Description & Operation: Notes

WARNING: This page is about a different car, the 1995 BMW 540i, 1995 BMW 530iT, 1995 BMW 530i, 1995 BMW 525iT, and 1995 BMW 525i. However, it is still accessible from the selected car via links, so may be relevant.

WARNING: To avoid injury from accidental air bag deployment, read and carefully follow all WARNINGS and SERVICE PRECAUTIONS.

The main components of Supplementary Restraint System (SRS) include an inflatable air bag/gas generator module, electronic SRS control unit, contact ring, energy capacitor/voltage transformer (integrated into SRS control unit), SRS warning light, driver-side knee bolster mounted under steering column, and in some vehicles, seat occupancy module, seat belt contacts, and Emergency Tensioning Retractor (ETR) seat belt assemblies. Although the SRS is similar for all vehicles, component locations and applications vary by model. See VEHICLE EQUIPMENT table. Passenger-side air bag module is installed inside dash, above glove box. See Fig 1. For SRS to operate properly, 3-point seat belts must be worn.

Fig 1: Locating SRS Components (Typical)

Courtesy of BMW OF NORTH AMERICA, INC.

VEHICLE EQUIPMENT

Application	Equipment
500 Series	Dual Air Bags, Seat Belt Contacts and Seat Occupancy Module

When SRS control unit senses an impact, it transmits a voltage signal to gas generator pellets in air bag modules and the ETR seat belt assemblies. The voltage signal ignites gas generator pellets to produce a quantity of non-toxic nitrogen gas.

In the air bag modules, the nitrogen gas passes through a tiny filter to fill and deploy the air bags in about 30 milliseconds. After the air bags inflate to maximum expansion, the nitrogen gas is released away from driver through 2 ventilation holes in rear of air bag. The air bag inflation and collapse sequence takes about 30-35 milliseconds.

In the Emergency Tensioning Retractor (ETR) seat belt assembly, the ignited nitrogen gas forces a piston to be driven downward in a tube attached to seat belt assembly. Piston has a cable attached to it. Cable pulls on seat belt reel, increasing seat belt tension and holding front seat occupant(s) tightly against seat backrest.