Principles of Operation

PRINCIPLES OF OPERATION

A supplemental restraint system (SRS) consists of a driver and passenger dual stage air bag module (containing an inflator/squib or gas generator and an air bag), safety belt pretensioners (containing an inflator/squib or gas generator), impact sensor, a restraints control module (RCM), a clockspring, an air bag warning indicator, occupant classification sensor (OCS) system and a passenger air bag deactivation (PAD) indicator. These components are all interconnected by a wiring harness and powered by the vehicle's battery. The RCM includes a backup power supply. This feature provides sufficient backup power to deploy the SRS components in the event that the ignition circuit is lost or damaged during impact. The backup power supply will deplete its stored energy approximately one minute after power and/or ground has been removed from the RCM. If a SRS fault exists, the RCM supplies a ground to flash a coded sequence which is called a LFC on the air bag warning indicator located in the instrument cluster (IC) module. Each LFC is flashed 5 times after which the air bag indictor will remain lit for the remainder of the key-on cycle. In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults.

In a frontal collision, the impact sensor located in the front of the vehicle detects sudden deceleration and sends an electrical signal to the RCM. The RCM uses the information from the impact sensor in the deployment determination. If the RCM determines that deployment is required, the RCM sends voltage and current to the squib(s) causing the solid chemical propellant to undergo a rapid chemical reaction. This controlled reaction produces harmless nitrogen gas that fills the air bag(s) and/or activates the safety belt pretensioners to remove slack from the safety belt(s).

The RCM communicates through the data link connector (DLC) the current and historical DTCs.

Air Bag Warning Indicator
The air bag warning indicator:
- is located in the instrument cluster (IC) module.
- prove out is a function of the restraints control module (RCM). The RCM will prove out the air bag warning indicator by illuminating it for 6 seconds and then turn off.
- will flash/illuminate based on the supplemental restraint system (SRS) fault.
- will illuminate continuously if the RCM is disconnected.

Clockspring
The clockspring:
- is mounted on the steering column, behind the steering wheel.
- provides a continuous electrical path from the driver air bag module to the restraints control module (RCM).

Driver Air Bag Module
The driver air bag module:
- is steering wheel mounted.
- will deploy upon receiving a flow of current from the restraints control module (RCM).
- has no subassemblies.

Electrical System
The electrical system that supports the air bag supplemental restraint system (SRS):
- receives power from the battery through the ignition circuit.
- provides the electrical path from the restraints control module (RCM) to the SRS components.
- provides the electrical ground path for the air bag warning indicator.
- provides the electrical path from the RCM to the data link connector (DLC).
- provides the electrical ground path from the instrument cluster (IC) module to the RCM.

Loops/Squibs
All deployable devices contain an initiating device called a squib. The squib is part of the deployment loop. Air bag/safety canopy modules can contain more than one squib, some vehicles may have up to 4 squibs in one air bag module. Squibs are often referred to as loops during the diagnostic process.

Occupant Classification Sensor (OCS)

NOTE: To identify between a production occupant classification sensor (OCS) system and an OCS system service kit, inspect the occupant classification system module (OCSM) electrical connector.

A production OCS system allows the disconnection of the OCS module electrical connector. An OCS system service kit has the OCSM electrical connector glued to the module, it cannot and should not be disconnected or altered. An OCS system service kit also has an in-line connector between the OCSM and the seat wiring harness.

The seat OCS system is found only on the front passenger seat. The front passenger seat OCS system is comprised of a silicone gel-filled bladder mounted in the seat cushion, a pressure sensor that is mounted to the seat frame and OCSM which is also mounted to the seat frame. Pressure is applied to the OCS system bladder when weight of any occupant or object in the front passenger seat is present. The pressure is then transferred through a tube, is sensed by the OCS system pressure sensor, then electronically communicated to the OCSM. Based on preprogrammed set points, the OCS module will inform the restraints control module (RCM), via a dedicated high-speed controller area network (HS-CAN), of the weight of any occupant or object in the front passenger seat. The RCM uses this information in determining if the passenger air bag module is to be deployed in the event of a deployable collision. The RCM may also use this information to illuminate/not illuminate the passenger air bag deactivation (PAD) indicator.

When an OCS system fault is present, the supplemental restraint system (SRS) defaults the passenger air bag module to ON (activated) regardless of the size of the occupant in the front passenger seat.

The OCS system components (seat cushion foam pad, bladder with pressure sensor and OCSM) are calibrated to each other and are serviced as an assembly. OCS system components are not to be installed separately.

The OCS system also interprets a variable voltage signal provided by the safety belt tension sensor to identify the presence of a correctly fastened child safety seat in the front passenger seat. The OCSM then communicates with the RCM, automatically deactivating the passenger air bag module. Refer to Safety Belt Tension Sensor.

The OCS system is also used for operation of the passenger Belt-Minder(R). For information on the passenger Belt-Minder(R) feature, refer to Seat Belt Systems. To deactivate or reactivate the passenger Belt-Minder(R) feature, refer to Instrument Panel, Gauges and Warning Indicators or the Owner's Literature.

Passenger Air Bag Deactivation (PAD) Indicator
The passenger air bag deactivation (PAD) indicator is a visual indicator used to inform the front seat occupants of the passenger air bag deactivation state. The PAD indicator is a stand-alone lamp installed into the vehicle instrument panel in a position visible to each front seat occupant.

The restraints control module (RCM) controls the state of the PAD indicator through a direct hardwire connection, based on information provided by the occupant classification sensor (OCS) system. The PAD indicator is lit to indicate the passenger air bag module is disabled. An exemption to this is when the front passenger seat is determined to be empty, and therefore indication of a deactivated passenger air bag module is not necessary. In all other cases, the PAD indicator is unlit when the passenger air bag module is enabled.

When the ignition switch is in the ON position, the PAD indicator prove-out period is initiated by the RCM. The RCM briefly activates the PAD indicator to prove-out the indicator function and verify to the front occupants proper functional operation of the PAD indicator.

The PAD indicator will be lit/unlit within 1.0 to 1.5 seconds of a change of state from the OCS system.

When an OCS system fault is present, the supplemental restraint system (SRS) defaults the passenger air bag module to enabled regardless of the size of the occupant in the front outboard passenger seat. The PAD indicator will be unlit. For information on the OCS System, refer to Occupant Classification Sensor (OCS).

The following tables indicate the passenger air bag status and the PAD indicator status based on the size of the front outboard passenger occupant and the passenger safety belt buckle switch status.

Passsenger Air Bag and PAD Indicator Status:

Passenger Air Bag Module
The passenger air bag module:
- will deploy upon receiving a flow of current from the restraints control module (RCM).
- is installed as an assembly.
- is mounted in the passenger side of the instrument panel.

Restraints Control Module (RCM)

WARNING:
- If a vehicle has been in a crash, inspect the restraints control module (RCM) and the impact sensor (if equipped) mounting areas for deformation. If damaged, restore the mounting areas to the original production configuration. A new RCM and sensors must be installed whether or not the air bags have deployed. Failure to follow these instructions may result in serious personal injury or death in a crash.
- Always tighten the fasteners of the restraints control module (RCM) and impact sensor (if equipped) to the specified torque. Failure to do so may result in incorrect restraint system operation, which increases the risk of personal injury or death in a crash.

NOTE: When installing a new restraints control module (RCM), always make sure the correct RCM is being installed. If an incorrect RCM is installed, erroneous DTCs will result.

The RCM is mounted on the center tunnel between the front seats. The RCM:
- supplies current flow to the inflators to deploy the air bags in the event of a deployable crash.
- activates the safety belt buckle pretensioners to remove slack from the safety belt.
- monitors the air bag supplemental restraint system (SRS) for faults.
- illuminates the air bag warning indicator if a fault is detected.
- illuminates the passenger air bag deactivation (PAD) indicator when conditions warrant illumination.
- flashes the air bag warning indicator to indicate the LFC detected.
- communicates through the data link connector (DLC) the current or historical DTCs.
- signals the instrument cluster (IC) module to activate a chime if the air bag warning indicator is not available and another SRS fault exists.
- contains an internal safing sensor which is not serviced separately.
- records deployment event data.

The RCM monitors the SRS for possible faults. If a fault is detected while the ignition switch is in the ON position, the RCM will illuminate the air bag warning indicator located in the IC module instrument cluster.

When the ignition is cycled (turned OFF and then ON), the air bag warning indicator will remain lit for 6 seconds and then go out. If an SRS fault exists, the air bag warning indicator will then flash the 2-digit LFC. The air bag warning indicator will flash the LFC 3 times, then it will remain illuminated for the rest of the key cycle. The RCM will also communicate the current and historical DTCs through the DLC, using a scan tool. If the air bag warning indicator does not function, and the system detects a fault condition, the RCM will signal the IC module to activate an audible chime. The chime is a series of 5 sets of 5 tone bursts. If the chime is heard, the SRS and the air bag warning indicator require repair.

LFCs are prioritized. If 2 or more faults occur at the same time, the fault having the highest priority will be displayed first. After that fault has been corrected, the next highest priority fault will be displayed.

The RCM includes a backup power supply. This feature provides sufficient energy to deploy the front air bags and safety belt buckle pretensioners in the event that the ignition circuit is damaged or lost in a collision before the RCM determines that air bag deployment is required. The backup power supply will deplete its stored energy approximately one minute after power and/or ground is removed from the module.

Safety Belt Buckle Switch
As part of the supplemental restraint system (SRS), the driver and front outboard passenger safety belt buckles are equipped with a Hall-effect switch. The safety belt buckle switches indicate to the restraints control module (RCM) whether the safety belt buckles are buckled or unbuckled.

The RCM uses this information in determining the deployment rate of the dual-stage driver and passenger air bag modules.

The safety belt buckle switches are also used for the driver safety belt warning system and the driver Belt Minder(R) and passenger Belt Minder(R) (if equipped). Refer to Seat Belt Systems and Instrument Panel, Gauges and Warning Indicators or the Owner's Literature.

Safety Belt Pretensioners
As part of the supplemental restraints system (SRS), the safety belt buckles are equipped with pretensioners. The safety belt buckle pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the restraints control module (RCM) when the module detects a frontal impact event exceeding a programmed limit and the safety belt is latched (buckled). The passenger safety belt pretensioner can deploy regardless of the presence of an occupant if the front outboard passenger safety belt is latched (buckled).

Safety Belt Tension Sensor
The safety belt tension sensor:
- is part of the front outboard passenger safety belt and retractor assembly.
- is located at the safety belt anchor point.
- is used in conjunction with the occupant classification sensor (OCS) system.
- is a 3-wire Hall-effect sensor that is part of the front passenger safety belt and retractor assembly.

The safety belt tension sensor is used by the OCS system to identify the presence of a child safety seat on the front outboard passenger seat, when the child safety seat is installed according to manufacturer instructions. The safety belt tension sensor senses the tension on the safety belt assembly then provides an output to the occupant classification system module (OCSM), indicating that the safety belt assembly is cinched. After sensing the weight applied to the seat by the occupant and using the safety belt tension sensor input, the OCS system determines how the occupant should be classified and communicates this information to the restraints control module (RCM). If the occupant is classified to be a child, the RCM will then automatically deactivate the passenger air bag module.

Seat Track Position Sensor
The seat track position sensor is a Hall-effect sensor located on the driver seat track. Based on programmed limits, the seat track position sensor will inform the restraints control module (RCM) of the driver seat position. The RCM uses this information in determining the deployment of the dual-stage driver air bag module and deployment of the collapsible column.

Secondary Air Bag Warning (Chime)
The secondary air bag warning chime, is an audible tone generated by the instrument cluster (IC) module. If the restraints control module (RCM) has detected a fault with the air bag warning indicator, a DTC will be stored in the memory of the RCM. Upon the RCM sensing that a supplemental restraint system (SRS) fault has been detected, the RCM will signal the instrument cluster module to sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps. The tone pattern will repeat periodically until the problem and warning light are repaired.