2001-06 GENERAL INFORMATION: Multiple Restraints System - Overview - X5

Multiple Restraint System (MRS III) Introduction

The X5 is equipped with the Multiple Restraint System (MRS III), which employs "SMART" technology. Smart technology refers to the control module's programming that allows for the deployment of airbags in stages, depending on the severity of the impact. Two-stage airbags are used for both the driver and front passenger, providing a softer cushioning effect when bags are triggered at lighter impacts.

The MRS III system is installed in E38/E39 and E46 Sedan vehicles as of 3/99 production, and in E46 Coupes as of 6/99 production.

MRS III control modules are manufactured by either Bosch or Temic. While the functional operation of both modules is the same, the control modules are not interchangeable from a replacement standpoint.

In addition to the use of two-stage airbags for the driver and passenger, the MRS III system includes the following features:

• The MRS III control module is linked to the K-Bus for coding and diagnosis.
• The MRS III includes a fuel pump cut-off feature in the event of an airbag deployment.
• Inert gas generators are used for all airbags and seat belt tensioners.
• The inert gas is a mixture of hydrogen (13.5%) and oxygen (86.5%).

Components

MRS III Control Module

The control module is mounted in the center console area on the driveshaft tunnel, below the center storage box. The X5 utilizes the TEMIC system, recognizable by the green 50-pin connector on the module.

The control module contains the processing electronics (Smart Technology) for triggering all airbags and pyrotechnic devices installed in the vehicle. Two electronic deceleration sensors are installed in the module for crash or impact detection.

[Image: MRS III Control Module. Courtesy of BMW OF NORTH AMERICA, INC.]

Satellite Sensors

The satellite sensors are mounted below the driver's and passenger's front seats on the seat frame. Their function is to detect the severity of side impacts and signal the MRS III control module via a pulse-modulated signal in the event of a crash. The control module uses this input signal, along with its internal impact sensor signal, to determine the deployment of the side/head airbags.

As with the control modules, the satellite sensors are manufacturer-specific. The Temic sensors in the X5 have a three-wire connector that will not interchange with Bosch sensors. Only two of the wires are used for the satellite sensor's operation. The signal for airbag deployment is carried over the power wire of the sensor.

[Image: Satellite Sensors. Courtesy of BMW OF NORTH AMERICA, INC.]

[Diagram: Satellite Sensors Communication Diagram. Shows driver's and passenger's side impact sensors connected to the MRS III module, with a crash signal output. Courtesy of BMW OF NORTH AMERICA, INC.]

Driver's Front Airbag

With the MRS III system, the driver's front airbag is a two-stage bag, similar to the passenger's front side bag introduced on the 1999 model E38/E39s. The complete assembly is mounted beneath the cover in the center of the steering wheel, as with previous airbags. The assembly now contains the inert gas generator chamber and two ignition stages (ignitors).

The airbag consists of:

• Accumulator/gas generator
• Two ignition capsules
• Propellant gas - 13.5% hydrogen/86.5% oxygen
• 64-liter airbag

[Images: Driver's Front Airbag. Shows the exterior of the airbag and its internal components. Courtesy of BMW OF NORTH AMERICA, INC.]

Passenger's Front Airbag

The passenger's front airbag is the same unit as installed on E38/E39 vehicles as of 9/98 production. The passenger's airbag consists of:

• Pressure accumulator/gas generator
• Two ignition capsules - for two-stage activation
• Propellant gas - 13.5% hydrogen/86.5% oxygen
• 105-liter airbag

[Image: Passenger's Front Airbag. Shows the airbag module and its components. Courtesy of BMW OF NORTH AMERICA, INC.]

[Image: Pressure Accumulator And Two Ignition Capsules. Courtesy of BMW OF NORTH AMERICA, INC.]

Side Airbags Front/Rear (Thorax)

The side airbags continue to be mounted in the door panels on the front and rear doors. Deployment of the side airbags is dependent on the triggering thresholds programmed in the MRS III control module, based on inputs from the satellite sensors and the internal crash sensor. The side airbags use the same cold gas inflation method as the driver's and passenger's front bags.

[Image: Side Airbags Front/Rear (Thorax). Courtesy of BMW OF NORTH AMERICA, INC.]

Head Airbag (ITS)

The head airbags are similar to the ITS bags used on the MRS II system. They are mounted from the "A" pillar up along the headliner and are anchored behind the "B" pillar. The ITS bags of the MRS III system are also the cold gas inflation type. The head airbags are always triggered with the side (Thorax) bags.

[Images: Head Airbag (ITS) (1 Of 2) and (2 Of 2). Illustrate the mounting and components of the head airbag system. Courtesy of BMW OF NORTH AMERICA, INC.]

Battery Safety Terminal (BST)

As with previous systems, the BST is used to disconnect the battery's "B+" connection to the engine compartment in the event of an airbag deployment. This safety measure helps prevent the possibility of a short circuit causing a fire.

[Diagram: Battery Safety Terminal (BST). Shows the BST's connection to the activation circuit, gas discharge tube, and power distribution centers. Courtesy of BMW OF NORTH AMERICA, INC.]

Seat Belt Tensioners

The seat belt tensioners are a new design and also use inert gas for triggering. The MRS III control module will deploy the seat belt tensioners based on programmed parameters during impact.

[Image: Seat Belt Tensioners. Shows a seat belt tensioner assembly. Courtesy of BMW OF NORTH AMERICA, INC.]

Seat Occupancy Sensor (SBE)

The SBE continues to be used as an input to the MRS III control module for detection of a front seat passenger. The module uses this input to determine seat belt tensioner and/or front airbag deployment.

[Image: Seat Occupancy Sensor (SBE). Courtesy of BMW OF NORTH AMERICA, INC.]

MRS III Operation

As with previous systems, the triggering thresholds are programmed in the MRS III control module. These thresholds are determined by BMW through crash and vehicle testing during the design and development of the vehicle. These thresholds will vary depending on the vehicle size and design.

There are several different thresholds for airbag and safety restraint deployment, including:

• Belt pre-tensioner threshold for activation of the seat belt tensioners.
• Airbag threshold #1 - the first level of activation for the two-stage front airbags, always deployed first when the front triggering threshold is reached.
• Airbag threshold #2 - the second level of the two-stage front airbags, can be deployed simultaneously or after a time delay, depending on the severity of the impact.
• Rear crash threshold - for activation of the seat belt tensioners with a rear impact.
• Battery safety terminal threshold - for activation of the BST with airbag deployment.
• Side airbag/ITS threshold - for deployment of the side and thorax airbags.

Triggering Thresholds - Two Stage Airbags

The programming of the MRS III includes four triggering thresholds for the two-stage front airbags. The triggering of the front airbags is also dependent on whether the seat belts are connected and if the front passenger seat is occupied. The triggering thresholds for the two-stage airbags are as follows:

If the signal from the SBE is defective on triggering, the MRS III will deploy as if the seat is occupied. If the signal from the seat belt contacts are defective, the MRS III will deploy as if the belts were not buckled.

[Diagram: MRS III I-P-O Communication Diagram. Illustrates the system's input, process, and output flow. Courtesy of BMW OF NORTH AMERICA, INC.]

Triggering Thresholds

Side Airbags/ITS: The triggering thresholds for the side airbags/ITS are dependent on the signals from the satellite sensors and the crash sensor in the MRS III control module. The triggering thresholds are independent of the belt tensioners.

Belt Tensioner: The triggering of the belt tensioners is dependent on the signal from the seat belt contact and the severity of the impact as detected by the control module.

Battery Safety Terminal: The BST will deploy in a frontal impact at threshold 2 or greater. The threshold for BST activation with a side impact is programmed separately in the side deployment criteria. The BST will also be deployed when the rear impact threshold is exceeded.

Fuel Pump Shut-Down: New to the MRS III system is the link via the K-Bus/CAN Bus to the Engine Control Module for deactivation of the fuel pump. The MRS III will signal the DME over the K-Bus through the instrument cluster and CAN Bus to shut off the fuel pump in the event that any crash threshold is exceeded.

Diagnosis

Diagnosis and troubleshooting of the MRS III system is fault-driven and can be accessed using the DIS Tester or MoDiC. The control module performs a self-test of the system every time the ignition is switched on, which includes the satellite sensors and seat occupancy sensor. Any faults with the system will cause the warning lamp in the instrument cluster to remain illuminated after the engine is started.

Installation of a new or replacement control module requires ZCS coding, also using the DIS or MoDiC. When servicing or replacing any MRS III components, always follow precautionary measures outlined in the repair manual of TIS. This includes disconnecting the battery prior to any repair or maintenance work being performed.

All airbag components are part number specific by model and require verification in the EPC to ensure the correct component is being installed.

[Diagram: Identifying MRS III/K BUS And DIS OR MODIC. Shows the connection between the MRS III, K-BUS, and diagnostic tools. Courtesy of BMW OF NORTH AMERICA, INC.]