System functions

The following safety system functions are described:

• Self-test
• Impact detection
• Activating the safety system
• Data storage on activation of the safety system

Self-test 

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

After switching on the ignition, the crash safety module or MRS control unit starts a self-test. During this time, the airbag warning lamp lights up (approx. 3 to 5 seconds). When the safety system is functional, the airbag warning lamp goes out.

• If no CAN message from the Crash Safety Module or MRS control unit is received at the instrument cluster the airbag indicator lamp lights up.
• The airbag indicator light remains on if the crash safety module or MRS control unit detects an existing fault or one that has been stored during the self-test or while the vehicle is in motion.
• If a fault is detected in the safety system, the operability of the safety system is partially maintained under the following conditions:
  • If a fault is detected in a circuit of the safety system, only the function of the circuit concerned is disabled. The remaining airbags, belt tensioners and Crash Active head restraints remain operational.
  • In the event of a fault in the circuit of the airbag warning lamp, the lamp does not light up in the self-test. With the precondition that no other fault is present, the safety system remains functional without restriction.
• The entire safety system is deactivated if there is an internal fault in the crash safety module or MRS control unit or in the voltage supply. (airbag warning and seat belt warning light light up, Check Control symbol in the liquid crystal display)

Impact detection 

The direction and severity of an impact is detected by the acceleration sensors in the crash safety module or MRS control unit and by external acceleration sensors. The crash safety module or MRS control unit process all the acceleration data.

Activating the safety system 

Comprehensive tests are used to define triggering thresholds for all possible types of accident. This results in different trigger thresholds for activation of the various restraint systems (such as airbags, belt tensioners, Crash Active head restraints).

The restraint systems are triggered only when 2 independent sensors detect the corresponding threshold. For example, a rear-end collision is detected by the longitudinal acceleration sensor in the airbag sensor in the driver's side B-pillar and by the longitudinal acceleration sensor in the crash safety module or MRS control unit. The crash safety module or MRS control unit calculates the direction of the impact and the accident severity from the data sent by the sensors. Depending upon the severity of the accident, the Crash Active head restraints and/or the safety battery terminal may be triggered, and the electric fuel pump may be deactivated.

Data storage on activation of the safety system 

When the restraint systems trigger, certain data are written to a read only memory in the crash safety module or MRS control unit. The data is relevant to accident research (no access for service).