Air Bag Systems: Description and Operation
SRS ComponentsAirbags
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 (0), seat belt tensioners (E), and front impact sensors (F). 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 the severity of a collision is at the margin, or threshold, that determines whether or not the airbags will deploy. In such cases, the seat belt will provide sufficient protection, and the supplemental protection offered by the airbag would be minimal.
Front Passenger's Weight Sensors ('07 model)
The ODS unit (G) is in the front passenger's seat-back. The seat weight sensors (H) are part of the seat base. The weight sensors detect the weight on the seat, and send the information to the ODS unit. 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 front passenger's airbag from deploying. When the front passenger's airbag is disabled, the 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.
Driver's Seat Position Sensor ('07 model)
The driver's seat position sensor (I) is under the driver's seat on the left side. When the driver's seat is moved to its full forward position, the deployment of the driver's airbag is moderated to decrease its force of impact during a front-end collision.
Side Airbag Cutoff Indicator/ODS Operation ('07 model)
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 an object (grocery bag, briefcase, purse, etc.) is in the seat. When the indicator comes on, 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 light should go off. There will be some delay between the occupant's repositioning, and when the indicator will turn on or off.
Passenger Airbag Cutoff Indicator ('07 model)
The indicator comes on if the weight of the front passenger 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.
Side Airbags
The side airbags (J) are in each front seat-back. They help protect the upper torso of the driver or front seat passenger during a moderate to severe side impact. Side impact sensors (first) (K) in each door sill and in the SRS unit detect such an impact and instantly inflate the driver's or the passenger's side airbag. Only one side airbag will deploy during a side impact. If the impact is on the passenger's side, the passenger's side airbag will deploy even if there is no passenger.
Side Curtain Airbag
The side curtain airbags (L) are in each side of the roof. They help protect the head of the driver or door side passenger during a moderate to severe side or impact. Side impact sensors (first) (K) in each front door sill, side impact sensors second) (N) in each rear door sill, and the SRS unit detect such an impact and instantly inflate the driver's or the passenger's side curtain airbag. A side impact at the front to middle of the vehicle causes the side curtain airbag and the side airbag on the impacted side to deploy. A side impact towards the rear of the vehicle causes only the side curtain airbag on the impacted side to deploy.
Seat Belt Tensioners
The seat belt tensioners are linked with the SRS airbags to further increase the effectiveness of the seat belt. In a front end collision, the tensioners instantly retract the belt firmly to secure the occupants in their seats.
OPDS
The side airbag system also includes an occupant position detection system (OPDS). This system consists of sensors (G) and a OPDS unit (M) in the front passenger's seat-back. The OPDS unit sends the signal to the SRS unit. If the OPDS unit determines that the front passenger is of small stature (for example, a child) and the front passenger is leaning into the side airbag deployment path, the SRS unit will automatically disable the passenger's side airbag. The SRS unit will also disable the airbag when the OPDS detects certain objects on the seat. When the side airbag is disabled, the side airbag cutoff indicator on the instrument panel alerts the driver that the passenger's side airbag will not deploy in a side impact. When the object is removed, or the passenger sits upright, the side airbag cutoff indicator will go off after a few seconds, alerting the driver that the passenger's side airbag will deploy in a side impact.
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
1. A front impact sensor must activate and send electric signals to the microprocessor.
2. The microprocessor must compute the signals and send them to 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 depending on the severity of the collision and whether the seat belt buckle switch is on or off, it sends the appropriate signals to the airbag inflator(s).
3. The 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 send them to the side airbag inflator(s). However, the microprocessor turns off the signals to the front passenger's side airbag if the OPDS unit determines that the front passenger's head is in the deployment path of the side airbag.
3. The inflator that received the signal must ignite and deploy the side airbag.
Side Curtain Airbag(s)
1. A side impact sensor must activate, and send electrical signals to the microprocessor.
2. The microprocessor must compute the signals and send them to the side curtain airbag inflator(s).
3. The inflator that received the signal must ignite and deploy the side curtain airbag.
Self-diagnostic System
A self-diagnostic circuit is built into the SRS unit; when the ignition switch is turned 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 if it 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 a DTC that relates to the cause of the malfunction, and the unit is connected to the data link circuit. This information can be read with the HDS when it is connected to the data link connector (DLC).
NOTE: Before disconnecting the battery for troubleshooting, make sure you have the anti-theft code for the audio and the navigation system, then write down the audio presets.
SRS Unit Inputs And Outputs At Connector A (28P):
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 B (28P)
SRS Unit Inputs And Outputs At Connector C (16P):
SRS Unit Inputs and Outputs at Connector C (16P)