SIR System Description and Operation
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SIR System Description and Operation
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The airbag system (SIR) complements the protection given by the seat belts. The SIR system includes a module for sensing and diagnostics (SDM), airbags, belt tensioners (retractor spool) and collision sensors. The SDM determines the seriousness of a collision using collision sensors placed in strategic places in the car. When the SDM detects a collision, it will process the information provided by the sensors to further justify air bag or belt tensioner deployment. The SDM will deploy the air bags and belt tensioners if it detects a collision of sufficient force. If the collision force is not sufficient to justify airbag deployment, the SDM can even then activate the belt tensioners. The SDM includes a sensory device that converts changes in the car's speed to electrical signals. The SDM then compares these signals with values that are stored in the memory. If the signal exceed the saved values the SDM evaluates the collision force and causes a current to pass through the front activation circuits to deploy the front airbags and belt tensioners or otherwise only the belt tensioners. The SDM constantly monitors the activation circuits for faults and will illuminate the airbag indicator if a fault is detected. The SDM continuously monitors the electrical components of the SIR system. Upon detection of a circuit malfunction, the SDM will set a DTC and inform the driver by illuminating the AIR BAG indicator. The steering column and knee bolsters 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.
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AIR BAG Indicator (Driver)
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The AIR BAG indicator, located in the instrument cluster, is used to notify the driver of SIR system malfunctions and to verify that the inflatable restraint sensing and diagnostic module (SDM) is communicating with the instrument cluster. When the ignition is turned ON, the SDM is supplied with ignition positive voltage. The instrument cluster will momentarily turn on the AIR BAG indicator. While the indicator is on, the SDM conducts tests on all SIR system components and circuits. If no malfunctions are detected, the SDM communicates with the instrument panel cluster via the serial data circuit and commands the air bag indicator off. The SDM continually monitors the air bag circuits by carrying out a number of checks. If a malfunction is detected the SDM will store a diagnostic trouble code (DTC) and command the instrument cluster to illuminate the AIR BAG indicator via 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.
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Inflatable restraint sensing and diagnostic module (SDM)
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The airbag sensing and diagnostic module (SDM) is a microprocessor and the control center for the supplemental inflatable restraint (SIR) system. The SDM contains internal sensors together with several external collision sensors that are mounted at strategic locations in the vehicle. In the event of a collision, the SDM compares the signals from the internal and external impact 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 illuminates the AIR BAG indicator located in the instrument cluster. 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 request the instrument cluster to illuminate the AIR BAG indicator, notifying the driver that a malfunction exists. In the event that ignition positive voltage is lost during a collision, the SDM maintains a 23-volt loop reserve for deployment of the air bags. It is important when disabling the SIR system for servicing or rescue operations to allow the 23-volt loop reserve to dissipate, which could take up to 1 minute.
This vehicle contains 8 air bags. The 8 air bags are located in the driver steering wheel (dual air bags), passenger instrument panel (passenger side) (dual air bags), driver seat side (B-pillar), passenger seat side (B-pillar), left rear seat side (C-pillar), right rear seat side (C-pillar), left roof rail, and right roof rail. Air bags contain a housing, inflatable air bag, two initiating devices (if dual air bags), canister of gas generating material and, in some cases, stored compressed gas. The deployment loops supply current to deploy the air bags. The steering wheel and passenger instrument panel air bags have two stages of deployment, which varies the amount of restraint to the occupant according to the collision severity. In the event of moderately severe frontal collisions the airbags are inflated to an extent less than maximum and which comprises the airbag step 1. In a more severe frontal collision the airbags are fully inflated which means that both steps 1 and 2 are activated. The current that flows through the airbag ignites the material in the container which causes rapid gas generation and in some cases the venting of 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 air bag is equipped with a shorting bar located in the connector. The shorting bar shorts the air bag deployment loop circuitry to prevent unwanted deployment of the air bag when it is disconnected.
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Seat Belt Pretensioners (Anchor and Retractor)
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The seat belt pretensioners (driver and passenger) consist of a housing, seat belt anchor (located in the seat), seat belt retractor (located in the B-pillar), 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 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 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 front air bags deploying, or they will deploy immediately before the front air bags 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.
This vehicle contains 6 impact sensors. The 6 impact sensors are located in the front of the vehicle (2, left and right), 2 in the B-pillars (left and right), and 2 in the C-pillars (left and right). The impact sensors contain a sensing device which monitors vehicle acceleration and velocity changes to detect side collisions that are severe enough to warrant air bag deployment. The impact sensors are not part of the deployment loop, but instead provide input to the 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 appropriate air bags.
The seat belt indicators are controlled through the inflatable restraint sensing and diagnostic module (SDM). For further information on seat belt indicators refer to
Seat Belt System Description and Operation
