Restraint systems (Operation): Description and operation

Operation

When the ignition is switched on, the airbag ECU 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 ECU. 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 ECU 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. Simultaneously, the airbag ECU 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 ECU.
• 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 ECU internal program status.

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 outputting the fire signals.

When deployment and collision event recording has been completed, the airbag ECU enters crash locked mode and illuminates the SRS warning lamp. In crash locked mode the airbag ECU is permanently disabled and must be replaced during subsequent repair action. Crash locked mode cannot be cleared using Testbook.

Diagnostics

While the ignition is on the diagnostic function of the airbag ECU monitors the SRS for faults. If a fault is detected, the airbag ECU 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. 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, only that circuit is disabled; 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.