How Do Modern Seatbelts Work?

Categories: Seat Belt Pretensioner Repair

Most people use seatbelts every day without thinking about them or asking the question: “How Do Seatbelts Work?” While we understand basic seatbelt function, the technology has evolved in recent years to include a device called “pretensioners”. As the name suggests, pretensioners are designed to tighten your seatbelt right before your body moves in an accident. To react in milliseconds, pretensioner technology is downright explosive. Pyrotechnic “gas-generating” charges are the key to ultra-quick reaction times. The force moves a mechanism that shortens the length of your belt. Boom!

The good ol’ days… Back when seatbelts wore YOU for safety. While seatbelt laws began to take effect in the mid-’80s, acceptance increased gradually. Technology has come a long way.

History: Before Seatbelts Became Mandatory

Before seatbelts became mandatory, many of us shared a perception that unrestricted movement while driving equated to freedom. A great example is what was affectionately called the “Tail Gunner” seat in the sixties and seventies. Parents allowed their children to hang out in the back of station wagons. Kids could face backward while in motion — make silly faces at passing motorists, roll around with friends and siblings, or lay down for a nap on a long trip. Of course, most of us survived driving around without seatbelts, but those who were unfortunate enough to be involved in collisions were tossed around like rag dolls.

According to the NHTSA, vehicles were required to feature seatbelts on January 1, 1968. That first law mandated that cars have seatbelts, but wearing seatbelts was optional. It wasn’t until 1984 that states required motorists to actually wear seatbelts. New York was the first state to pass such a law. TRIVIA: According to The Center Square, New Hampshire is the only state that does not have a mandatory 18 and over seat belt law as of 2023.

According to the NHTSA, New Hampshire (green) is the only state where motor vehicle occupants aged 18 and up are not required to wear a seat belt. Yellow states only apply secondary enforcement which means you can only get ticketed if pulled over for another reason. Orange and red states apply some iteration of primary enforcement.

Modern Tech: Inertia Locking Retractors

Seatbelt technology advanced in various stages throughout history and around the world. For example, Swedish engineer Nils Bohlin invented the 3-point belt for Volvo in 1959. The novel design did not catch on in the United States until the 70s. More Trivia: Before working for Volvo, Bohlin designed ejector seats for the military. Three advancements separate modern seatbelts from early designs: Locking retractors, pretensioners, and load limiters. Instead of strapping yourself in, retractors feature spring-loaded inertia reels that allow for some freedom of movement. In the event of rapid deceleration including a panic stop or collision, the mechanism locks to hold the occupant in place. The inertial locking retractor is a simple yet effective safety feature that is still part of every modern seatbelt. The pretensioner is where things get interesting.

The Mercedes Benz S-class has a long track record of being first to market with safety innovations. Introduced in 1981, the W126 chassis was the first mass-produced vehicle to be factory equipped with seat belt pre-tensioners. Airbags were standard equipment as well.

Pyrotechnics: Wait.. My Seatbelts Contain Explosives?

Technically, but the explosions are really, really small and contained. There is no fire during the explosion, the reaction is purely chemical in nature. What if your seatbelt could actually sense a collision and instantly hold you more tightly? That extra security exists and is commonplace. First introduced on the W126 1981 Mercedes Benz S-Class, this tried and true technology utilizes tiny explosive gas charges that, when deployed, shorten a seat belt’s length. There are different types.

seat belt pretensioner with 3 connectors
Here’s a great example of a triple-stage pretensioner. The three circles call out three separate pyro sensor connectors. The retractor pretensioner has two sensors and the anchor pretensioner (the tube at the bottom) has one sensor.

Pretensioner Types

Buckle Pretensioner: A pyrotechnic gas generator propels a piston in a direction that shortens the belt/buckle/tensioner cable assembly.

Anchor Pretensioner: Very similar to a buckle pretensioner, this self-contained device shortens the anchor assembly by way of a pyrotechnic gas generator.

Retractor Pretensioner: The pyrotechnic gas generator in this slick design forces the seatbelt reel to reverse-wind itself which, in turn, tightens the belt.

Multi-Stage Pretensioners: Some vehicles feature combinations of the above pretensioner types with the ability to deploy in stages.

In this video, we showcase numerous pretensioners including retractor-style, anchor-type, and buckle configurations.

In all cases, the belt assembly locks in the tensioned position after deployment and cannot be unlocked without taking apart the assembly and replacing the pyrotechnic charge. MyAirbags was one of the first companies to restore post-collision seatbelts. It is critical to note that servicing pyro sensors requires special licensing by the ATF. Likewise, the shipping of refurbished pretensioners with active pyro fuses requires special Haz-Mat shipping procedures. Do not attempt to disassemble or otherwise tamper with a post-collision seat belt pretensioner as serious injury may occur. The assemblies need to be replaced or better yet — repaired by professionals like the team at MyAirbags.

Due to the nature of their pyrotechnic charges, MyAirbags packs and ships your restored seat belt pretensioners with specially marked boxes and handling. Conversely, if you had seat belt pretensioners repaired by another company and your parts DON’T show up in special Haz-Mat packaging, something is fishy. The shop either shipped your parts illegally or may have failed to repair your parts.

Repairing Pretensioners Is A Serious Business

Maybe you’re in the business of repairing post-collision vehicles or just curious about how your own car is being fixed while at a body shop. If your seatbelt pretensioners have been deployed, it is important to have them restored properly. MyAirBags is fully licensed by the ATF to repair pretensioners. We’re also authorized to ship your repaired units with special Haz-Mat packaging. In choosing to work with a seat belt restoration vendor, be sure to verify that they are properly licensed. Another critical qualification, we use only OEM components at MyAirbags. Some of the offshore knock-off parts used by our competitors are downright dangerous. Lastly, be suspicious if your repaired seat belt pretensioners are returned to you in non-Haz-Mat packaging. It means that the vendor is either deceiving the shipping companies or might be faking your repairs.

With up to ten airbags plus pyro sensors at each seatbelt, it’s amazing that most of us are surrounded by small pyrotechnic charges while driving our cars. The technology is thoroughly concealed and works passively upon impact. Very few motorists are aware of their presence, but it’s good to know how pretensioners work.