Principles of Operation: Notes

WARNING: This page is about a different car, the 2008 Mercury Mariner and 2008 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

A supplemental restraint system (SRS) consists of a driver and passenger dual stage air bag module, safety belt buckle pretensioners, safety belt retractor pretensioners, safety canopies (containing an inflator/squib or gas generator and an air bag), impact sensors, 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. The RCM performs an internal, external circuitry and component "self-test" during each startup. If a SRS fault exists, the air bag warning indicator will illuminate and remain illuminated for the rest of the key cycle.

In addition to the self-test at start up, the RCM continuously monitors all of its external and internal circuitry for faults. The RCM communicates through the data link connector (DLC) the current and historical DTCs on the high-speed controller area network (HS-CAN). The RCM also communicates over the HS-CAN to the instrument cluster (IC) module, PCM and the occupant classification system module (OCSM).

In a frontal collision, the front impact severity sensor located in the front of the vehicle detects sudden vehicle deceleration and sends an electrical signal to the RCM. The RCM uses the information from the front impact severity sensor and the OCSM in deployment determination. If the RCM determines that air bag(s), safety belt buckle pretensioner(s), safety belt retractor pretensioner(s) and/or safety canopy 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)/safety canopies and/or activates the safety belt pretensioners to control the tension of the safety belt(s).