Air Bag Systems: Description and Operation

SRS System Description

SRS Components

Airbags

The SRS is a supplemental safety device which, when used with the seat belt, is designed to help protect the driver and the front passenger in a frontal impact exceeding a certain set limit. The system consists of the SRS unit, including the impact sensor (A), the cable reel (B), the driver's airbag (C), the front passenger's airbag (D), the side airbags (E), the side curtain airbags (F), the seat belt tensioners (G), the side impact sensors (first) (H), the front impact sensors (I), the rear safing sensor (J), and the side impact sensors (second) (K).

Since the driver's and front passenger's airbags use the same sensors, both normally inflate at the same time. However, it is possible for only one airbag to inflate. This can occur when collision severity is near threshold for airbag deployment. In such case, the SRS will only deploy airbags when the protection provided by the seat belt is insufficient.

Front Passenger's Weight Sensors

The front passenger's weight sensors (L) are part of the seat. The front passenger's weight sensors detect the weight on the seat, and send the information to the ODS unit (M). If the total weight is about 66 lbs (30 kg) or less, the ODS unit sends a signal to the SRS unit to prevent the front passenger's airbag from deploying. When the front passenger's airbag is disabled, the front passenger airbag cutoff indicator on the center panel comes on to alert the driver that the front passenger's airbag will not deploy in a front-end collision.

NOTE: The sensors only detect the weight on the seat. The sensors do not detect the weight of the passenger's legs or arms that may be resting on the floor or armrests.

Driver's Seat Position Sensor

The driver's seat position sensor (N) is under the driver's seat on the left side. When the driver's seat is moved to forward most position, the deployment of the driver's airbag is moderated to decrease its force of impact during a front-end collision.






Rear Safing Sensor

The rear safing sensor is located under the rear seat. The rear safing sensor does the same basic function as the safing sensor in the SRS unit. It measures sideways G force, such as the force the vehicle would receive in a side collision in the rear, and sends that information to the SRS unit. The SRS unit uses that information, and the information from the second side impact sensors to determine which side is being impacted and the severity of impact. If the threshold is met, the SRS unit deploys the side airbag, the side curtain airbag and the seat belt tensioner on that side.

Side Airbag Cutoff Indicator/OPDS Operation

The OPDS sensors (O) is located in the front passenger's seat-back. The ODS unit detects front passenger's position on the seat by using the OPDS sensors, then sends the signals to the SRS unit. The indicator comes on when the front passenger's seat is occupied by a small adult or child who is leaning into the deployment path, or when an object (grocery bag, briefcase, purse, etc.) is in the seat. This indicates the passenger's side airbag is off and will not deploy; there is no problem with the side airbag. If the passenger sits upright or you remove the object from the seat, the indicator should go off. There will be some delay between the occupant's repositioning, and when the indicator will turn on or off.

Front Passenger's Airbag Cutoff Indicator/Front Passenger's Weight Sensor Operation

The indicator comes on if the weight on the front passenger seat is about 66 lbs (30 kg) or less. This indicates the passenger's front airbag is off and will not deploy. The front airbag is shut off to reduce the chance of airbag-caused injuries.

SRS Operation

The main circuit in the SRS unit senses and analyzes the force of impact and, if necessary, ignites the inflator charges. If the 12 volt battery voltage is too low or power is disconnected due to the impact, the voltage regulator and the back-up power circuit will keep voltage at a constant level.

For the SRS to operate

Front Seat Belt Tensioners

(1) A front impact sensor or the side impact sensor must activate and send electric signals to the microprocessor.

(2) The microprocessor must compute the signals and trigger the tensioners.

(3) The charges must ignite and deploy the tensioners.

Driver's and Front Passenger's Airbag(s)

(1) The front impact sensors must activate, and send electric signals to the microprocessor.

(2) The microprocessor must compute the signals and trigger the airbag inflator(s).

(3) The triggered inflators that received signals must ignite and deploy the airbags.

Side Airbag(s)

(1) A side impact sensor must activate, and send electric signals to the microprocessor.

(2) The microprocessor must compute the signals and trigger the side airbag inflator(s). However, the microprocessor does not trigger the front passenger's side airbag if the SRS unit determines that the front passenger's head is in the deployment path of the side airbag.

(3) The triggered inflators that receive the signal must ignite and deploy the side airbags.

Side Curtain Airbag(s)

(1) The side impact sensors or the rear safing sensor must activate and send electrical signals to the microprocessor.

(2) The microprocessor must compute the signals and trigger the side curtain airbag and side airbag inflator(s).

(3) The triggered inflators that receives the signals must ignite and deploy the side curtain airbag and side airbag at the same time.






Self-Diagnostic System

A self-diagnostic circuit is built into the SRS unit; when the ignition switch is turned to ON (II), the SRS indicator comes on and goes off after about 6 seconds if the system is operating normally. If the indicator does not come on, or if it does not go off after 6 seconds, or if it comes on while driving, it indicates a problem in the system. The system must be inspected and repaired as soon as possible.

For better serviceability, the SRS unit memory stores DTCs related to the cause of the malfunction. The SRS unit is connected to the data link connector (DLC). This information can be read with the HDS when it is connected to the DLC Reading and Clearing Diagnostic Trouble Codes.

NOTE: Before you disconnect the negative cable from the 12 volt battery for troubleshooting, review 12 Volt Battery Terminal Disconnection and Reconnection Procedures.

SRS Unit Inputs and Outputs at Connector A (39P)














SRS Unit Inputs and Outputs at Connector B (39P)