Description & Operation

During a frontal collision of sufficient force, the SIR system deploys air bag from center of the steering wheel. System consists of 2 main sub-systems: the deployment loop and the Diagnostic Energy Reserve Module (DERM).

Main function of deployment loop is to supply current to inflator (air bag) module in the steering wheel. Deployment loop consists of an arming sensor, SIR coil assembly, inflator module and 2 discriminating sensors. The arming sensor switches power through the SIR coil assembly to the inflator module. Discriminating sensors provide a ground for the deployment loop. The arming sensor and at least one of the discriminating sensors must activate at the same time for the deployment loop to be complete.

SIR coil assembly provides a continuous path for current in the circuit between the steering column and the inflator module (assembly is similar in function to a horn brush and slip ring assembly). A coil of wire inside the assembly winds up when the steering wheel is turned in one direction, and then unwinds when turned in the opposite direction.

Main function of DERM is to supply the deployment loop with a 36-Volt Loop Reserve (36-VLR) for 10 minutes in case the ignition feed to the arming sensor is lost during a collision. Another function of the DERM is to monitor circuit and component faults within the system. The DERM applies a small amount of current through the deployment loop to monitor the voltage drop across each component.

A resistor module near the DERM limits the amount of this current flow through the deployment loop. If voltages are not within expected limits, DERM turns on the AIR BAG warning (telltale) light on instrument cluster and stores a fault code. Fault codes can be retrieved from the DERM using a Scan tester at the ALDL connector.