Principles of Operation
Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS)
Refer to Wiring Diagram Set 46, Air Bag for schematic and connector information. Diagrams By Number
Principles of Operation
The Restraints Control Module (RCM) continuously receives/monitors inputs from the following Supplemental Restraint System (SRS) components.
- Front impact severity sensor (left frontal restraints sensor)
- Driver safety belt buckle switch
- Passenger Air Bag Deactivation (PAD) switch
If the RCM detects sudden vehicle deceleration and/or lateral deceleration based on all the information received from all sensors and switches and determines that deployment is required, the RCM sends voltage and current to deploy the appropriate SRS components.
During a frontal crash, the RCM may deploy the following SRS components, based on crash severity and impact sensor input.
- Safety belt buckle pretensioner(s)
- Driver/passenger single-stage air bag
The fact that the safety belt buckle pretensioners or air bags did not activate for both front seat occupants in a collision does not mean that something is wrong with the system.
The RCM performs a self-test of the complete SRS during each startup. If a SRS fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the ignition cycle. In addition to the self-test at start up, the RCM continuously monitors all of its SRS components and circuitry for correct operation.
Air Bag Warning Indicator
The air bag warning indicator:
- located in the Instrument Cluster (IC) module will prove out by lighting for 6 seconds and then turn off.
- will flash and/or illuminate based on the message the IC module receives from the RCM.
- will illuminate if the IC module does not receive a message from the RCM.
Clockspring
The clockspring:
- allows for continuous electrical connections between the driver air bag module and the RCM when the steering wheel is turned.
Driver Air Bag Module
The driver air bag module:
- is a single stage air bag and will deploy depending on vehicle impact severity and sensor input.
High Speed Controller Area Network (HS-CAN)
This vehicle utilizes a communication system called a High Speed Controller Area Network (HS-CAN). The RCM communicates with various modules for required information about the vehicle. Refer to Information Bus.
Passenger Air Bag Module
The passenger air bag module:
- is a single stage air bag and will deploy depending on vehicle impact severity and sensor input.
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.
NOTE: Prior to removal of the RCM, it is necessary to upload module configuration information to the scan tool. This information needs to be downloaded into the new RCM module once installed. Refer to Information Bus.
The RCM carries out the following functions:
- Deploys the air bag(s) in the event of a deployable collision
- Activates the safety belt buckle pretensioners to remove slack from the safety belt in the event of a deployable collision
- Monitors the SRS for faults
- Sends a message to the IC module to illuminate the air bag warning indicator if a fault is detected
- Communicates through the Data Link Connector (DLC) the current and/or historical DTCs
The RCM monitors the SRS for possible faults. If a fault is detected, the RCM will request the IC module to illuminate the air bag warning indicator. When the ignition is turned OFF and then ON, the IC module will prove out the air bag warning indicator by lighting for 6 seconds. If no faults are detected by the RCM the IC module will turn the air bag warning indicator off and it will remain off. If the RCM detects a fault, it will send a message to the IC module to turn the air bag warning indicator on and it will remain on for the rest of the key cycle. If the RCM requests illumination of the air bag warning indicator and the air bag warning indicator does not function, the IC module will automatically 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.
The RCM includes a backup power supply. The backup power supply will 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. The backup power supply will deplete its stored energy approximately one minute after the battery ground cable is disconnected.
The RCM will also communicate the on-demand (current) and continuous (historical) DTCs through the DLC to the scan tool.
Safety Belt Buckle Switches
The driver safety belt buckle is comprised of integrated circuits called a Hall-effect switch. The safety belt buckle switch indicates to the RCM whether the safety belt are buckled or unbuckled.
The RCM supplies current to the safety belt buckle switch. Current flows through the switch, buckled or unbuckled to ground. The RCM will sense the difference in this current draw, approximately 6 mA (unbuckled) or 15 mA (buckled). If the RCM detects current out of this range, it will set a DTC.
The RCM communicates the driver safety belt buckle switch status to the IC module, which monitors the information to control the safety belt warning indicator. For information on the safety belt warning indicator, refer to Instrument Panel, Gauges and Warning Indicators.
Safety Belt Buckle Pretensioners
The safety belt buckle pretensioners remove excess slack from the safety belt webbing. The pretensioners are activated by the RCM when the module detects a collision event force exceeding a programmed limit.
Secondary Air Bag Warning (Chime)
The secondary air bag warning chime is an audible chime located in and controlled by the IC module. If a fault is detected with the air bag warning indicator a DTC will be stored in memory of the IC module. Upon receiving the message from the RCM that a SRS fault has been detected, the IC module will sound the secondary air bag warning chime in a pattern of 5 sets of 5 beeps.
Impact Sensors (Restraints Sensor)
For this vehicle line, the SRS uses one satellite sensor in addition to the RCM. The RCM is mounted beneath the driver seat. All vehicles have one front impact severity sensor located in the front-center area of the vehicle, behind the grille mounted on the radiator support. Mounting orientation is critical for correct operation of all impact sensors.
Self Test/Continuous Memory and Clear DTCs
During vehicle operation, the RCM will detect and store both intermittent (continuous memory) and hard failure DTCs in non-volatile memory.
The DTC strategy employed by the RCM incorporates a time-out scheme for determining when a concern exists in the system. This requires a concern to mature for up to one minute in the system before the RCM will store a DTC. For the RCM to determine that a concern no longer exists, the concern must be absent for up to one minute. The actual detection time-outs vary with each DTC.
Once 127 ignition cycles have been recorded by the RCM since the concern was last detected, all CMDTCs will automatically be removed from memory.
Lamp Fault Codes (LFCs)
This vehicle supports Lamp Fault Codes (LFCs) only when the RCM is in plant mode. A new RCM installed to a vehicle will be in plant mode until:
- Programmable Module Installation (PMI) has been carried out.
- the RCM sees a fault-free SRS (no DTCs present).
If a LFC is present after completing PMI, a fault is present in the SRS and on-demand DTCs must be retrieved and diagnosed.
Prove Out Procedure
When the ignition is first turned to the ON or RUN position, the IC module and the RCM will perform a prove out by checking internal and external circuity, internal components and functions. The IC module illuminate the air bag warning indicator continuously for approximately 6 seconds and then turn off and remain off if no SRS fault is present. If, during the RCM prove out, the RCM has detected a failure within the SRS, it will send a message to the IC module to illuminate/flash the air bag warning indicator and it will remain illuminated for the remaining key cycle. If an SRS fault is present, the air bag warning indicator will:
- fail to light.
- remain lit continuously.
- flash.
The air bag warning indicator may not illuminate to indicate a fault is present until approximately 30 seconds after the ignition switch has been turned from the OFF to the ON position. This is the time required for the RCM to complete the testing of the SRS. If the air bag warning indicator is inoperative and an SRS fault exists, a chime will sound in a pattern of 5 sets of 5 beeps. If this occurs, the air bag warning indicator will need to be repaired before diagnosis can continue.