Air Bag Systems: Description and Operation
SRS System Description
SRS Components
Airbags
The SRS is a safety device which, when used with the seat belt, is designed to help protect the driver and front passenger in a frontal impact exceeding a certain set limit. The system consists of the SRS unit, including safing sensor and impact sensor (A), the cable reel (B), the driver's airbag (C), the front passenger's airbag (D), side airbags (E), side curtain airbags (F), seat belt tensioners (G), seat belt buckle tensioners (H), side impact sensors (first) (I), front impact sensors (J), roll rate sensor (K) and side impact sensors (second) (L). 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 the 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 (N) are part of the front passenger's 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 65 lbs (30 kg) or less, the ODS unit sends a signal to the SRS unit to prevent the passenger's airbag from deploying. When the passenger's airbag is disabled, the passenger airbag cutoff indicator (P) 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 (O) is under the driver's seat on the left side. When the driver's seat is moved all the way forward, the deployment of the driver's airbag is moderated to decrease its force of impact during a front-end collision.
Roll Rate Sensor
The roll rate sensor is located under the front passenger's seat. It detects the amount of roll of the vehicle and sends the information to the SRS unit. The SRS unit uses this information to determine if a vehicle rollover is imminent. If so, it deploys both side curtain airbags and the front seat belt tensioners.
Side Airbag Cutoff Indicator/ODS Operation
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 moves to another seat, 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.
Passenger's Airbag Cutoff Indicator/Seat Weight Sensor Operation
The indicator comes on if the weight on the front passenger's seat is about 65 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 judges the force of impact and, if necessary, ignites the inflator charges. If 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:
Seat Belt Tensioners and Seat Belt Buckle Tensioners
(1) A front impact sensor side impact sensor or the roll rate 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) A front impact sensor must activate and send electric signals to the microprocessor.
(2) The microprocessor must compute the signals and trigger the airbag inflator(s).
(3) The trigger inflators that received the 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 the signal must ignite and deploy the side airbags.
Side Curtain Airbag(s)
(1) A side impact sensor 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 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 does not go off after 6 seconds, or 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 a DTC that relates to the cause of the malfunction, and the 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 battery for troubleshooting, review Battery Terminal Disconnection and Reconnection. Procedures
SRS Unit Inputs and Outputs at Connector A (28P)
SRS Unit Inputs and Outputs at Connector B (28P)
SRS Unit Inputs and Outputs at Connector C (16P)
