BMW (SRS) Self Diagnostic System

OPERATION
Each time the ignition is switched "On", the Supplemental Restraint System (SRS) does a self-test. This is indicated by illumination of the SRS Warning Lamp for five seconds, after which the lamp extinguishes (standby mode) if no faults are detected. Along with internal checks, the integrity of the following external circuits is checked:

- All igniter circuits for shorts to ground or B+, and resistance range
- SRS Warning Lamp for open or shorts to ground or B+
- Safety Belt Buckle Contacts for open, shorted, and resistance range
- Seat Occupancy Detector (SBE) message validity (when installed)

Standby Mode
While in standby mode, the SRS control module constantly monitors the system for defects. When a defect is detected, the control module will respond in one of three ways.

Three Types Of Defects
- "Minor" faults (such as seat buckle contact failure) which do not prevent the system from operating. These faults illuminate the SRS warning lamp for about two minutes, and then the lamp goes out.

- "Non-critical" faults (such as igniter resistance) which do not prevent airbag deployment. In this case, the warning lamp is illuminated continuously, but the system is still fully operational.

- "Critical" faults, such as accelerometer failure, which would not allow proper system operation in the event of an impact. The warning lamp is illuminated continuously, the Processor sets a fault and disables the system via the IGNITION TRIGGER SHUTDOWN.

The control module non-volatile memory has the capacity to store a minimum of 10 different defects, along with corresponding recording of how long each defect was active. Diagnostic and Programming modes are used with the MoDiC or BMW compatible tester to initiate testing and read out diagnostic trouble (fault) codes.