Air Bag Systems: Description and Operation

The Supplemental Restraint System (SRS) offers driver and front passenger protection in addition to seat belts by deploying steering wheel and instrument panel air bags if the vehicle is in a frontal collision of sufficient force up to 30° off vehicle centerline.
System components include: Sensing and Diagnostic Module (SDM), driver's and passenger's air bag modules, contact coil assembly, SRS wiring harness, air bag warning lamp, and forward discriminating sensor.

SENSING AND DIAGNOSIS MODULE (SDM)
The SDM sensing device converts vehicle velocity changes into electrical signals. These signals are processed and compared to memory values. If a signal exceeds the specified value stored, an additional signal processing is performed and compared to the memory value. When two of the generated signals exceed the stored value or when one of the generated signals exceeds the stored value and the forward discriminating sensor closes, current is supplied to deploy the air bag modules. The SDM also monitors SRS components and stores Diagnostic Trouble Codes (DTCs) if a system condition is encountered. Once a DTC has been stored, the SDM will light the air bag warning lamp.


AIR BAG MODULE
Driver's
Located in the steering wheel, the driver's air bag module consists of an inflatable bag and inflator (a canister of gas generating material and initiating device). Current passing from the SDM through the initiator ignites the inflator material, producing a gas that rapidly inflates the bag.


Passenger's
Located in the instrument panel, the passenger's air bag module consists of an inflatable bag, an inflator (a canister of gas and initiating device) and a pressure sensing device. Current passing from the SDM through the initiator generates pressure, depressing a plunger and releasing the stored gas that rapidly inflates the bag. The pressure sensing devices ensures there is adequate pressure to inflate the bag.


CONTRACT COIL
Combined with the combination switch assembly on the steering column, the contact coil assembly has two deployment loop coils and a horn circuit coil. This allows steering wheel rotation while maintaining continuous deployment loop contact.


FORWARD DISCRIMINATING SENSOR
Located in the front of the vehicle, the forward discriminating sensor consists of a sensing element, open switch contacts and diagnostic resistor. If there is a severe vehicle velocity change warranting air bag deployment, the sensing element closes the contacts. Connected in parallel with the switch, the diagnostic resistor will causes a voltage drop in conjunction with the SDM resistors, allowing the SDM to monitor the forward discriminating sensor and associated circuitry.