General System Description

WARNING: The SRS is not a substitute for wearing a seat belt.

PURPOSE
The Central Airbag (ZAE) Supplemental Restraint System (SRS) provides additional protection for the head and upper body of the driver and front passenger via two inflateable airbags, in the event of a specific frontal impact. The knees of the front occupants are protected by two energy absorbing bolsters, one underneath the instrument panel and the other incorporated into the glove box.








OVERVIEW OF KEY FEATURES
The ZAE module incorporates integral impact sensing in the form of a piezoelectric accelerometer and a safing-sensor, eliminating the need for external wheelhouse-mounted mechanical impact sensors. The system utilizes contacts in the safety belt latch, and also monitors the passenger Seat Occupancy Detector (SBE) system. This information is processed by the ZAE module to adjust deployment thresholds and alter deployment strategy as appropriate for the impact situation.

The ZAE module is capable of deploying the driver's and front passenger's airbag, as well as a pyrotechnic safety belt buckle tensioners for the driver and front passenger.

NOTE: Pyrotechnic safety belt tensioners are not available on current models. The ZAE module, however, has the capability to deploy such devices. The belt tensioners used in current model year vehicles are of the purely mechanical type, and are not activated by a control module.

The ZAE module can be used to deploy SRS devices for three separate collisions. It may not be necessary to replace a ZAE SRS control unit until after the third impact resulting in a deployment (unless physically damaged). Impact recording is automatically performed by the ZAE module, and deployment information is recorded in non-volatile (permanent) memory. After the third impact triggering deployment, the ZAE module "locks" itself permanently and sets a special diagnostic trouble (fault "100") code. In this condition, the ZAE module can no longer be reset, and must be replaced.

Only one ZAE hardware version for all BMW vehicles and models is available. All replacement "spare parts" ZAE control modules delivered are not coded and must be coded before use. This is accomplished with the MoDiC and Coding software version 10.0 or later.

All two pin connectors in the igniter supply leads have a special feature that short circuits the two pins when the connector is disconnected. This prevents unintentional airbag inflation during repairs and installations.





Contact Ring
The electrical connection between the steering wheel/steering column is accomplished by a "continuous loop" contact ring. This contact ring is made up of a wire ribbon that will coil and uncoil when the steering wheel is turned. This assures perfect electrical contact to the steering wheel under all conditions.

NOTE: Arrow points to locking screw that keeps new contact ring in center position. Special care must be taken when working in this area.





Safety Belt Tensioner
In addition to the SRS, front seats are equipped with mechanical safety belt tensioners which are designed to take up slack in the safety belts in the event of a frontal collision.

A frontal impact triggers the mechanical impact sensors, releasing the pre-tensioned springs, pulling the belt receptacles down approximately 55 mm. This tightens the lap and shoulder belts, if fastened.





The Seat Occupancy Detector (SBE)
The SBE informs the ZAE module whether or not the front passenger seat is occupied. The SBE system is not just a simple pressure switch, however, and has its own integral control module in the passenger seat bottom cushion. The control module is nondiagnosable (except through ZAE fault codes). The safety belt latch contacts and associated circuitry are incorporated into the female belt latch receptacles. Circuits use fixed resistances on both the switch open and switch closed contacts. This allows the ZAE module to detect safety belt latched, safety belt not latched, or a failure in the form of either a shorted or open circuit.

Communication with the ZAE module is accomplished with a single direction serial data-bus. The SBE transmits 20 digital messages per second; each message is seven bits long. Three of these bits are used to control communication between the SBE and the ZAE. The four remaining bits contain the actual "seat" information, and can communicate as to whether the seat is occupied, not occupied, or whether there is a "shorted' type failure or "open" type failure. The ZAE will only recognize the seat as being unoccupied if it receives a continuous, error-free string of 20 "unoccupied' messages from the SBE. Once the seat is considered occupied, this status is held in the control module for approximately two minutes.

If a SBE failure occurs, the ZAE assumes the seat is occupied (fail-safe).

NOTE: The passenger seat will be considered occupied if the sensor mat is loaded with a force of approximately 20 pounds or more. Therefore, items such as briefcases and grocery bags should be securely stowed instead of being carried on the front seat. In addition, the weight of a child restraint seat might also be sufficient to cause the sensor mat to consider the front passenger seat occupied.





OPERATION
Standby Mode
The SRS system is designed to operate only in frontal collisions, but the safety belt must still be worn if the driver is to benefit from its full protective function. In minor collisions, a roll over, or in the event of a side or rear-end impact, the safety belts are the sole means of protection. The diagram illustrates the impact area designated to activate the SRS.

The system is operational at all ignition switch positions except "Off", and is constantly monitored by the control module (Standby Mode).

Each time the SRS is switched "On", the system does a self-test. This is indicated by illumination of the SRS Warning Lamp for five seconds, after which the lamp goes out 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)

There are three types of faults:

- "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 faults, along with corresponding recording of how long each fault was active. Diagnostic and Programming modes are used with the MoDiC or equivalent tester to initiate testing, read out diagnostic trouble (fault) codes, read out ZAE identity, and program vehicle-specific information.

Driver Airbag:

Passenger Airbag:

Deployment
With the use of an accelerometer type impact sensor instead of a contact type, the ZAE - SRS module can distinguish between different magnitudes of impact. This information, coupled with inputs from belt latches and the SBE, allows the ZAE module to employ a "tiered" deployment strategy in impact situations preventing unnecessary deployment of the front passenger airbag.

The most important aspect, however, of this "smart" deployment strategy is the ability to raise the airbag deployment threshold (impact tier) if the safety belt is in use. This allows the entire restraint system to respond to an impact appropriately, considering magnitude of impact, the occupancy of the vehicle, and the driver and passenger safety belt use. Note that driver and passenger situations are analyzed and handled independently. The impact tiers and the strategy for the drivers seat is listed above. The table that follows shows the effect of the various possible inputs and impact scenarios for the front passenger seat.

To avoid unwarranted airbag inflation under normal driving conditions at least two sensors, one of which must always be the safing sensor, must be activated simultaneously.





At impact igniters, which are triggered by the ZAE - SRS module, generate the ignition temperature necessary for the solid fuel in the gas generators to burn. The gas generated is passed through a filter, cooled slightly and then used to inflate the airbags.

As the airbags are filled with this gas they bursts through predetermined breaking points in the cover. Deflation takes place immediately through two vent holes in the side of the airbag facing away from the driver.

The inflated airbags may reduce injuries to the head and upper part of the body during frontal impact.