Air Bag Systems: Description and Operation
WARNING: The SRS is not a substitute for wearing a seat belt.
PURPOSE
The Supplemental Restraint System (SRS) provides additional protection for the head and upper part of the driver's body, via an inflateable airbag, in the event of a specific frontal impact. The drivers knees are protected by an energy absorbing bolster underneath the instrument panel.
CONSTRUCTION
a - SRS steering wheel with impact shell and padded cap, in which airbag, gas generator and ignition pill are integrated.
b - Contact ring - guarantees power supply to ignition pill.
c - Two crash sensors (front left and right on wheel house) and safety switch (left).
d - Electronic diagnosis unit (control module) underneath instrument panel trim with integrated safing sensor (prevents unwanted activation).
e - SRS indicator lamp integrated in check control.
f - Knee guard
The electric power for the system, which is required for the igniting and monitoring electronics, is taken from the vehicle battery. A voltage transformer, which boosts the ignition voltage, is integrated in the control unit. In the event of battery failure, power for ignition is supplied by a capacitor within the control module.
To assure positive ground contact, the diagnostic module unit has its own central ground connection. No other connections must be made to this ground connection.
OPERATION
Standby Mode
The SRS system is designed to operate only in frontal collisions, but the safety belt must still be worn if the driver is to benefit from its full protective function. In minor collisions, a roll over, or in the event of a side or rear-end impact, the safety belts are the sole means of protection. The diagram illustrates the impact area designated to activate the SRS.
The system is operational at all ignition switch positions except "Off", and is constantly monitored by the control module (Standby Mode). An indicator lamp (SRS) on the Check Control panel verifies proper operation of the system.
The indicator lamp should illuminate for approx. 6 seconds on starting the vehicle and then extinguish, indicating that the system is ready for operation. The system is faulty and should be checked immediately if:
- The SRS indicator lamp does not illuminate when the engine is started.
- The SRS indicator lamp extinguishes for a brief period after the 6 second test and then illuminates once again or flashes.
- The SRS indicator lamp illuminates or flashes suddenly when driving.
If faults occur in the SRS they will be recorded in the control module defect memory. From there they may then be retrieved during system diagnosis via a series of coded flashes from the indicator lamp.
Deployment
As soon as there is an impact with sufficient force (equal to vehicle traveling more than 11 mph. against a solid object) in the designated area, the weighted roller (3) makes contact with the spring contact (2) completing the electrical circuit necessary for system operation.
To avoid unwarranted airbag inflation under normal driving conditions at least two sensors, one of which must always be the safing sensor, must be activated simultaneously. Discharging of the capacitor then causes the igniter in the gas generator to be activated.
The igniter generates the ignition temperature necessary for the solid fuel in the gas generator to burn (approximately 30 milliseconds). The gas generated is passed through a filter, cooled slightly and then used to inflate the airbag.
As the airbag is filled with this gas it then bursts through predetermined breaking points in the cover. The inflated airbag may reduce injuries to the head and upper part of the body upon impact.
The volume of the airbag is approximately 70 liters, approx. 2.5 cu. ft.
Deflation takes place immediately through two vent holes in the side of the airbag facing away from the driver.
All of the two pin connectors in the igniter supply lead have a special feature that short circuits the two pins when the connector is disconnected. This prevents unintentional airbag inflation during repairs and installations.
After deployment of the SRS in an accident it will be necessary to at least replace the airbag module, contact ring, airbag wiring harness, crash sensors and control module. Non-triggered airbag parts, which are damaged, may not be repaired and must also always be replaced.
The electrical connection between the steering wheel/steering column is accomplished by a "continuous loop" contact ring. This contact ring is made up of a wire ribbon that will coil and uncoil when the steering wheel is turned. This assures perfect electrical contact to the steering wheel under all conditions.
NOTE: Arrow points to locking screw that keeps new contact ring in center position. Special care must be taken when working in this area.