Seat Belt Tensioner: Description and Operation

Seat Belt Tensioner:

Front seating position seat belt tensioners supplement the standard driver and optional passenger air-bags for all versions of this model. When equipped, the seat belt tensioner is integral to the front seat belt and retractor units, which are secured to the inner B-pillars of the vehicle. Each retractor is concealed beneath the molded plastic B-pillar trim. The seat belt tensioner consists primarily of a tubular steel housing, a piston, a cable, a torsion bar, and a small pyrotechnically activated gas generator. All of these components are located on one side of the retractor spool on the outside of the retractor housing except for the torsion bar, which serves as the spindle upon which the retractor spool rotates. The seat belt tensioner is controlled by the Airbag Control Module (ACM) and is connected to the vehicle electrical system through a dedicated take out of the vehicle wire harness by a keyed, snap-fit molded plastic connector insulator to ensure a secure connection.

The seat belt tensioner cannot be repaired and, if faulty or damaged, the entire front seat belt and retractor unit must be replaced. If the front airbags have been deployed, the seat belt tensioners have also been deployed. The seat belt tensioner is not intended for reuse and must be replaced following a deployment. A growling or grinding sound while attempting to operate the seat belt retractor is a sure indication that the seat belt tensioner has been deployed and requires replacement.

Seat Belt Tensioner Operation:

The seat belt tensioners are deployed in conjunction with the airbags by a signal generated by the Airbag Control Module (ACM) through the driver and passenger seat belt tensioner line 1 and line 2 (or squib) circuits. When the ACM sends the proper electrical signal to the tensioners, the electrical energy generates enough heat to initiate a small pyrotechnic gas generator. The gas generator is installed so that it directs its expelled gas at the underside of the piston in the cylinder. The piston is connected to a cable that is wound around a pulley on one end of the retractor spool. As the gas expands, it propels the piston through the tube, which pulls the cable and rotates the retractor spool to take up slack in the seat belt webbing.

Once a seat belt tensioning sequence has been completed, the forward momentum of the occupant results in deformation of the torsion bar. Removing excess slack from the front seat belts not only keeps the occupants properly positioned for an airbag deployment following a frontal impact of the vehicle, but also helps to reduce injuries that the occupant might otherwise experience in a frontal impact event as a result of harmful contact with the steering wheel, steering column, instrument panel and/or windshield. The torsion bar is designed to deform in order to control the loads being applied to the occupant by the seat belt during a frontal impact, further reducing the potential for occupant injuries.

The ACM monitors the condition of the seat belt tensioners through circuit resistance. The ACM will illuminate the airbag indicator in the Electro Mechanical Instrument Cluster (EMIC) and store a Diagnostic Trouble Code (DTC) for any fault that is detected. For proper diagnosis of the seat belt tensioners, a DRBIII(R) scan tool is required. Refer to the appropriate diagnostic information.