Part I
Crash Safety
Crash safety
The 4th generation ACSM ("Advanced Crash Safety Module") discharges the following functions:
- Detecting an accident situation that is critical for the occupants
- Activating the necessary restraint systems (selectively depending on the severity of the accident and accident type)
Brief component description
Descriptions of the following safety-system components are provided:
NOTICE: There are differences in the equipment of the safety system depending on the series, model, vehicle equipment and national-market version.
The following description does not go into detail about all possible equipment ranges of the safety system depending on the series, model, vehicle equipment, optional equipment and national-market version.
ACSM: Crash safety module
The Crash Safety Module (ACSM) is the control unit for the safety system. No sensors are installed in the Crash Safety Module.
The Crash Safety Module performs a self test after the ignition is switched on. During this time, the airbag indicator light lights up (approx. 3 to 5 seconds). The airbag indicator lamp goes out as soon as the safety system is ready for operation.
All gas generators and sensors of the safety system are connected directly to the Crash Safety Module. The Crash Safety Module evaluates the acceleration data (longitudinal and lateral acceleration) of the sensors and thus determines the direction and severity of the accident in a collision. 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 (airbags, seatbelt tensioners, etc.). When an impact occurs the Crash Safety System decides whether deployment is required and which gas generators (belt tensioners, airbags, crash-active head restraints, etc.) must be activated. The Crash Safety Module also triggers the safety battery terminal. The restraint systems are triggered only when 2 independent sensors detect the corresponding threshold.
The Crash Safety Module sends a message to other bus users when the restraint systems are triggered. The individual control units execute various functions according to the severity of the accident. For example, the footwell module (FRM) or front electronic module (FEM) activates the interior lights and hazard warning flashers.
The Crash Safety Module transmits a signal to the Combox over a direct wire. The Combox automatically initiates an emergency call which also includes the location of the vehicle.
When the restraint systems trigger, certain data are written to a read only memory in the Crash Safety Module. The data is relevant to accident research (no access for service). After 3 crash signals, the data memory is full. The airbag warning lamp lights up. The Crash Safety Module must be replaced.
For the seat belt warning, the signals of the belt buckle switch are picked up separately.
The crash safety module 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 crash safety module.
Front door airbag sensor, left and front door airbag sensor, right
The 2 airbag sensors are pressure sensors. The airbag sensors are fitted in each of the inner door panels. The airbag sensor consists of the following components:
- Pressure sensor for detection of a side-on crash
- Electronic module for signal processing and for data interchange
If the door outer skin is pressed inwards in a side-on collision, the door interior is reduced in size. This increases the pressure in the door interior. This increase in pressure is measured by the pressure sensor. The measured pressure values are digitized in the electronic module. The electronic chip transmits a message to the Crash Safety Module (transmitted in cycles). The message is processed in the Crash Safety Module (ACSM).
Airbag sensors on left and right B-pillars
Airbag sensors are fitted on the left-hand and right-hand B-pillars at the bottom in the area of the sill. Each airbag sensor contains a lateral acceleration sensor and a longitudinal acceleration sensor. The longitudinal acceleration sensor detects frontal or rear-end collisions. The lateral acceleration sensor acceleration sensor detects a side impact.
Airbag front sensor left and right
Front airbag sensors are bolted onto the left-hand and right-hand brace of the front panel. The 2 front airbag sensors are longitudinal acceleration sensors. The sensors detect frontal or rear-end collisions.
Seat belt buckle switch
The belt buckle switches indicate whether the seat belts have been fastened or not. The seat belt buckle switches receive voltage from the Crash Safety Module (ACSM). The current consumption of the switch is the signal for the switch position (seat belt fastened or not fastened). The seat belt buckle switches are permanently monitored whenever power is present at Terminal 15 and the status is wake-up line ON.
Whether or not the belt tensioner on the front passenger side is activated depends on the status signal of the seat belt buckle switch (seat belt fastened or not fastened). If the seat belt is inserted in the seat belt buckle switch, the belt tensioner is also activated, if needed (only with head-on or rear-end collision). If the front passenger seat is detected as occupied via the signals from the seat occupancy mat without the belt buckle switch sending a signal, a seat belt warning is issued.
Seat position sensor (US version)
According to the regulations of the FMVSS (Federal Motor Vehicle Safety Standard), a 5th percentile woman must be distinguished from a 50th percentile man on the drivers seat. The seating position sensor detects the airbag range designed for a person with regard to the seat adjustment in longitudinal direction.
The seating position sensor is a Hall sensor and it is supplied with voltage via the Crash Safety Module.
The signal of the seating position sensor also influences the delayed triggering of stage 2 of the inflator assembly (driver's airbag and front passenger airbag) as well as the ignition squib for opening the discharge hole.
If the vehicle is equipped with a memory seat, the seat-position sensor is omitted.
Front passenger seat occupancy detection via seat-occupancy mat with pressure sensors (European version)
The front passenger seat occupancy detection detects whether the seat is occupied or not. The seat occupancy mat consists of conductors and pressure sensors. The Crash Safety Module receives and processes the signals from the pressure sensors.
The following graphic shows the seat-occupancy mat using example: F25.
The seat-occupancy mat is installed between the cover and foam insert of the seat.
The following graphic shows the seat-occupancy mat using example: F20.
The seat-occupancy mat is installed on the underside of the foam insert.
When a weight is placed on the seat, resistance in the seat occupancy mat changes. The seat is detected as not occupied up to a weight of approximately 12 kg (only applies if the seat-occupancy mat is installed between the cover and the foam insert of the seat). The seat occupancy detection function, however, also depends on the type of seat. e.g.: The material tension in a leather seat or a taut sports seat is already interpreted as pressure on the sensors. In such cases, a weight below 12 kg can trigger the "seat occupied" condition.
The signal is transmitted to the Crash Safety Module (ACSM) in a direct wire. The signal is only required for the seat belt warning. For example, if the passenger's seat is recognised as being occupied without the seat belt buckle switch sending a signal, a fasten belt warning will be emitted.
Switch for front passenger airbag deactivation (European version)
The switch setting is selected using the integral code in the remote-control unit or the ID transmitter. The indicator lamp for front passenger airbag deactivation lights up when the switch is OFF.
In the switch position "OFF", the following airbags on the passenger's side are not deployed:
- Front passenger airbag
- Side airbag (in the armrest side section of the passengers seat)
- Crash active headrest
- Knee airbag (if present)
Front passenger seat occupancy detection via seat-occupancy mat with capacitive sensor system (US version)
According to the regulations of the NHTSA (National Highway Traffic Safety Administration), a small child in the child restraint system specifically designed for the purpose on the front passenger seat must be detected automatically. If the passenger's seat is classified as "occupied" by a child restraint system specifically designed for the purpose, the Crash Safety Module (ACSM) automatically disables the airbags on the passenger's side (passenger's airbag, side airbag, knee airbag). The indicator light for front passenger airbag deactivation lights up.
The following graphic shows the seat-occupancy mat using example: F25.
The front passenger seat occupancy detection consists of a seat-occupancy mat with a capacitive sensor system. The occupancy of the front passenger seat is detected by measuring the resistance of the human body.
Classification of the occupancy status is based on the evaluation of the capacitive resistance. The electronic evaluation unit picks up the change and determines the corresponding status.
The airbags on the front-front passenger side are deactivated automatically when a small child is detected in the child restraint system provided for this purpose. The indicator light for front passenger airbag deactivation lights up. If a person of adequate size sitting correctly is detected, the airbags on the front passenger side are activated. The indicator light for the front passenger airbag deactivation does not light up.
The front passenger airbag could deactivate with a youth or adult seated in certain positions (indicator light for passenger airbag deactivation comes on). In these cases, the seat position must be changed until the indicator light for the front passenger airbag deactivation goes out (airbags on the front passenger side are activated).
The electronic processing circuit relays status information to the Crash Safety Module in cyclical transmissions. Potential error conditions are stored in the Crash Safety Module's fault memory.
If the seat occupancy mat is defective, the airbags on the front passenger side are automatically disabled (front passenger airbag, knee airbag, side airbag). The indicator light for front passenger airbag deactivation lights up.