Air Bag Systems: Description and Operation
SIR System Description and Operation
SIR System Overview
The SIR System supplements the protection offered by the occupant seat belt system (2). The SIR System contains various inflator modules located throughout the vehicle, i.e. inflatable restraint steering wheel module (1) and the inflatable restraint instrument panel (I/P) module (1). Each inflator module has a deployment loop that is controlled by the inflatable restraint sensing and diagnostic module (SDM), which is mounted inside of the vehicle. The SDM determines the severity of a collision and commands deployment of each inflator module. The SDM performs continuous diagnostic monitoring of the SIR System electrical components. Upon detection of a circuit malfunction, the SDM will set a diagnostic trouble (DTC) and inform the driver by commanding the instrument panel cluster (IPC) to turn the AIR BAG indicator ON. The steering column and knee bolsters (3) are designed to absorb energy and compress during frontal collisions in order to limit leg movement and decrease the chance on injury to the driver and front 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 instrument panel (I/P) module
* Inflatable restraint sensing and diagnostic module (SDM)
* Inflatable Restraint Passenger Presence System (PPS)
* Inflatable restraint Passenger AIR BAG ON/OFF indicator
* Inflatable restraint steering wheel module
* Inflatable restraint steering wheel module coil
* Steering wheel and column
* Inflatable restraint front end sensor
* Inflatable restraint seat belt retractor pretensioners
* Inflatable restraint wiring harnesses
* Driver and passenger knee bolsters
A frontal collision of sufficient force will deploy the frontal air bags. The SDM contains a sensing device that converts vehicle velocity changes to an electrical signal. In the event of a frontal collision, the SDM receives a signal from the electronic frontal sensor which assists the SDM in determining the severity of the collision. The SDM compares these signals to a value stored in memory. When the generated signals exceed the stored value, the SDM will cause current to flow through the frontal deployment loops simultaneously deploying the frontal air bags. The SDM, I/P module, steering wheel module, steering wheel module coil, seat belt pretensioner and the connecting wires makeup 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. This SDM has 2 fused power inputs; one fuse is for the battery voltage and the other fuse is for the ignition voltage. The SDM uses vehicle battery voltage as its main power input. The SDM then uses the vehicles GMLAN Serial Data Communication line and the ignition voltage logic input for enabling or disabling the SIR deployment loops. 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 or seat belt pretensioners. 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 records the SIR system status when a deployment occurs and turns the AIR BAG indicator located in the IPC ON. As soon as 3 distinct deployment commands (representing different events) have been issued to any belt pretensioner, or the SDM commands any front and side air bag to deploy once, the SDM shall be considered to not be reusable. 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 (PPS)
Important: The Passenger Presence System (PPS) is a calibrated unit. When replacing the assembly, all parts in the service kit must remain together. Do not mix any of the old parts with the new parts. 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) module. The PPS consist of an electronic control module, silicone filled sensor pad, pressure sensor, seat belt tension retractor sensor, wiring harness, and PASSENGER AIR BAG ON/OFF indicators. The silicone filled sensor pad is located under the passenger seat foam cushion and is connected by a hose clamped to the pressure sensor. The weight of the occupant sitting in the front passenger seat is measured as a pressure change within the bladder by the pressure sensor. The pressure sensor sends a voltage signal to the PPS module. If the pressure from the occupants 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 pressure from the occupants weight is higher than a specified value, the PPS module will send an enable signal to the SDM to enable the I/P module. 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 I/P. The PPS monitors itself for faults and will set DTCs if a fault is detected. The PPS will also notify the SDM of a fault and the SDM will request the instrument panel cluster (IPC) to turn ON the AIR BAG indicator located on the IPC. 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 in the center of I/P.
Inflatable Restraint PASSENGER AIR BAG ON/OFF Indicator
The PASSENGER AIR BAG ON/OFF indicators located in the center of the instrument panel cluster (IPC), is used to notify the driver and passenger when the instrument panel (I/P) air bag is enable or disable. The PASSENGER AIR BAG ON/OFF indicators are also used to display any faults/DTCs, when the sensing and diagnostic module (SDM) sends a command to the Passenger Presence System (PPS), the PPS will flash the ON/OFF indicators. The PPS will flash the first number of the DTC using the OFF indicator. This will be an ODD number, then flash the second number of the DTC using the ON indicator. For more information on this flash procedure refer to Passenger Presence System Flash Code Procedures. Component Tests and General Diagnostics
AIR BAG Indicator
The AIR BAG indicator, located in the instrument panel cluster (IPC), is used to notify the driver of Supplemental Inflatable Restraint (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 tests on all SIR system components and circuits. If no malfunctions are detected, the SDM will communicate with the IPC through the GMLAN 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 GMLAN 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, 2 initiating devices, canister of gas generating material and, in some cases, stored compressed gas. The 2 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 2 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 module 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 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 2 or more current-carrying coils. The coils allow the rotation of the steering wheel while maintaining continuous electrical contact between the steering wheel module deployment loop and the steering wheel module. Two coil wires are used for the steering wheel module deployment loop. Additional coil wires are used for accessories that are 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 steering wheel module when the connector is disconnected.
Steering Wheel and Column
The steering wheel and column are designed to absorb energy when driver contact is made with steering wheel or inflated module. In a collision, the driver may contact the steering wheel directly or load the steering wheel and column through the inflated module. When the driver applies load to the inflator module or the steering wheel, the column will compress downward, absorbing some of the impact and helping to reduce bodily injuries to the driver. The steering wheel and column must be inspected for damages after a collision.
Inflatable Restraint Front End Sensor
The front end sensor is equipped on vehicles to supplement the SIR System performance. The front end sensor is an electronic sensor and is not part of the deployment loops, but instead provides an input to the sensing and diagnostic module (SDM). The front end sensor can assist in determining the severity of some frontal collisions. The SDM contains a microprocessor which 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 frontal deployment loops deploying the frontal air bags.
Inflatable Restraint Seat Belt Retractor Pretensioner
The seat belt pretensioner modules contain a housing, a seat belt retractor, the seat belt webbing, an initiating device, and a canister of gas generating material. 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) will cause current to flow through the deployment loops to the initiator. Current passing through the initiator ignites the material in the canister producing a rapid generation of gas and the release of compressed gas, if present. The gas produced from this reaction deploys the seat belt pretensioner and retracts the seat belt webbing, which removes slack in the seat belt.
Depending on the severity of the collision, the seat belt pretension may deploy without the frontal inflator modules deploying, or they will deploy immediately before the frontal inflator modules deploy. As soon as 3 distinct deployment commands, representing different events, have been issued to any belt pretensioner, the SDM shall be considered to not be reusable.
Each seat belt pretensioner is equipped with a shorting bar located on the connector of the pretensioner. The shorting bar shorts the seat belt pretensioner deployment loop circuitry to prevent unwanted deployment of the pretensioner when servicing the seat belt pretensioner.
Inflatable Restraint Wiring Harness
The inflatable restraint wiring harness connects the inflator modules, the sensing and diagnostic module (SDM), the deployment loops, and the serial data circuit together using weather-packed connectors. SIR System connectors are yellow for easy identification. When repairing SIR wiring harnesses, follow the proper testing and repair procedures listed in the Service Data.
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 collision, the front seat occupants legs may come in contact with the knee bolsters. The knee bolsters are designed to crush and deform, absorbing some of the impact and helping 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 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 seat belt retractor pretensioners (left/right)
* Inflatable restraint side impact sensors (SIS) (left/right)
* Inflatable restraint roof rail modules (left/right)
* 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 side impact sensor (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 makeup 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 modules 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 side air bags.