Components

MRS/MRS II COMPONENTS

CONTROL MODULE
The control module is located along the center line of the vehicle, generally in the area of the center console depending on model and year. It contains the following circuitry:
- Electronic acceleration sensor for crash detection
- Processing electronics/final stages for airbag activation
- Self diagnostics for self test and system monitoring
- Internal mechanical safety switch - must close in conjunction with acceleration sensor before airbag activation







Replacement control modules must be coded when installed in the vehicle using the DIS tester or MoDiC.

FRONT PASSENGER AIR BAG ASSEMBLY




The front airbag is designed to protect the head and upper part of the body in the event of a frontal impact. The assemblies consist of a gas generator, ignition pill and the folded airbag.

SIDE AIR BAG ASSEMBLY (FRONT/REAR)




The side airbag is designed to protect the upper part of the body (chest/thorax) in the event of a side impact. The assembly consists of a gas generator, ignition pill and folded airbag. The assembly is bolted to the inner door panel and the trim cover incorporates a break away airbag cover.

2-STAGE PASSENGER AIRBAG
The E38 and E39 received the 2-stage airbag system as of 9/98 production.




The passenger's airbag is the only modification to the system. The remainder of the MRS II systems are carried over from the 1998 models. The system was modified as follows:
- New pressure accumulator generator
- New airbag - volume of 105 liters
- Two ignition capsules - for two stage activation
- Propellant gas made from 13.5% hydrogen and 86.5% oxygen
- Deployed units can be disposed of as scrap metal

The use of two ignition stages, coupled with the lower volume and new propellant, optimizes the deployment of the airbag and makes it less aggressive when the airbag inflates.

The main component is the gas generator which mounts below the airbag on the passenger's side of the dashboard. The gas generator consists of a pressure chamber with an ignition set at each end. A hose connects the airbag with the pressure chamber.

PRESSURE CHAMBER
The propellant charge in the chamber is a mixture of approximately 13.5% Hydrogen and 86.5% Oxygen under pressure.

IGNITION STAGE ONE




The main ignition capsule consists of the ignitor, combustion chamber, the impact pin and the sealing plate.

IGNITION STAGE TWO




The second ignition capsule consists of an ignitor, ball seal and combustion tube.

AIRBAG ACTIVATION




Ignition stage one is always the first to be ignited during an impact. Ignition stage two is ignited after a time threshold that is programmed in the MRS II control module. The two stage activation is optimized by the airbag's inflation curve for each model.

When the ignition capsule is ignited, the impact pin is pushed against the sealing plate and the passageway for inflating the airbag is opened. The flame from the ignition capsule ignites the propellant gas and the expanding gas is forced through the tube into the airbag.

After the timed threshold, the second stage is ignited and the ball seal is forced off of its seat. The flame from the second stage ignites the propellant on the opposite side of the pressure chamber and causes further expansion of the gasses. This ensures that the required volume of gas is produced by the pressure chamber to fully inflate the airbag.

After inflation, the gasses (steam) are allowed to escape from the airbag as with previous generations. The escaping gasses are not harmful to the passengers of the vehicle.

HEAD AIRBAG ASSEMBLY (ITS)




The ITS is designed to protect the head in the event of a side impact. The assembly consists of a hermetically sealed rubber tube that is encased by a cross woven tubular nylon material, a gas generator and ignition pill.

The ITS is mounted from the "A" pillar , at the front, to the roof panel slightly behind the "B" pillar. The ignitor is mounted to the "A" pillar and is connected to the ITS through a reinforced sealed tube. The ITS is stored behind the roof panel trim over the front doors.

When triggered, the inflation charge causes the diameter of the tube to expand and its length to shorten. As the tube expands, it is forced out of the stored position and it drops over the side door glass to protect the occupants head against impact. The ITS remains fully inflated for several seconds after it is triggered due to the sealed expansion tube. This allows the ITS to continue to provide protection in the event of a secondary impact. The tube will deflate after the gas cools.

The expandable tube of the ITS is the same for all models. However the securing strap at the "B" pillar end is a different length for the different models. For this reason, the ITS is part number specific for each model and cannot be interchanged.

BATTERY SAFETY TERMINAL (BST)







The BST is designed to disconnect the battery from the starter, generator and B+ terminal in the engine compartment during collisions. This passive safety component is designed to reduce the possibility of short circuits to ground occurring from these high amperage circuits.

However, other systems that require power to function during and after a crash continue to receive power from the positive battery post. These circuits include:
- ABS - provides continued operation if the vehicle is still in motion
- MRS II - remains operational for crash protection
- General Module - for unlocking the vehicle and interior light activation
- Lamp/Check module - for hazard warning light activation
- Telephone - provides communication after the crash
- Board Monitor - with navigation system for communication with Cross Country Group for assistance

BST CONSTRUCTION







The BST is an encapsulated pyrotechnic device that is similar to an airbag in operation. The BST contains an ignitor capsule and discharge tube that is mounted in front of the high amperage B+ cable. The B+ cable contains a tapered press fit contact that mounts behind the gas discharge tube.

During activation, the tapered B+ cable contact is forced away from the battery post and held away by spring tabs within the capsule.

BST ACTIVATION







Once activated, the ignitor generates a gas charge which is directed down the internal discharge tube

This causes the tapered end of the B+ cable contact to dislodge from its seated position and immediately open the circuit.







The force of the charge continues to push the cable contact away from its seated contact. The spring tabs of the housing are compresses as the contact pushes to the end of its travel.







When the contact hits the end stop of the housing, it bounces back against the return spring which prevent it from closing the contact with the battery.

SIDE IMPACT SENSORS




Electronic (piezoelectric) sensors are used for side impact detection. They are mounted at the left and right footwell cross members in front of the driver's and passenger's front seats. The sensors produce a pulse width modulated input signal to the MRS control module for side impact detection. The pulse width of the signal decreases in proportion to the severity of the impact.




The control module uses the internal crash as a plausibility check for deployment of the side impact and ITS airbags. Both sensors must detect that the trigger threshold has been exceeded before the bags will deploy.

SEAT OCCUPANCY SENSOR (SBE)
An occupancy sensor pad is incorporated into the front passenger's seat cushion. The pad is a foil contact sensor that is open when the seat is not occupied. When the seat cushion is pressed by the weight of a passenger, the sensor circuit produces a resistance that varies with the weight and/or movement.




This variable resistance signal is input to the Seat Occupancy Module (SBE). The SBE is a small signal converter module, mounted under the passenger's seat. It converts the analog signal, from the seat pad into a square wave modulated output signal that is sent to the MRS control module. The MRS control module uses this signal for air bag triggering in the event of a collision.

SEAT BELT TENSIONERS
Pyrotechnic seat belt tensioners are used as a method of reducing the spooling effect of the seat belts during a collision.




The seat belt latch is connected to the pyrotechnic device through a cable. The control module is responsible for triggering the belt tensioner based on its programmed parameters.

The pyrotechnic device is similar to an airbag assembly in that when the control module triggers the belt tensioner, a small gas charge is ignited in the tensioner assembly and the pressure forces the cable to pull the latch tight.

SEAT BELT SWITCHES
The seat belt switch signals are input signals used by the control module for its deployment logic. Either switch contacts or hall sensors are used for this input, depending on the model year.