Air Bag Systems: Description and Operation

The control module (ECU) with integral safing sensor and diagnostic capability is installed by the RH 'A' post. The ECU also incorporates independent back-up battery power supplies to ensure airbag deployment in the event of loss of main battery power in a crash. Two primary crash sensors hardwired to the airbags, are fitted forward of the radiator, inboard of the head lamps. Electronic extension of the primary sensors (dwell enhancement) incorporated in the ECU, maintains deployment circuitry in the event of loss of primary sensor signals. A disarm feature also incorporated in the ECU, isolates deployment power from the airbag system until a primary sensor closes. The airbag system will only be activated when the ignition switch is in position II or III and the air bags will only deploy if at least one primary sensor and the safing sensor are made.

Before any airbag/SRS service is performed, at least one minute must elapse after disconnection of the battery positive cable, to allow dissipation of back-up power supply energy.

The airbag/SRS system ECU continuously monitors all system components and wiring connections for faults. If a fault occurs in the airbag/SRS readiness indication line, a code written into the non-volatile memory of the ECU is transmitted to the system cluster to display a continuous LED warning. In the event of a further fault being detected during this condition, an audible tone comprising five sets of five bleeps will sound. The readiness indicator line fault must be corrected before fault codes can be determined.

If a fault creates a risk of spurious airbag/SRS deployment, the ECU will attempt to remove deployment power by rupturing the 10A battery fuse via a solid state switch. To aid fault diagnosis, the ECU will not rupture the fuse again until the fault is rectified and cleared. The 10A battery fuse must not be renewed until the airbag/SRS system has been de-activated. The fuse (F4) is located in the fuse-box at the driver's end of the fascia.