Seat belt tensioners front and rear

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Volvo S80, 2005 Volvo S80, 2004 Volvo S80, 2003 Volvo S80, and 2002 Volvo S80. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Locating Front And Rear Seat Belt Tensioners

Courtesy of VOLVO CARS OF NORTH AMERICA.

There are five seat belt tensioners in the car, two for the front seats and three for the rear seat. They contain an explosive charge which tensions the seat belts when deployed. The seat belt tensioner tensions the seat belt to reduce the forward movement of the body. This also reduces the forces on the chest from the airbag and seat belt.

The seat belt tensioners are separated from each other electrically. This means that each seat belt tensioner has a separate individual power supply and ground from the control module. Information on whether the seat belt is engaged or not is sent from the seat belt buckle. The way in which this is done varies depending on the model year:

Model year 1999-2002: The front seat belt buckles transmit a signal to the central electronic module (CEM). The central electronic module (CEM) then sends the data onwards on the control area network (CAN) to the supplemental restraint system (SRS) module
Model year 2003-: The front seat belt buckles are directly connected to the supplemental restraint system module (SRS).

The supplemental restraint system module (SRS) determines whether to deploy the seat belt tensioners based on whether the seat belts are being used. The seat belt tensioner will not be activated if the seat belt is not engaged. However, if the central electronic module (CEM) reads off incorrect data for the front seat belt buckles, the seat belt tensioners will be activated and the air bag will be deployed. Deployment of the airbags occurs in the same way as when the seat belts are not fastened.

The seat belt tensioner consists of a pyrotechnical charge and a cylinder with a piston. The piston is connected to a steel cable which is secured to the seat belt reel. When the control module activates the seat belt tensioner the piston is forced downwards by the expanding gas. The piston pulls in the steel cable which reels in the belt.

NOTE:

Never use an ohmmeter or similar tool to test airbags or seat belt tensioners. Disconnect airbags or seat belt tensioners before testing them. Failure to do so may result in accidental deployment. This could result in serious injury or death.