbelt system does not keep the occupant in the proper position during the forces of a crash. In order for frontal airbags to reduce or eliminate injury, the occupant must be seated, belted and in the correct position in the vehicle.

Photo By Ron Moore The tube-style pre-tensioner fires during a collision to pull the slack out of the lap/shoulderbelt assembly.

Pre-tensioners may activate mechanically by the force of deceleration or may be activated by a small electrical current similar to that which deploys frontal airbags. In order to pull the slack out of the seatbelt, the pre-tensioners must be directly connected to the seatbelt system.

There are two pre-tensioner types. One design attaches at the seatbelt buckle end; the other type attaches to the retractor spool mechanism. Either type of pre-tensioner system will tighten the belt during crashes that involve frontal airbag deployment only.

Pre-tensioners working at the buckle typically use a small explosive charge to generate a pressurized gas. The pressure of the gas forces a piston down deep into a tube that is about the size of a cigar. As the piston slides down the tube, it pulls a cable which pulls the seatbelt buckle downward. As the buckle moves down, it snugs the seatbelt being worn by the occupant.

Photo By Ron Moore This view of a 1999 Volvo S80 display vehicle reveals how B-pillar cuts are typically made where pre-tensioners are now located.

The second more common type of pre-tensioner is the retractor-type. There are rotary-piston designs and piston-tube designs. With a piston tube system, a small chemical charge again generates gas to propel a piston down a small tube. The piston is attached to the retractor spool of the seatbelt with a cable. The cable “backwinds” the retractor spool, snugging the seatbelt. With the rotary piston design, the piston actually spins the large seatbelt spool backwards to snug the belt against the occupant.

In a crash test performed at the Vehicle Research Center of the Insurance Institute for Highway Safety, engineers found that a Volvo pre-tensioner retracted two inches of seatbelt slack at the moment of impact.

You will know that the seatbelt pre-tensioners have deployed when you arrive at a crash and see deployed frontal airbags. With the buckle-type pre-tensioner, you can confirm this by noticing that the top of the seatbelt buckle is at or below the top of the seat cushion.

If your patient is still seated and belted when you conduct your primary survey, a good sign confirming that a pre-tensioner has activated along with the frontal airbags is the presence of some slight tension in the lap/shoulder belt system.

Photo By Ron Moore Rescuers must learn to remove interior roof pillar trim before cutting. This Volvo S80 B-pillar has both a pre-tensioner and a side-impact airbag crash sensor.

Pre-tensioners are available for front driver and passenger seatbelts. Selected model vehicles also provide pre-tensioners for rear seat passengers as well. The new Volvo S80, for example, has five lap/shoulder seatbelts, two for the front seat occupants and three for the rear. All five positions have seatbelt pre-tensioners. The rear pre-tensioners are located under the rear window shelf.

If in a crash situation the front airbags have not deployed, it should be anticipated that none of the pre-tensioners have activated either. Use the same precautions you do for undeployed airbags. Disconnecting power from the battery drains the airbag’s electrical system capacitor and also disables the pre-tensioner’s electrical system if it is an electrically fired design.

Extrication crews should avoid cutting into a seatbelt pre-tensioner unit. To do this, we must first locate the units. Typically they are on or inside the vehicle’s B-pillar or are secured to the rocker channel at the seatbelt spool mechanism.

The best way to identify them is to strip away the interior plastic trim of the B-pillar before cutting into it. Any cutting or spreading along the rocker channel that is near the seatbelt spool should also be done with caution.

General Motors publishes an 18-page booklet explaining its airbag systems and their pre-tensioners. The publication, titled Air Bags and Pre-Tensioners in GM Vehicles, is available free of charge by calling the toll-free telephone number 800-393-4831 and following voice prompt 2. The booklet is item number 22008.22-Q2 and is free of charge for single copies to an agency.

TABLE OF PRE-TENSIONERS (1999 model year)
Acura 3.2RL & 3.5RL Seatbelt pre-tensioner (2) at front three-point seatbelts. Audi A-4 Seatbelt pre-tensioner (5) at all seatbelts. Audi A-6 Seatbelt pre-tensioner (5) at seatbelts. Audi A-8 Seatbelt pre-tensioner (5) at all seatbelts. BMW 3-series (323 & 328 models represent six vehicle types) Seatbelt pre-tensioner (4), all three-point seatbelts, at female buckle. BMW 5-series (528 & 540 models represent five vehicle types) Seatbelt pre-tensioner (5), all three-point seatbelts, at female buckle. BMW 7-series (740 & 750 models represent five vehicle types) Seatbelt pre-tensioner (5), all three-point seatbelts, at female buckle. BMW M-series (M, M3 & M5 models represent five vehicle types) Seatbelt pre-tensioner (5), all three-point seatbelts, at female buckle. BMW Z-series (Z3 models represent three vehicle types) Seatbelt pre-tensioner (2), at female buckle. Cadillac Catera Seatbelt pre-tensioner (2), front three-point seatbelts. Cadillac Seville Seatbelt pre-tensioner (2), front three-point seatbelts. Chevrolet Prism LSi Seatbelt pre-tensioner (2), front three-point seatbelts. Chevrolet Venture minivan Seatbelt pre-tensioner (2), front three-point seatbelts. Ford Econoline van Seatbelt pre-tensioner (2), front three-point seatbelts. Infiniti G20 Seatbelt pre-tensioner (2), front three-point seatbelts. Infiniti I30 Seatbelt pre-tensioner (2), front three-point seatbelts. Infiniti I45 Seatbelt pre-tensioner (2), front three-point seatbelts. Jaguar XJ, XK, Vanden Plas and S-series - 2000 model year (models represent seven vehicle types) Seatbelt pre-tensioner (2), front three-point seatbelts, 1995 and newer. Land Rover Discovery II SUV Seatbelt pre-tensioner (2), front three-point seatbelts. Lexus 300 & 400 sedan series(models represent six vehicle types) Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes C-class (models represent 3 vehicle types) Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes CLK-320, 430 coupes Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes CLK Cabriolet 320 two-door convertible Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes E-class (models represent five vehicle types) Seatbelt pre-tensioner (2), front three-point seatbelts.		Mercedes S-class (models represent two vehicle types) Model year 2000 - Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes SL-class coupe & roadster (models represent four vehicle types) Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes SLK 230 Kompressor coupe/roadster Seatbelt pre-tensioner (2), front three-point seatbelts. Mercedes ML320, ML 430 SUV Seatbelt pre-tensioner (2), front three-point seatbelts. Oldsmobile Silhouette minivan Seatbelt pre-tensioner (2), front three-point seatbelts. Pontiac Montana minivan Seatbelt pre-tensioner (2), front three-point seatbelts. Rolls-Royce Seatbelt pre-tensioner (2), front three-point seatbelts, 1995 & newer. Saab 9-3 & 9-5 models Seatbelt pre-tensioner (2), front three-point seatbelts. Subaru Outback (model year 2000) Seatbelt pre-tensioner (2), front three-point seatbelts. Toyota Corolla Seatbelt pre-tensioner (2), front three-point seatbelts. Toyota Sienna minivan Seatbelt pre-tensioner (2), front three-point seatbelts. Toyota Solara Seatbelt pre-tensioner (2), front three-point seatbelts. Toyota Tacoma & Pre-Runner pickups Seatbelt pre-tensioner (2), front three-point seatbelts. Volkswagen New Beetle Seatbelt pre-tensioner (2), front three-point seatbelts. Volkswagen Cabrio Seatbelt pre-tensioner (2), front three-point seatbelts. Volkswagen Golf Seatbelt pre-tensioner (2), front three-point seatbelts. Volkswagen GTI Seatbelt pre-tensioner (2), front three-point seatbelts. Volkswagen GLX Jetta Seatbelt pre-tensioner (4), front and rear three-point seatbelts. Volkswagen Passat Seatbelt pre-tensioner (4), front & rear three-point seatbelts. Volkswagen Passat GLS wagon Seatbelt pre-tensioner (2), front three-point seatbelts. Volvo S40, S70, V40 & V70 Seatbelt pre-tensioner (4), front & rear three-point seatbelts. Volvo C70 convertible Seatbelt pre-tensioner (4), front three-point seatbelts. Volvo S80 Seatbelt pre-tensioner (5), all three-point seatbelt locations.