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 front passenger in a impact exceeding a certain set limit. The system consists of the SRS unit including the safing sensor and 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 system 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 base. 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 passenger's airbag from deploying. When the passenger's airbag is disabled, the passenger's airbag cutoff indicator (N) on the passenger's vent 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 to the 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 middle of 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 the side that is impacted and the force. 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 (P) 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 if 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.
Front Passenger Airbag Cutoff Indicator/Front Passenger's Weight Sensor Operation
The indicator comes on if the weight of the front passenger is about 66 lbs (30 kg) or less. This indicates the front passenger's airbag is off and will not deploy. The front passenger's 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 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
(1) The front impact sensor, the side impact sensor, or the rear safing 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 sensor must activate and send electric signals to the microprocessor.
(2) The microprocessor must compute the signals and trigger the airbag inflators.
(3) The triggered inflators that received signals must ignite and deploy the airbags.
Side Airbag(s)
(1) The side impact sensor must activate and send electric signals to the microprocessor.
(2) The microprocessor must compute the signals and trigger the side airbag inflators. 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 inflators.
(3) The triggered inflators 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 an abnormality in the system. The system must be inspected and repaired as soon as possible.
For better serviceability, the SRS unit memory stores any 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 battery for troubleshooting, refer to Battery Terminal Disconnection and Reconnection Procedures.
E-Pretensioner
When at a risk of a collision, the E-pretensioner system, working with the collision mitigation braking system (CMBS), retracts the driver's and front passenger's seat belt several times slightly to provide a tactile warning to the driver. And when a collision is imminent, the E-pretensioner system retracts the seat belt tightly to help reduce injury from the impact. The E-pretensioner system also works when the driver brakes hard and the brake assist system is turned on.
CMBS and E-Pretensioner Operation
Approaching a vehicle ahead: When driving, the millimeter wave radar detects a vehicle ahead, and the system provides audible and visual warnings if it detects a risk of collision.
Closing on a vehicle ahead: The system applies light braking, and increases seat belt tension slightly to provide a tactile warning to the driver. The front passenger does not feel the tactile warning.
Collision unavoidable: The system activates strong braking, and securely tightens both the driver's and front passenger's seat belts to help reduce injury caused by the impact.
E-Pretensioner
The mechanical portion of the E-pretensioner consists of the motor and the gear sections. The collision mitigation braking system and the brake assist system sends signals to the motor in the retractor to retract or rewind the belt. If the system judges that the driving situation is safe, the tension on the seat belts is released.
The E-pretensioners used in conjunction with the conventional tensioners.
E-Pretensioner Unit
The E-pretensioner unit operates the motor-driving controls based on F-CAN data it receives from the ACC unit, VSA modulator-control unit, and the PCM. Also the E-pretensioner unit receives seat belt buckle switch signals from the SRS unit by an interactive communication function. The E-pretensioner unit has a self-diagnostic function, and if it detects a malfunction, it sends its information to the SRS unit. The SRS indicator is then turned on by the SRS unit.
SRS Unit Inputs and Outputs at Connector A (39P)
SRS Unit Inputs and Outputs at Connector B (39P)
