Isuzu/Mazda/Subaru SRS Operation Isuzu 1995-2011 DESCRIPTION & OPERATION Amigo, Axiom, Rodeo, Trooper & VehiCROSS The SRS consist of the Sensing and Diagnostic Module (SDM), SRS clockspring, driver's air bag module, passenger's air bag module, air bag warning lamp and SRS wiring harness. The function of the deployment loops is to supply current to the air bag assemblies in the event of a front end collision of sufficient force up to 30° off the vehicle centerline. The air bag assemblies are only supplied enough current to deploy when the SDM detects vehicle velocity changes severe enough to require deployment. The SDM also monitors the SRS electrical airbag components. When an electrical component malfunction occurs, the SDM will illuminate the air bag warning lamp and store a diagnostic trouble code in the SDM memory. The trouble codes stored in the SDM memory can be accessed through the use of a suitable scan tool. The air bag assembly consists of an inflatable air bag and an explosive pyrotechnic inflator. The explosive pyrotechnic inflator is a canister of gas generating material and an initiating device. When deployment current is supplied to the air bag module, the current passing through the explosive pyrotechnic inflator which ignites the gas generating material. The gas produced by the gas generating material rapidly inflates the air bag module. The SRS clockspring is used to maintain continuous contact between the driver's air bag module and the deployment loop, while allowing for steering wheel rotation. The sensing device of the SDM converts vehicle velocity changes to electrical signals. These electrical signals are compared to a value stored in the SDM memory. When the electrical signal exceeds the value stored in the SDM memory, sufficient electrical current will be directed through the SRS wiring harness assembly to deploy air bag assemblies. The SDM contains a 24 Volt Loop Reserve (VLR) energy supply to provide deployment energy when ignition voltage or 12 VSS is lost in a frontal collision. Impulse & StylusThe development loop and the Diagnostic Energy Reserve Module (DERM) are the two main portions of the Supplemental Inflatable Restraint (SIR) system. The deployment loop portion supplies current, through the SIR module located in the steering wheel, which is used to deploy the air bag in the event of an front end collision of sufficient force. The deployment loop consists of the arming sensor, clockspring, SIR module and discriminating sensors (forward sensor and passenger compartment sensor). The arming sensor switches power to the SIR module on the high side of the deployment loop. Either of the discriminating sensors may provide ground to the SIR module on the low side of the deployment loop. The SIR module will only be supplied sufficient current to deploy when the arming sensor and at least one of the two discriminating sensors closes simultaneously for three milliseconds or more. The Diagnostic Energy Reserve Module (DERM) supplies the deployment loop with a 36-Volt Loop Reserve (36VLR) to assure sufficient power to deploy the air bag for several seconds after ignition voltage or 12 VSS is lost in a collision. Deployment will still take place without the DERM connected, if ignition voltage or 12 VSS is present and is greater than 7.5 volts at the arming sensor. The DERM is utilized in conjunction with the resistors in the arming sensor, discriminating sensor and resistor module. These resistors make it possible for the DERM to detect circuit and component defects within the deployment loop. The DERM monitors the voltage drop across each component within the loop during normal non-deployment conditions. If the monitored voltages fall outside of preprogrammed limits, the DERM will store a faulty code and illuminate the INFL REST indicator lamp. Side Impact airbags are also used on some 2003 and up models. The side impact AB is located inside the front seat backrest. ------------------------------------------------------------------------------ MAZDA THEORY OF OPERATION 1986-92 MIATA, 626, MX-6 & RX-7 The following models are equipped with a driver airbag component. The airbag system (SRS) is needed as a supplement to the driver side seat belt. The air bag will deploy when the vehicle is involved in a front end accident of sufficient force. Restraint is provided by an inflatable airbag stored in the steering wheel pad. All Other Models 1992-2011 The airbag system (SRS) is intended as a supplement to the driver and passenger side seat belts. The airbag system (SRS) will deploy for both driver and passenger when the vehicle is involved in a front end accident of sufficient force. Restraint for the driver is provided by an inflatable air bag stored in the steering wheel pad. Restraint for the passenger is provided by an inflatable airbag stored in the dashboard. airbag system (SRS) WARNING LAMP The warning lamp will indicate a problem or failure of the airbag system (SRS). The warning lamp will illuminate when the ignition switch is placed in the On position or after cranking the engine. The warning lamp should go Off after approximately 6 seconds if system is functioning properly. If the warning lamp fails to illuminate, remains On or flashes, there is a problem in the airbag system (SRS). BACKUP BATTERY A backup power supply stores sufficient power to operate the airbag system (SRS) if battery power is lost during a crash. The backup power supply momentarily holds enough charge to trigger the igniter after the current supply has been interrupted. It will not hold a charge indefinitely. CLOCKSPRING Ensures uninterrupted electrical connection to airbag module while allowing rotation of steering wheel. CRASH SENSORS The crash sensors detect crash impact and complete electrical circuit to the airbag module. Crash sensors consist of a bias magnet, plated sensing mass and electrical contacts. S and D type sensors are used in this system. The construction of the sensors is the same but, due to a difference is strength of their bias magnet, their shock sensitivity is different. DIAGNOSTIC MODULE or SRS MODULE The control module or SRS Module contains a computer that monitors the airbag electrical components and connections. The module performs an electrical circuit self-inspection and activates the warning lamp if a fault or condition is detected. System electrical conditions can be determined by the coded warning lamp display. If an airbag condition occurs and the warning lamp is malfunctioning, an audible alarm built into the module will sound to indicate the need for service. The airbag is intended as a supplement to the driver and passenger seat belts. The airbag will deploy for both driver and passenger (if equipped) when the vehicle is involved in a front end accident of sufficient force. Restraint for the driver is provided by an inflatable airbag stored in the steering wheel pad, and for the passenger by a bag in the instrument panel. ------------------------------------------------------------------------------ SUBARU THEORY OF OPERATION 1992-97 The Supplemental Restraint System (SRS) consists of a control unit, front sensors, a safety sensor built into the control unit and driver's and, on 1994-97 models, passenger's air bag modules. On SVX, 1992-94 Impreza and 1992-97 Legacy models, the front sensors and the safety sensors are connected in parallel respectively. The front sensors and safety sensor are connected in series, so that the air bag will inflate if at least one front sensor and at least one safety sensor detect an impact at the same time. On 1995-97 Impreza models, the electric sensor and safety sensor are connected in series, so that the air bag will inflate if the two sensors detect an impact at the same time. 1998-2011 The air bag system consists of an air bag control unit, electric sensor and safety sensor built into the control unit, front left-hand and right-hand sub-sensors, driver's and passenger's air bag modules, side air bag sensors and side air bag modules. A front impact will cause the safety sensor, electric sensor and sub-sensor to input an impact signal to the Central Processor Unit (CPU) or air bag control unit. That unit will then determine on the basis of the sensor input values whether or not to deploy the driver's and passenger's front air bags. A side impact signal that exceeds a predetermined value will cause the air bag on the impacted side to inflate. AIR BAG CONTROL UNIT The air bag control unit detects vehicle deceleration rate from electrical signals it receives from the sensors. When the vehicle deceleration rate exceeds a predetermined value, the module will deploy the appropriate air bag(s). A back-up power supply and boosting circuit is incorporated into the control unit to provide deployment voltage to the air bag system circuit in case of battery damage or voltage drop during a collision. The control unit also has a built-in self-diagnosis function. If a fault is present in the air bag system, the control unit will light an instrument panel air bag warning lamp and store a fault code in the module's memory. SUB-SENSOR FORESTER Front sub-sensors are mounted ahead of the left-hand and right-hand front wheel openings. A frontal impact exceeding a predetermined value will cause the mass in the sensor to rotate forward, closing the sensor contacts. When the sensor contacts close, an impact signal is input to the air bag control. IMPREZA, LEGACY & OUTBACK Front left-hand and right-hand front sub-sensors are installed ahead of the front wheel apron wall. The front sub-sensors are of the pendulum type. A frontal impact exceeding a predetermined value will cause the mass in the sensor to rotate forward, closing the sensor contacts. When the sensor contacts close, an impact signal is input to the air bag control unit. SAFETY SENSORS & ELECTRIC SENSORS The safety sensor and electric sensor are built into the air bag control unit. The safety sensor is of the pendulum type. A frontal impact (or side impact on models equipped with side air bags) exceeding a predetermined value will cause the mass in the safety sensor to rotate forward, closing the sensor contacts. When the safety sensor contacts close, an impact signal is input to the air bag control unit. This sensor is of the semiconductor type, Fig. 5. The electrical sensor senses deceleration at collision by the change of the electrical resistance and the impact sensing circuit. On SVX and 1992-97 Legacy models, front sensors are installed on either front wheel apron wall. If the roller type sensor receives a frontal impact exceeding a certain set limit, it rotates to turn the switch on. On 1995-97 Impreza models, the electric sensor is built into the control unit. This sensor is of the semi-conductor type and senses deceleration at collision by the change of electrical resistance and the impact sensing circuit, Fig. 7. The safety sensor is also built into the control unit. A piston type safety sensor is used. SIDE IMPACT AIR BAG SENSOR FORESTER, IMPREZA, LEGACY & OUTBACK The side impact air bag sensors are located on the left-hand and right-hand inner center pillars. When an impact greater than the sensor's set limit occurs, it will send an ignition signal to the air bag control unit. The side impact air bag module on the impacted side of the vehicle will deploy. DRIVER'S AIR BAG MODULE The air bag module is located at the center of the steering wheel and contains the air bag and inflator. If a frontal collision occurs, the inflator produces a large volume of gas to rapidly inflate the air bag. PASSENGER'S AIR BAG MODULE The air bag module is located on the passenger side of the instrument panel and contains the air bag and inflator. If a frontal collision occurs, the inflator produces a large volume of gas to rapidly inflate the air bag. SIDE IMPACT AIR BAG MODULE The side impact air bag module is located at outer side of the front seat backrest and contains an air bag and inflator. If a side collision occurs, the inflator produces a large volume of gas to rapidly inflate the air bag for that side of the vehicle. STEERING ROLL CONNECTOR The steering roll connector is located between the steering column and steering wheel. A flat cable stored in a spiral form transmits the control unit electrical signal to the steering wheel from the body harness. AIR BAG CONTROL UNIT CONNECTOR The air bag control unit uses a connector with double lock and coupling error detection mechanisms. If coupling is incomplete, the air bag warning lamp will light.