The multiple restraint systems MRS 3 and MRS4 consist of a control unit, two sensors for detecting a side collision, one sensor for detecting the seat occupancy of the front passenger seat and gas generators for activating the airbags.
The multiple restraint system MRS4 is a further development of the MRS3 system.
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Components
Control unit
All gas generators and sensors are connected with the MRS control unit. The control unit collects the data of the sensors and evaluates it. In the event of a crash in the front or side area, the internal control unit logic determines whether an airbag deployment is necessary and which airbags must be activated, if necessary.
For data acquisition of the crash, acceleration sensors housed in the MRS control unit assess the accident situation using the occurring deceleration values. In the event of a side collision, the external side sensors (satellites) are additionally evaluated.
In addition to all inputs and outputs, the multiple restraint system control unit also monitors the internal components. Possible fault states are permanently stored in the control unit and displayed to the driver via the airbag indicator light.
Firing circuits
The multiple restraint system currently consist of - with maximum equipment - the following 15 ignition circuits:- | Driver airbag (stage 1) |
- | Front passenger airbag (stage 1) |
- | Driver's airbag (stage 2) |
- | Front passenger airbag (stage 2) |
- | Side airbag, front left |
- | Side airbag, front right |
- | ITS left (ITS = Inflatable Tubular Structure) |
- | ITS right |
- | Side airbag, rear left |
- | Side airbag, rear right |
- | Head airbag, rear left |
- | Head airbag, rear right |
- | Belt tensioner, driver's side |
- | Belt tensioner, passenger's side |
- | Battery disconnection |
Airbag indicator light (AWL)
The airbag indicator light indicates the operability of the entire multiple restraint system to the driver.
The MRS control unit starts a self-test after the ignition has been switched on. During this time (approx. 3-5 seconds), the airbag indicator light illuminates and then goes out if no fault was found.
If the multiple restraint system control unit detects a current or stored fault during the self-test or while the vehicle is being driven, the airbag indicator light lights up continuously.
Seat belt buckle query (USA only)
A seat belt buckle query is installed for vehicles with the USA national-market version.
![Firing Circuit Driver Airbag Stage 2 Firing Circuit Driver Airbag Stage 2](/uploads/1/2/6/3/126334841/423496469.jpg)
Via a switch in the seat belt buckle, the multiple restraint system receives information about whether the seat belt was fastened.
Using the seat belt query, the control unit can decide whether the belt tensioner needs to be activated for a crash and whether the airbag needs to be activated with interval stage 1 (belt not fastened) or interval stage 2 (belt fastened).
Satellites (sensors for side airbags)
The satellites are sensors that are attached in the side areas to the cross members below the driver and front passenger seats. They are connected with the multiple restraint system control unit via a data line.
In the event of a side collision, an activation of all airbags of the side of the vehicle facing the crash can occur.
An activation of the side and head airbags only occurs, however, if both the sensor as well as the multiple restraint control unit have detected an acceleration.
Seat occupancy detector (SBE)
A seat occupancy mat is located under the front passenger seat surface that changes its electrical resistance as the weight load increases. The seat occupancy electronics evaluate this change and determines the ”seat occupied” or ”seat unoccupied” status. The result is then sent to the multiple restraint system control unit.
According to this information, the control unit can - in the event of a head-on collision - activate the ignition circuit for the front passenger airbag and belt tensioner when the front passenger seat is occupied.
Bus connection
The multiple restraint system control unit is connected to the body bus for all series in which a body bus is present. For series that do not have a body bus (Z3 and Z3 Coupé), there is a design with connection to the diagnostic bus (TxD line).
The diagnosis and encoding occurs via the bus.
Function
The multiple restraint system has the task of determining the type of crash with the help of the sensors. Within a split-second, the information is then evaluated, forwarded to the corresponding gas generators and finally, the airbags are activated.
Using the recorded acceleration values, the size of the crash is determined and one or multiple ignition circuits are activated depending on this.
NOTE: Even with extreme driving situations, no undesired activation of the airbag may occur.
Front airbags
The front airbags (driver and front passenger airbag) support the function of the seat belts and prevent the driver's and front passengers heads from hitting against the steering wheel resp. the dashboard during a head-on collision.
If the control unit detects that the front passenger seat is not occupied, the front passenger airbag and the belt tensioner of the front passenger side are not activated.
Two-stage airbags are used for the driver and front passenger.
Depending on the detected size of the crash, only interval state 1 of the front airbag is activated or interval state 1 and interval stage 2 are activated. There is a variable delay period between the activation of the two airbag interval stages which is also determined via the acceleration values.
The USA national market version also additionally differentiates whether the passengers have their seat belts fastened. If the passengers do not have their seat belt fastened, the triggering threshold is reduced so that the airbags are activated earlier in interval stage 1 and interval stage 1/2.
Side airbags, inflatable tubular structure front, head airbags rear
The side and head airbags protect the passengers in the event of a side collision. The side airbag above all serves to support the body in the side chest area. ITS front (ITS = Inflatable Tubular Structure) and head airbags rear help prevent injuries to the head and neck area.
For all side airbags, ITS and rear head airbags, the left and right vehicle sides are handled separately.
In the event of a side collision, all side airbags, ITS and head airbag of the side facing the crash are simultaneously activated.
The following applies for the activation of the airbag on the front passenger side during a side collision:
- System MRS3, supplier Temic: The activation of the airbags (side airbag, ITS, head airbag) occurs independently from the seat occupancy of the front passenger seat.
- System MRS3, supplier Bosch and MRS4: No activation of the side airbags for unoccupied front passenger seat.
Seat belt tensioner
The belt tensioner ensures during a collision that the seat belt fits considerably tighter and keeps the body securely in the seat. Within a split-second, the seat belt buckle - aided by the gas pressure - is retracted and shoulder and lap belt are thus simultaneously tightened. This prevents slipping under the lap belt from occurring to a large extent.
The activation of the belt tensioner occurs both for a frontal as well as a rear-ended crash if the triggering threshold for the belt tensioner activation was exceeded.
Safety battery terminal (battery terminal disconnection)
To reduce the risk of fire in the event of a head-on collision, the battery terminal disconnection ignition circuit is also activated.
A separate dedicated gas generator (safety battery terminal) disconnects the high current line (terminal 30) from the battery to the starter motor/alternator.
E46 4-cylinder modelsLeft hand drive vehicle
Since no increased risk of fire can be observed when the battery is installed in the engine compartment, the safety battery terminal is dispensed with as of March 2002 (03/2002) for all E46 left hand drive vehicle models with 4-cylinder engines.
The MRS in the workshop
Caution!
All work on the airbag must only be conducted with the battery disconnected!
Connect and disconnect control unit, sensors and inflator assemblies (gas generators) only with the battery disconnected!
The MRS control unit contains vehicle-specific data and must therefore be encoded before commissioning!
Removal and installation in other vehicles is prohibited!