7: MRS Control Unit

R50, R52, R53
MRS Control Unit

Installation location
The MRS control unit is bolted onto the transmission tunnel in the centre of the vehicle.

Transmission Tunnel:

Key Explanation
1 MRS control unit

Construction
Two acceleration sensors are accommodated in the MRS control unit in order to detect impacts.
The acceleration sensors are arranged at 90 degrees to each other. The longitudinal acceleration sensor is on the vehicle's longitudinal axis. The lateral acceleration sensor is at right angles to the vehicle's longitudinal axis.
The MRS control unit contains an ignition capacitor. If the power supply is interrupted in the event of an impact, the ignition capacitor serves as an energy reserve for the MRS control unit.
The MRS control unit is connected to the vehicle wiring harness by 2 connectors.

Pin assignment X10179, 26-pin

Pins 1 To 26:

Pin assignment X18069, 54-pin

Pins 1 To 30:

Pins 31 To 54:

How it works
The information below tells you how the control unit works:
- Impact detection
The MRS control unit can detect an impact from any angle (360�).
The direction and severity of an impact is detected by the acceleration sensors in the MRS control unit and the external acceleration/pressure sensors.
- External acceleration sensors:
Front airbag sensors
B-pillar airbag sensors
- External pressure sensors:
Airbag sensors in the doors
All data is processed in the microprocessor in the MRS control unit. This is used to determine the accident severity.

- Storing data when the multiple restraint system is triggered
When the restraint systems are triggered, specific data is recorded in the non-erasable fault memory in the MRS control unit.
The following data is stored:
- System time
The time is required so that the various system-relevant processes can be configured after the MRS control unit is first started.
- Status of the seat-occupancy detector
- Status of the seat-belt-lock switch
- Information about the accident severity
- Triggering of the ignition output stages

- Message to other control units
If the restraint systems are triggered, the MRS control unit sends a message to the other control units. The following functions are performed by the relevant control units depending on the severity of the accident:
- Electric fuel pump is switched off (via DME control module)
- Central locking is unlocked (via general module)
- Interior lights are switched on (via general module)
- Hazard warning lights are switched on (via general module)

- Self-test
In addition to all the inputs and outputs, the MRS control unit also monitors the internal construction components (self test). Any faults are stored in the MRS control unit. In the event of a system fault or a component fault, the airbag warning lamp lights up.