BMW provides a Supplemental Restraint System (SRS) for the protection of the driver. An air bag is used in conjunction with the standard seatbelt to lessen the possibility of injury in case of a frontal, or near frontal, collision. The E36 3 Series and the E34 5 Series also provide an automatic belt tensioner system to take up slack from the seatbelt at the same time as air bag deployment.
The SRS can only be of use when used in conjunction with the standard seatbelt. This is why it is considered supplemental. Use of the standard seatbelt is mandatory for the best possible protection from all the safety devices built into a BMW vehicle.
See Figure 1
In the case of a frontal collision, as defined by an area 30 degrees left and right of the centerline of the vehicle and traveling at least 12 mph (20 km/h), the air bag will deploy and if equipped, the seatbelt tensioner will operate. This will provide a cushion for the upper body of the driver and will draw the seatbelts tight providing additional restraint.
The system consists of crash sensors, an air bag in the steering wheel, a control unit, steering wheel contact ring and seatbelt tensioners on vehicles so equipped. All the components and wiring are colored orange to indicate the component being part of the SRS.
When either of the sensors are activated by a collision with a solid object and the safing sensor is also activated, the control unit sends current to the air bag and the seatbelt retractors. The current sets off the ignitors within the air bag and seatbelt retractors. The solid fuel burns, generating a harmless gas that fills the air bag extremely quickly. The solid fuel in the seatbelt retractor burns and the resulting gas pushes a piston down, taking the slack out of the seatbelt.
The air bag deploys from the center pad of the steering wheel. It fills with gas to full inflation within 30 milliseconds. The deployment sound is that of a loud bang. The inflation rate is so quick, that by the time the occupant strikes the air bag, the air bag is already deflating. This allows a controlled release of energy, preventing injury to the occupant. The occupant will only realize that the air bag deployed after the air bag is fully deflated and has done its job.
Some common misconceptions about air bags have crept into the common knowledge and should be corrected. The air bag will not suffocate the occupant, nor will it block the view of the occupant; the inflation and deflation times are too quick for this to occur. The dust produced after the air bag is deployed is not harmful; it is a combination of the burnt fuel and the talc powder that some air bags are packed in to facilitate smooth deployment. A parking lot tap will not deploy the air bag, unless it is a frontal collision above 12 mph (20 km/h). An air bag is not to be used in lieu of a seatbelt; it must be used in conjunction with a seatbelt. Being rear ended will not deploy the air bag. The air bag can not protect the occupant in a side impact, only the seatbelt can provide any measure of restraint.
Always wear your seatbelt! If it is good enough for use by every racecar driver, it should be good enough for you.
When the ignition switch is activated, the SRS light will illuminate for 6 seconds and extinguish. This indicates normal operation. If the light does not illuminate, illuminates after the 6 second period or flashes for 5 minutes during driving then stays lit, there is a problem with the SRS. Have the system checked by a qualified BMW technician. The SRS does not lend itself to being serviced by a non qualified person. If the SRS is activated, the system must be replaced, as it can only be used once.
The control module is located under the left side of the dashboard on the E30 3 Series and the E34 5 Series. The control module on the E36 3 Series is located behind the glove compartment.
The control module monitors the entire SRS and will warn the driver of malfunctions by illuminating the SRS lamp on the dashboard. The control module monitors the operation of the crash sensors and will activate the air bag and seatbelt retractors.Crash Sensors
See Figures 2 and 3
The crash sensors are mounted on the left and right inner fenders. The sensors will complete a circuit to the control module if the impact is great enough. The sensors must be mounted with the arrows facing forward. they should not be tampered with or disassembled.
The control unit a safing sensor built into it. The safing sensor only allow deployment in cases where all criteria for deployment are met. As a result, during an accident, the safing sensor and at least one of the inner fender mounted sensors must be activated.Air Bag
The air bag is packed in the center pad of the steering wheel assembly. There is an ignitor and solid fuel that produces the gas to fill the air bag. As the fuel starts to burn and the air bag begins to expand, the air bag breaks through the outer skin of the steering wheel center pad. The air bag unfolds and fills to full expansion. Two vents on the back side allow the gases to exit the air bag and release energy.
The air bag is fully deployed within 30 milliseconds and is deflating by the time the occupant strikes the bag. This is to control the levels of forces that the occupant is subjected to and to provide the best possible level of occupant protection.Seat Belt Tensioners
The are 2 styles of seatbelt tensioners. The E34 5 Series uses a pyrotechnic style ignitor and solid fuel, similar to the air bag. The E36 3 Series uses a mechanical system that operates independently of the air bag. The seatbelt tensioners operate on both the drivers and passenger seatbelts.
On the E34 5 Series, at the same time the air bag is deployed, the ignitor and solid fuel of the seatbelt tensioner is deployed. The gases produced by the burning fuel in the seatbelt tensioner force a piston down, pulling a steel cable that is attached to the seatbelt reel. The seatbelt is pulled taut, providing a more positive restraint for the occupant
The mechanical system on the E36 3 Series is similar in operation to the pyrotechnic system of the E34 5 Series, except the solid fuel and ignitor is replaced by a spring and inertia lock. If the force of the accident is great enough, the inertia lock built into the seatbelt reel assembly, will release the spring, pulling the seatbelt taut.Steering Wheel Contact Ring
The purpose of the steering wheel contact ring is to provide a continuous electrical path to the ignitor in the air bag. The contact ring allows rotation of the steering wheel while providing the electrical path in an uninterrupted fashion.
DISARMING THE SYSTEM
- Place the ignition switch in the OFF position.
- Disconnect the negative battery terminal and cover the battery terminal to prevent accidental contact.
- Disconnect the crash sensors in the engine compartment.
- Remove the lower cover of the steering column and disconnect the orange connector. The E30 3 Series has a small panel on the bottom of the steering column that when pulled down, holds the connector.
- On the E34 5 Series, disconnect the seatbelt tensioner connectors.
ENABLING THE SYSTEM
- Place the ignition switch in the OFF position.
- Connect the sensors, the steering column connector and the seatbelt tensioner connectors.
- Connect the negative battery terminal.
- Place the ignition switch in the ON position. Check that the SRS light illuminates for 6 seconds and extinguishes. If it illuminates in any other pattern, there is a problem that needs to be rectified by a qualified BMW technician.