Air Bag Systems: Description and Operation
SIR System Description and Operation
SIR System Overview
The supplemental inflatable restraint (SIR) system supplements the protection offered by the occupants Seat Belt System (2). The SIR system may contain several inflator modules located throughout the vehicle, i.e. steering wheel module (1), instrument panel (I/P) module (1), or roof rail modules. In addition to inflator modules, the vehicle may contain seat belt pretensioners that tighten the seat belt in the event of a collision, thus reducing the distance between the occupant and the seat belt when an inflator module is deployed. Each inflator module has a deployment loop that is controlled by the sensing and diagnostic module (SDM) mounted inside the vehicle. The SDM determines the severity of a collision with the assistance of various sensor inputs located at strategic points on the vehicle. When the SDM detects a collision, it will process the information provided by the sensors to further support air bag or pretensioner deployment. The SDM will deploy the frontal air bags and pretensioners if it detects a collision of sufficient force. If the force of the impact is not sufficient to warrant inflator module deployment, the SDM may still deploy the seat belt pretensioners. The SDM performs continuous diagnostic monitoring of the SIR system electrical components. Upon detection of a circuit malfunction, the SDM will set a DTC and inform the driver by turning the AIR BAG indicator ON. The steering column (1) and knee bolsters (3) are designed to absorb energy and compress during frontal collisions in order to limit leg movement and decrease the chance of injury to the driver and passenger.
Frontal SIR System Description
The frontal supplemental inflatable restraint (SIR) system consists of the following components:
* AIR BAG indicator located in the instrument panel cluster (IPC)
* Inflatable restraint sensing and diagnostic module (SDM)
* Inflatable restraint passenger presence system (PPS)
* Inflatable restraint Passenger AIR BAG ON/OFF indicator
* Inflatable restraint instrument panel (I/P) module
* Inflatable restraint steering wheel module
* Inflatable restraint steering wheel module coil
* Inflatable restraint seat belt retractor pretensioners
* Inflatable restraint wiring harnesses
* Steering wheel and column
* Driver and front passenger knee bolsters
A frontal collision of sufficient force will deploy the frontal air bags and/or pretensioners. The sensing and diagnostic module (SDM) contains a sensing device that converts vehicle velocity changes to an electrical signal. The SDM compares these signals to values stored in memory. If the signals exceed a stored value, the SDM will determine the severity of the impact and either cause current to flow through the frontal deployment loops deploying the frontal air bags and pretensioners, or it will deploy the pretensioners only. The SDM, I/P module, steering wheel module, steering wheel module coil, seat belt retractor pretensioners, and the connecting wires make up the frontal deployment loops. The SDM continuously monitors the deployment loops for malfunctions and turns the AIR BAG indicator ON if a fault is detected.
Inflatable Restraint Sensing and Diagnostic Module (SDM)
The sensing and diagnostic module (SDM) is a microprocessor and the control center for the supplemental inflatable restraint (SIR) system. The SDM contains internal sensors along with several external sensors, if equipped, mounted at strategic locations on the vehicle. In the event of a collision, the SDM compares the signals from the internal and external sensors to a value stored in memory. When the generated signals exceed the stored value, the SDM will cause current to flow through the appropriate deployment loops to deploy the air bags. The SDM records the SIR system status when a deployment occurs and turns the AIR BAG indicator located in the IPC ON. The SDM performs continuous diagnostic monitoring of the SIR system electrical components and circuitry when the ignition is turned ON. If the SDM detects a malfunction, a DTC will be stored and the SDM will command the AIR BAG indicator ON, notifying the driver that a malfunction exist. In the event that ignition positive voltage is lost during a collision, the SDM maintains a 23-volt loop reserve (23 VLR) for deployment of the air bags. It is important when disabling the SIR system for servicing or rescue operations to allow the 23 VLR to dissipate, which could take up to 1 minute.
Inflatable Restraint Passenger Presence System
Important: The passenger presence system (PPS), heated seat element (if equipped), and the seat bottom foam cushion is a calibrated unit and cannot be service separately. After repairing or replacing the PPS, the system must be rezeroed in order to function properly.
The PPS is used to monitor the weight of an occupant on the front outboard passenger seat and communicate the status to the sensing and diagnostic module (SDM) whether to enable or suppress the deployment of the instrument panel (I/P) inflator module. The PPS consist of an electronic control module, sensor mat, heated seat element (if equipped), wiring harness, and PASSENGER AIR BAG ON/OFF indicators. The sensor is made up of several flexible conductive metal strips placed underneath the seat cushion trim. These sensor strips transmit and receive a low-level electric field. The weight of the occupant sitting in the front passenger seat is measured as a change in current flow within the sensor mat. If the sensor determines that the occupant weight is less than a specified value, the PPS module will send a suppress signal to the SDM to disable the I/P module. If the sensor determines the occupant weight is higher than a specified value, the PPS module will send an enable signal to the SDM to enable the I/P. The PPS module will notify the customer of the enable/disable status by turning on one of the PASSENGER AIR BAG ON/OFF indicators located in the center of the instrument panel cluster (IPC). The PPS monitors itself for faults and will display flash diagnostic trouble codes (DTCs) when a fault is detected and the SDM commands it by using the PASSENGER AIR BAG ON/OFF indicators. The PPS will also notify the SDM of a fault and the SDM will request the IPC to turn the AIR BAG indicator located on the IPC ON. To determine what DTCs have been set by the PPS, the scan tool is used to command the SDM to request the PPS to flash the DTCs using the PASSENGER AIR BAG ON/OFF indicators located on the I/P.
Inflatable Restraint PASSENGER AIR BAG ON/OFF indicator
The PASSENGER AIR BAG ON/OFF indicators is used to notify the driver and passenger when the I/P and the right front air bag is enable or disable.
AIR BAG Indicator
The AIR BAG indicator, located in the instrument panel cluster (IPC) is used to notify the driver of SIR system malfunctions and to verify that the sensing and diagnostic module (SDM) is communicating with the IPC. When the ignition is turned ON, the SDM is supplied with ignition positive voltage. The SDM requests the IPC to flash the AIR BAG indicator seven times. While flashing the indicator, the SDM conducts test on all SIR system components and circuits. If no malfunctions are detected the SDM will communicate with the IPC through the class 2 serial data circuit and command the AIR BAG indicator OFF. The SDM provides continuous monitoring of the air bag circuits by conducting a sequence of checks. If a malfunction is detected the SDM will store a diagnostic trouble code (DTC) and command the IPC to turn the AIR BAG indicator ON via class 2 serial data. The presence of a SIR system malfunction could result in non-deployment of the air bags or deployment in conditions less severe than intended. The AIR BAG indicator will remain ON until the malfunction has been repaired.
Dual Stage Inflator Modules
Dual stage inflator modules contain a housing, inflatable air bag, two initiating devices, canister of gas generating material and, in some cases, stored compressed gas. The two initiators are part of the frontal deployment loop. The function of the frontal deployment loops are to supply current through the steering wheel and instrument panel (I/P) inflator modules to deploy the air bags. The inflator modules have two stages of deployment, which varies the amount of restraint to the occupant according to the collision severity. For moderate frontal collisions the inflator modules deploy at less than full deployment (low deployment) which consists of stage 1 of the inflator module. For more severe frontal collisions a full deployment is initiated which consists of stage 1 and stage 2 of the inflator module. The current passing through the initiators ignites the material in the canister producing a rapid generation of gas and is some cases, the release of compressed gas. The gas produced from this reaction rapidly inflates the air bag. Once the air bag is inflated it quickly deflates through the air bag vent holes and/or the bag fabric.
Each dual stage inflator modules is equipped with a shorting bar located in the connectors of the module. The shorting bar shorts the inflator module deployment loop circuitry to prevent unwanted deployment of the air bag when it is disconnected.
Inflatable Restraint Seat Belt Retractor Pretensioners
The seat belt retractor pretensioners consist of a housing, a seat belt retractor, the seat belt webbing, an initiator, and a canister of gas generating materials. The initiator is part of the seat belt pretensioner deployment loop. When the vehicle is involved in a collision of sufficient force, the sensing and diagnostic module (SDM) causes current to flow through the seat belt deployment loops to the initiator. Current passing through the initiator ignites the material in the canister producing a rapid generation of gas. The gas produced from this reaction deploys the seat belt pretensioners and retracts the seat belt webbing, which removes all of the slack in the seat belts. Depending on the severity of the collision, the seat belt pretensioners may deploy without the frontal inflator modules deploying, or they will deploy immediately before the frontal inflator modules deploy. Each seat belt pretensioner is equipped with a shorting bar that is located in the connector of the seat belt pretensioner. The shorting bar shorts the seat belt pretensioner circuitry to prevent unwanted deployment of the seat belt pretensioner when the connector is disconnected.
Inflatable Restraint Steering Wheel Module Coil
The steering wheel module coil is attached to the steering column and is located under the steering wheel. The steering wheel module coil consists of two or more current-carrying coils. The coils allow the rotation of the steering wheel while maintaining continuous electrical contact between the driver deployment loop and the steering wheel module. Two or four, if equipped with dual stage air bags, coil wires are used for the steering wheel module deployment loop. Additional coil wires are used for accessories attached to the steering wheel depending on the vehicle model. The steering wheel module coil connector is located near the base of the steering column. The connector contains a shorting bar that shorts the steering wheel module coil deployment loop circuitry to prevent unwanted deployment of the air bag when servicing the inflator module.
Steering Column and Wheel
The steering wheel and columns are designed to absorb energy when driver contact is made with the steering wheel or inflated air bag. In a frontal collision the driver may come in contact with the steering wheel directly or load the steering wheel and column through the inflated air bag. When the driver applies load to the air bag or the steering wheel the column will compress downward absorbing some of the impact, helping to reduce bodily injuries to the driver. The steering wheel and column must be inspected for damages after a collision.
Knee Bolster
The knee bolsters are designed to help restrain the lower torso of front seat occupants by absorbing the energy through the front seat occupants upper legs. In a frontal collision the front seat occupant legs may come in contact with the knee bolsters. The knee bolsters are designed to crush or deform, absorbing some of the impact, which helps to reduce bodily injuries. The driver and passenger knee bolsters are located in the lower part of the instrument panel and must be inspected for damages after a collision.
Side SIR System Description
The side supplemental inflatable restraint (SIR) system consists of the following components:
* AIR BAG indicator located in the instrument panel cluster (IPC)
* Inflatable restraint sensing and diagnostic module (SDM)
* Inflatable restraint side impact sensors (SIS)
* Inflatable restraint roof rail modules
* Inflatable restraint wiring harnesses
Inflatable Restraint Roof Rail Modules
The roof rail modules are located under the headliner extending from the front windshield pillar to the rear window pillar. The roof rail modules contain a housing, inflatable air bag, initiating device, and a canister of gas generating material. The initiator is part of the roof rail module deployment loop. When a side impact of sufficient force occurs the SIS detects the impact and sends a signal to the sensing and diagnostic module (SDM). The SDM compares the signal received from the SIS to a value stored in memory. When the generated signal exceeds the stored value, the SDM will cause current to flow through the side deployment loop deploying the roof rail air bags. The SDM, roof rail modules, and the connecting wires make up the side deployment loops. The SDM continuously monitors the deployment loops for malfunctions and turns the AIR BAG indicator ON if a fault is present.
Each roof rail module is equipped with a shorting bar located on the connector of the module. The shorting bar shorts the roof rail module deployment loop circuitry to prevent unwanted deployment of the air bag when servicing the inflator module.
Inflatable Restraint Side Impact Sensor (SIS)
The side impact sensor (SIS) contains a sensing device which monitors vehicle acceleration and velocity changes to detect side collisions that are severe enough to warrant air bag deployment. The SIS is not part of the deployment loop, but instead provides an input to the sensing and diagnostic module (SDM). The SDM contains a microprocessor that performs calculations using the measured accelerations and compares these calculations to a value stored in memory. When the generated calculations exceed the stored value, the SDM will cause current to flow through the deployment loops deploying the roof rail module air bags.
Inflatable Restraint Wiring Harness
The inflatable restraint wiring harnesses connect the inflators modules, sensing and diagnostic module (SDM), deployment loops, and class 2 serial data together using weather pack connectors. SIR system connectors are yellow in color for easy identification. When repairing the SIR wiring harnesses follow the proper testing and wiring repair procedures listed in this manual.