Operation

Operation

General
When the ignition is switched on, the airbag DCU performs a bulb check of the SRS warning lamp as part of the power up procedure. The lamp should be extinguished after approximately 5 seconds, indicating that the system is fully operational. If the lamp remains illuminated, a fault has been detected and repair action is required.

While the ignition is on, data from the single point sensor is continuously monitored by the airbag DCU. If the data from the single point sensor indicates vehicle deceleration is at or above the preset limit, and the mechanical safing sensor is closed, the DCU interprets this as a collision that requires deployment of the airbags and retraction of the seat belt pretensioners. It then activates transistors to send fire signals to the airbag modules and the seat belt pretensioners. On pre 2002 MY vehicles, the airbag DCU simultaneously records in memory the following information:
- The error code of the last permanent fault (if any) detected before the collision.
- Internal program information about the collision as seen by the airbag DCU.
- The diagnostic status of the airbag and seat belt pretensioner circuits before deployment.
- The voltage of each power backup capacitor before deployment.
- Information on the airbag DCU internal program status.

NOTE: If external power is lost during the collision, recording of the last three above items only occurs if there is sufficient power in the backup capacitors after output of the fire signals.

After deployment, the airbag DCU enters a crash locked mode and illuminates the SRS warning lamp. In the crash locked mode the airbag DCU is permanently disabled and must be replaced during subsequent repair action. Crash locked mode cannot be cleared using T4.

Diagnostics
While the ignition is on the diagnostic function of the airbag DCU monitors the SRS for faults. If a fault is detected, the airbag DCU stores a related fault code in memory and switches the earth output to illuminate the SRS warning lamp. With a supply voltage range fault, the warning lamp is illuminated only for the duration of the fault. With all other faults, including intermittent faults, the warning lamp is illuminated for the remainder of the drive cycle. At the next ignition on, if the fault is still present the warning lamp remains illuminated after the lamp check; if the fault does not recur, the warning lamp extinguishes but the fault code remains stored in memory. On pre 2002 MY vehicles, an intermittent fault will be cleared from memory if 40 drive cycles are completed without its recurrence.

After detecting a fault, the system may retain some operational capability. If a fault is detected in an airbag or pretensioner circuit, the circuit may be disabled, depending on the fault; the other airbag and pretensioner circuits remain operational and their related components will still be deployed in a collision. If an internal or power supply fault is detected, the complete system will be disabled. If a fault exists in the SRS warning lamp circuit, the lamp will not illuminate during the lamp check at ignition on, but, provided there are no other faults, the system will otherwise be fully operational.

Fault code retrieval and fault diagnosis of the SRS can only be done using T4. Additional SRS information that can be read using T4 is the:
- Airbag DCU bar code.
- Evolution number of the hardware and software, and the algorithm level.
- Status of the crash locked mode.
- Vehicle Identification Number (VIN) data.

Diagnostic checks performed by the airbag DCU include:
- Monitoring of the airbag and pretensioner circuits for open/short circuits.
- Internal errors.
- Supply voltage (limits are: 8.6 to 19.0 V at power up, 6.0 to 19.0 V during drive cycle up to 2002 MY; 10.0 to 16.5 during normal operation from 2002 MY onwards).