Air Bag Systems: Description and Operation

With BOSCH (Engine Management System)

The Supplementary Restraint System (SRS) provides enhanced passive protection for the driver and front passenger in the event of a serious collision.

SRS Diagnostic Control Unit (Z151)
The power supply to the SRS Diagnostic Control Unit (Z151) is fed via fuse 23 in the engine compartment fuse box, when the ignition is turned to position II. The SRS warning lamp in the Instrument Cluster (Z142) will be illuminated for a period of approximately 5 seconds after the ignition switch is tuned to position II, while the SRS Diagnostic Control Unit (Z151) performs a system integrity check. If a fault should be detected, the relevant fault code will be stored in the SRS Diagnostic Control Unit (Z151) memory and the warning lamp will remain illuminated for the remainder of the ignition cycle.

The SRS Diagnostic Control Unit (Z151) monitors the following components during the power-up phase and for the complete ignition cycle.
- Accelerometers
- Safing sensor
- SRS Diagnostic Control Unit (Z151) microprocessor
- Front airbags
- Side airbags
- Seatbelt pre-tensioners
- Front crash-sensors (NAS only)
- SRS warning lamps

The SRS Diagnostic Control Unit (Z151) contains a main sensor which is the primary device for detecting the presence of a crash condition, and a safing sensor which is used for confirmation of a crash condition. This confirmation signal is provided to prevent unintentional detonation of SRS system components. If in the event of a frontal or front angled collision, both sensors exceed the trigger values stored in the SRS Diagnostic Control Unit (Z151) memory, electronic switches are activated which allow electrical current to be supplied to the airbags and seatbelt pre-tensioners. The airbags and seatbelt pre-tensioners are then activated by a control signal from the SRS Diagnostic Control Unit (Z151).


WARNING: Only fully trained and authorized personnel should be allowed to work with SRS components.

Unintentional detonation of an SRS system component or the consequent failure of a component in a crash condition may cause serious injury.