Seat Belt Tensioner. Basic Information and Safety Precautions (BMW 3 Series E46)

The seat belt tensioner is installed in the front seats. It ensures that the seat belt fits tightly to the body during a frontal impact and prevents the passenger from sliding under the belt. The tensioner is activated in the event of a severe frontal impact by the electronic airbag/belt tensioner control unit. Depending on the severity of the accident, the control unit activates either only the belt tensioner or also the airbag.

A small charge is ignited in the belt tensioner by means of a control device. The gases formed during the explosion push out a piston in the tensioner tube, which is connected to the belt buckle via a cable. In this case, the belt buckle is pulled downwards in such a way that the idle movement of the shoulder and pelvic belt is reduced.

A deployed belt tensioner is indicated by the airbag/belt tensioner indicator lamp lighting up. When deployed, the seat belt operates as if the tensioner were missing. The belt tensioner does not require maintenance. After deployment, it must be replaced as a set.

Before disposing of a faulty belt tensioner, it must be forcibly rendered harmless (work of the service station).

To prevent accidental deployment of the seat belt tensioner and airbag, you must read the safety rules for handling the seat belt, refer to Section Power steering (servo control).

Safety precautions when handling the seat belt tensioner

Replacement and testing of the belt tensioner/airbag system must be carried out at a service station. No design changes are allowed.



If the belt tensioner has been triggered as a result of an accident, it must be replaced with a new one (work of the service station).

Before working on the belt tensioner, disconnect the negative cable (-) of the battery. Then insulate the negative pole of the battery to prevent accidental contact with it.

A removed but unused belt tensioner should be kept locked up during long-term vehicle repairs.

The belt tensioner unit and control unit are sensitive to impacts. If they fall from a height of more than 50 cm, they must not be reinstalled.

When welding, the welding machine terminal must be connected directly to the welded part. In any case, the control device must be disconnected from the cable.

The airbag/belt tensioner control unit is located under the center console. To avoid malfunctions, do not keep magnetizing objects near it.

The belt tensioner unit and the safety device must not be exposed to temperatures above +90°C, even briefly. Damaged components cannot be repaired. The components of the airbag/belt tensioner system must not be opened. Before installation, check again that the negative cable (-) is disconnected and the negative terminal of the battery is covered.

Never remove or dispose of the belt tensioner yourself. There is a risk of explosion!

System check

When the ignition is turned on, the airbag/belt tensioner system indicator lamp should come on and go out after 4 seconds.

If the indicator lamp does not light when the ignition is turned on, this indicates a fault in the system. If the lamp does not go out or lights up while driving, there is a fault in the airbag/belt tensioner system. It should be noted that the system is not operational.