Restraints and Safety Systems: Description and Operation
650404 (099)
Multiple restraint system 5
E87, E90, E91, E92
Introduction
The multiple restraint system 5 (MRS) is a direct development of the multiple restraint system fitted to BMW vehicles.
> - E87
> - E90, E91, E92
[system overview ...]
The MRS performs the following functions:
- Detects an accident situation which could be critical for the occupants
- Activates the necessary restraint systems (selectively, depending on the severity and type of accident)
New features in comparison with the previous MRS (E83):
- Side airbags are installed in the side sections of the front-seat backrests
Brief description of components
The following sensors detect the direction and severity of an impact:
- Acceleration sensors in the MRS control unit
A lateral acceleration sensor and a longitudinal acceleration sensor are integrated into the MRS control unit. The sensors are arranged at 90 degrees to each other.
- Airbag sensor on left-hand B-pillar and right-hand B-pillar
Each of the airbag sensors on the B-pillars contains a lateral-acceleration sensor and a longitudinal-acceleration sensor. The sensors are arranged at 90 degrees to each other.
- Airbag front sensors (US version only)
The 2 front airbag sensors are longitudinal acceleration sensors.
- Front left door airbag sensor and front right door airbag sensor (US version only)
The 2 airbag sensors are pressure sensors.
The following components provide input signals for the multiple restraint system (MRS):
- Seat-belt buckle switch for driver's seat and front-passenger seat
The seat-belt buckle switches provide information whether or not the seat is occupied.
The seat-belt buckle switches are receive power from the MRS control unit.
The power consumption of the switch forms the signal for the switch position (seat belt engaged or not engaged).
The signal from the seat-belt buckle switch decides whether or not the seat belt tensioner is actuated when it is needed (only in the event of a frontal/rear-end collision).
The seat belt-lock switches on the driver and front passenger seats are permanently monitored when terminal R is ON.
When terminal 15 is ON, a seat-belt warning lamp lights up and a warning sounds if the driver and passenger seat belts are not fastened.
- Passenger-seat occupancy detector
The passenger seat-occupancy detector recognises whether the seat is occupied or not. When it is loaded with a weight of more than approx. 12 kg, the resistance of the seat occupancy mat changes. The signal is sent to the MRS control unit via a direct line.
If the front passenger seat is recognised as being occupied without the seat belt buckle switch sending a signal, a fasten belt warning will be emitted.
Whether or not the belt tensioner on the front passenger side is activated depends on the status signal of the seat belt buckle switch (seat belt fastened or not fastened). If the seat belt has been inserted into the seat-belt buckle switch, the belt tensioner is also activated.
If the multiple restraint system (MRS) triggers the airbags and the front-passenger seat is unoccupied, the front-passenger airbag and the side airbag on the front-passenger side will not be triggered. The head airbag is triggered.
> - E87 from 09/2005
If the multiple restraint system (MRS) triggers the airbags, the front-passenger airbag and the side airbag on the front-passenger side will be triggered even if the front-passenger seat is unoccupied.
- Passenger seat occupancy detector (US version)
In accordance with the stipulations of the National Highway Traffic Safety Administration (NHTSA) a small child in a child restraint system on the front-passenger seat must be automatically detected. Upon detection, the airbags on the passenger side are automatically deactivated (front-passenger airbag and side airbag). The indicator lamp for front-passenger airbag deactivation lights up.
The seat-occupancy detector is based on an evaluation of the surface pressure generated when the seat is loaded.
The following control units are involved in the function of the MRS:
- MRS control unit
All gas generators and sensors in the multiple restraint system (MRS) are connected directly to the MRS control unit.
The MRS control unit evaluates the data it receives from the sensors. In the event of a collision (head-on, rear-end or side impact), the MRS control unit will decide whether the seat-belt tensioners and airbags need to be triggered, and which airbags need to be triggered.
- DME or DDE: Digital engine electronics or digital diesel electronics
When an airbag is triggered, the electric fuel pump is switched off by the DME or DDE
- Instrument cluster
The instrument cluster generates the optical and audible seat-belt warning.
The MRS control unit controls the seat-belt warning using a Check-Control message.
The MRS control unit sends the status information to the instrument cluster via the body CAN following terminal R ON. Depending on the status, the optical and audible warning is triggered when terminal 15 is ON.
- JBE: Junction box electronics
The JBE is the data interface (= the gateway) between the K-CAN and the PT-CAN. (K-CAN = body CAN; PT-CAN = powertrain CAN.)
The junction box consists of the JBE and the power distributor.
The JBE is connected to the MRS control unit via the K-CAN.
The JBE is the executing control unit for the central locking system. (The CAS control unit is the master control unit for the central locking system. )
The JBE is responsible for controlling all central locking drives.
The central locking is unlocked in the event of an accident exceeding a certain level of severity.
- CAS: Car access system
The MRS control unit receives power from the CAS control unit (when terminal R is ON).
The CAS control unit is the master control unit for the central locking system. (The JBE is the executing control unit for the central locking system.)
The central locking is unlocked in the event of an accident exceeding a certain level of severity.
- FRM: Footwell module
The footwell module (FRM) controls the interior lighting and the exterior lighting.
The interior lighting and the hazard warning lights come on automatically in the event of an accident exceeding a certain level of severity.
The following components are activated by the MRS control unit:
- Triggering circuit for the airbags
With maximum equipment, the MRS (Multiple Restraint System) comprises the following triggering circuits:
- Driver's airbag (stage 1)
- Front-passenger airbag (stage 1)
- Driver's airbag (stage 2)
- Front-passenger airbag (stage 2)
- Side airbag in driver's seat
- Side airbag in front front-passenger seat
- Head airbag, left
- Right-hand head airbag
- Belt tensioner, front left
- Belt tensioner, front right
- Safety battery terminal
- Belt tensioner, rear left
- Belt tensioner, rear right
Driver and front-passenger airbag
The driver and front-passenger airbag reduces the risk of injury to the occupants' head and chest area in the event of a head-on collision.
The driver airbag is located under the centre pad on the steering wheel. The front-passenger airbag is integrated into the instrument panel above the glove box.
The gas generators in the airbags are triggered as needed, depending on the severity of the accident.
Side airbag
For the first time, BMW has installed side airbags in the side sections of the front seat backrests.
The relocation into the backrest side section has the advantage that there are no longer airbags in the doors, allowing the engineers more freedom when designing the doors.
Both versions -airbags in doors and airbags in seats- satisfy the strict legal requirements with regard to occupant safety to the same degree.
Side airbags reduce the risk of injury to the occupants' pelvis and torso in the event of a side impact.
Head airbag
The head airbag prevents the head making direct contact with the side structure or an object which penetrates the vehicle.
[more]
Belt tensioners
As a rule, the seat belt will not be sitting completely tight around the body: This seat belt slack makes sure that the occupants have adequate comfort of movement.
To prevent the negative effect that this loose belt can have, the pyrotechnic seat-belt tensioner will pull the belt buckle down by several centimetres in the event of a frontal or rear-end collision. This pulls the seat belt tight into the occupant's shoulder and pelvis.
Safety battery terminal
Depending on the severity of the accident, the safety battery terminal disconnects the starter motor and the alternator from the battery. This minimises the risk of short circuits in a serious accident. The trigger signal comes from the MRS control unit.
- Airbag indicator lamp
The airbag warning lamp shows the operational capability of the multiple restraint system (MRS). The airbag indicator lamp in the instrument cluster is activated by the MRS control unit (via the K-CAN).
After the ignition is switched on, the MRS performs a self test. During this time, the airbag indicator lamp will light up (approximately 3 to 5 seconds). When the system is ready for operation, the airbag indicator lamp goes out.
If the instrument cluster has not received a CAN message, the airbag indicator lamp will light up.
If, during the self test or while the vehicle is in motion, the MRS control unit detects an existing fault or one that has been stored, the airbag warning lamp remains on.
If the MRS detects a fault, the operational capability is partially maintained, subject to the following:
- If a fault is detected in an MRS circuit, only the affected circuit is deactivated. The other airbags and belt tensioners remain operational.
- If there is a fault in the airbag warning lamp circuit, the lamp does not light up during the self-test. If there is no other fault, the MRS remains fully operational.
If there is an internal fault in the MRS control unit or in the power supply, the entire system is deactivated. (The airbag and seat-belt indicator lamps light up, visual display in the LCD)
- Seat belt indicator lamp
The seat belt indicator lamp is the visual seat belt warning. The visual seat belt warning instructs the occupants of the vehicle to fasten their seat belts. Terminal 15 ON activates the visual seat belt warning.
The visual seat belt warning is issued as follows:
- Seat-belt indicator lamp in the instrument cluster (required by law)
- Check-Control message in the LCD
- Deactivating front-passenger airbag and side airbag on front-passenger side (not in US version)
If Option 470 "ISOFIX mounting for child seat" (front) has been installed, the airbags can be deactivated via the switch for passenger airbag deactivation (as on the E85).
In switch position OFF, the following airbags on the front-passenger side will not be triggered: front-passenger airbag and side airbag.
The indicator lamp for front-passenger airbag deactivation (front-passenger airbag OFF lamp) lights up.
The switch is located on the right-hand edge of the dashboard. The switch is accessible when the front-passenger door is open. The switch is operated using the key integrated into the remote control.
- Indicator lamp for passenger airbag deactivation (US version; option 470 ISOFIX mounting for child seat)
If the warning lamp for passenger-airbag deactivation (passenger airbag off lamp) is lit, the following airbags on the passenger side have been deactivated: front-passenger airbag and side airbag.
The "Passenger Airbag OFF" lamp is constantly monitored by the MRS control unit. A fault in the power supply or a defective lamp is stored in the MRS control unit's fault memory. If this occurs, the airbag warning lamp is switched on.
The "Passenger Airbag OFF" lamp is in the roof function centre.
- Shutting off the fuel pump
Depending on accident severity, the fuel supply may also be cut off.
- TCU: Telematics control unit
The emergency call is transmitted via the TCU. The TCU is connected to the MRS control unit by the instrumentation bus (I-bus).
Mechanical components in the multiple restraint system (MRS):
- Belt force limiters on seat belts
The seat belts are the primary restraint system for all occupants. Each of the 5 seats is fitted with a three-point safety belt.
To minimise the load on the chest area of the rear passengers in the event of a severe head-on collision, the front seat belts are fitted with belt force limiters as standard. The belt force limiters ensure that the belt strap can unroll if the load exceeds a certain level. The risk of injury due to the belt force acting on the body is therefore reduced.
- Passive knee protection (US version only)
Not every state in the USA has a legal requirement for seat belts to be worn. Therefore, US versions have additional impact protection in the knee area of the driver and front passenger.
In the event of an impact, this knee protection provides a flat support for the knee if the front passengers are not wearing their seat belt. This causes the forwards movement of the upper body to be controlled towards the relevant airbag (driver or passenger airbag).
System functions
The following system functions of the multiple restraint system (MRS) are described:
- Triggering the multiple restraint system
- Visual and audible seat belt warning
- Emergency call (EURO version)
- Emergency call (US version)
Triggering the multiple restraint system
Extensive testing has allowed the triggering thresholds to be determined for all possible types of accidents.
The following triggering thresholds were set for activating the different restraint systems:
- Seat belt tensioners
- Front airbags
- Side airbags
- Head airbags
- Safety battery terminal
- Electric fuel pump
The following examples illustrate how the various restraint systems are triggered:
- Head-on collision
The belt tensioner and the front airbags are activated if frontal impact is sufficient to exceed the triggering thresholds in the MRS control unit.
The belt tensioners have a lower triggering threshold than the front airbags. This means that depending on accident severity, only the belt tensioner may be activated by the MRS control unit.
If the seat-belt buckle switch is faulty, the MRS control unit will attempt to activate the belt tensioner in spite of the error message.
If the seat-occupancy detector is faulty, the control unit assumes that the passenger seat is occupied. The restraint systems are activated. Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
- Side impact
If the triggering threshold in the MRS control unit is exceeded, the side airbag and the head airbag on the side affected by the impact are activated.
Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
- Front diagonal impact
The airbags and belt tensioners are triggered depending on the rate of vehicle deceleration in a longitudinal or lateral direction.
If several triggering thresholds are exceeded, the corresponding restraint systems will be triggered for frontal collision and/or side impact (front airbag, side airbag, head airbag). Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
- Rear-end collision
The belt tensioners are activated if rear impact is sufficient to exceed the triggering threshold in the MRS control unit.
Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
- Rear diagonal impact
The airbags (side airbags) and belt tensioners are triggered depending on the rate of vehicle deceleration. If a number of triggering thresholds are exceeded, the appropriate restraint systems are activated.
Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
- Rollover
The MRS control unit does not detect the vehicle rolling over. If the vehicle rolls over in a collision, causing the triggering thresholds in the MRS control unit to be exceeded, the appropriate restraint systems are activated.
Depending on accident severity, the safety battery terminal is activated and the electric fuel pump switched off.
Visual and audible seat belt warning
For the seat belt warning, the signals of the 2 seat belt buckle switches (driver's belt/front-passenger belt) are monitored separately.
If the seat occupancy detector detects that the passenger seat is occupied, the front passenger's belt must be inserted into the seat belt buckle. This switches off the seat belt warning.
The following situations are taken into account in respect of the issuing of a seat belt warning:
- Front seat belt not fastened and distance driven less than 200 m
If less than 200 m has been driven, only a visual seat belt warning is issued (the seat belt indicator lamp lights up). The vehicle may for example be driven out of a garage without triggering an audible seat belt warning.
- Front seat belt not fastened and distance driven more than 200 m
The seat-belt indicator lamp and the visual display in the LCD light up (visual seat-belt warning). The audible seat-belt warning is activated for approximately 90 seconds.
Once the seat belt has been fastened, the visual seat belt warning disappears. The audible seat belt warning is deactivated.
If the seat belt has not been fastened by the end of the audible seat belt warning, the seat belt indicator lamp lights up.
- Unfastening the front seat belt when the vehicle is in motion
If a seat belt is unfastened when the vehicle is in motion, the seat belt warning lamp will light up immediately.
The visual display also lights up in the LCD after approximately 15 seconds. An audible seat belt warning sounds for approximately 90 seconds.
Once the seat belt has been fastened, the visual seat belt warning disappears. The audible seat belt warning is deactivated.
If the seat belt has not been fastened by the end of the audible seat belt warning, the seat belt indicator lamp lights up.
Note: Deactivating the acoustic seat belt warning
The acoustic seat-belt warning can be deactivated using coding.
This does not affect a legally stipulated seat belt warning.
Emergency call (EURO version)
- Automatic emergency call:
In vehicles equipped with BMW Assist, the multiple restraint system (MRS) has an additional function: the automatic emergency call.
In the event of an accident exceeding a certain level of severity, BMW Assist automatically initiates an emergency call. The vehicle data and its exact location, which is determined by the navigation system, are transmitted to the emergency call centre.
Emergency call (US version)
- Manual emergency call
A manual emergency call is initiated by the emergency call button. The emergency call button is directly connected to the telephone. Pressing the emergency call button establishes a voice connection with the provider. The emergency call button is integrated into the roof control panel (FZD).
- Automatic emergency call:
If the accident is of corresponding severity, the MRS control unit sends a signal to the Telematics Control Unit (TCU). The TCU initiates an emergency call which also contains the location of the vehicle. The TCU receives the location determination for the navigation system. If a navigation system has not been installed, the location is determined by an internal GPS receiver in the TCU.
Notes for service staff
- General note:
- Diagnosis:
- Encoding/programming: ---
Subject to change.