Air Bag Control Module: Description and Operation

Deployment Loop and Diagnostic Energy Reserve Module
The main portions of the Supplemental Inflatable Restraint (SIR) system are the development loop and the Diagnostic Energy Reserve Module (DERM). The main function of the deployment loop is to supply current through the SIR module in the steering wheel which will cause deployment of the airbag in the event of an accident severe enough to warrant deployment.

The arming sensor, coil, SIR module and discriminating sensors (forward sensor and passenger compartment sensor) make up the deployment loop. The arming sensor switches power to the SIR module on the high side of the deployment loop. Either of the discriminating sensors can supply ground to the SIR module on the low side of the deployment loop. The SIR module is only supplied sufficient current to deploy when the arming sensor and at least one of the two discriminating sensors closes simultaneously for three milliseconds or more.

The main function of the Diagnostic Energy Reserve Module (DERM) is to supply the deployment loop with a 36 volt loop reserve (36VLR) to assure sufficient power to deploy the airbag for several seconds after ignition voltage is lost in an accident. Deployment can still take place without the DERM connected if ignition voltage is present and is greater than 7.5 volts at the arming sensor.

Another function of the DERM is electrical system diagnostics. The DERM is utilized in conjunction with the resistors in the arming sensor, discriminating sensor and resistor module. These resistors make it possible for the DERM to detect circuit and component faults within the deployment loop. The DERM monitors the voltage drop across each component within the loop during normal non-deployment conditions. If the monitored voltages fall out-side of expected limits the DERM will indicate a fault through the storage of a malfunction code and the illumination of the ``Inflatable Restraint'' indicator lamp.