RESTRAINT SYSTEMS
DESCRIPTION AND OPERATION
9
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.