A seat belt pretensioner is a device affixed to either the seat belt retractor or the seat belt latch (buckle) assembly that draws the belt tight immediately after a crash.
During the impact in a crash, special crash sensors detect the sudden deceleration. The body control module (computer) receives the data from the crash sensors and, when necessary, deploys the airbags. But first, it triggers the seat belt pretensioner.
A pretensioner can be located inside of the seat belt retractor (where the belt is spring loaded and stored) or on the latch assembly. Inside the pretensioner is an explosive charge. When the charge detonates, it causes either the belt to draw tight or the latch assembly to shorten. In either case, the effect is that the belt draws tighter across the shoulder, rib cage, and hips of the occupant. This secures them in the proper alignment in preparation for airbag deployment.
The procedure to replace a seat belt pretensioner depends, for the most part, on whether the pretensioner is part of the seat belt retractor mechanism or the latch assembly. For the former, a technician must remove interior trim panels for access to the seat belt retractor. The fasteners that hold the ends of the seat belt and the retractor in place must be carefully removed. The entire seat belt, retractor, and pretensioner are all replaced as an assembly, and the fasteners are torqued to manufacturer’s specifications.
When the pretensioner is attached to the seat belt latch, the technician might need to remove the entire seat for access to the component. In either case, whether the pretensioner is in the retractor or the latch, it will be necessary to properly disarm and rearm the SRS with a scan tool. The SRS will not reset if the components are not in sound shape or if the procedure has not been done properly.
Because the seat belts (and therefore the pretensioners) are integrated into the Supplemental Restraint System (SRS), the body control module is continuously monitoring the system for potential problems. If there is an issue with the pretensioner, it is damaged or has previously cycled, the computer will detect the issue and turn on the SRS warning light on the dashboard. A subsequent scan of the diagnostic code should reveal a problem with the pretensioner, and it will be impossible to clear the code until the pretensioner has been replaced.
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