Principles of Operation: Notes

WARNING: This page is about a different car, the 2008 Ford RV Cutaway, 2008 Ford Econoline, 2008 Ford E450 Super Duty, and 2008 Ford Cutaway. 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 single stage air bag module (containing an inflator/squib or gas generator and an air bag), safety belt buckle pretensioners (containing an inflator/squib or gas generator), impact sensor, a restraints control module (RCM), a clockspring, an air bag warning 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 sends a message to the instrument cluster (IC) module to flash a coded sequence which is called a LFC on the air bag warning indicator. Each LFC is flashed 3 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 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 fact that the pretensioners or airbags did not activate for both front seat occupants in a collision does not mean that something is wrong with the system.

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 IC module and PCM.