Supplemental Restraint System - New Version

Group 72
Equipment and
Accessories

Bulletin Number
72 01 93 (2137)

Woodcliff Lake, NJ
October 1994
Product Engineering

This Technical Reference Information bulletin replaces T.R.I. 72 01 93 (2137) dated November 1993, which should be removed from your T.R.I. binder and discarded. This bulletin adds information on Deployment Thresholds and Seat Occupancy Detection.

SUBJECT:
Central Airbag Electronics Module - Supplemental Restraint System (ZAE - SRS)

MODELS:
1994/1995 MY E31/E34/E36





Situation:
The '94 MY and later E31, E34, and E36 vehicles are equipped with a new version of SRS for the passenger side front seat as well as the driver side. The system's control module is called ZAE (Central Airbag Electronics Module), and features integral impact sensors.

This control module has the capability to deploy both driver and front passenger airbags, as well as pyrotechnic safety belt tensioners on models so equipped (planned with E38). New features were phased in starting during 3/94 E31 and 6/94 E34/E36 production, including safety belt latch contacts. With this latest implementation, different deployment thresholds are possible based on belt use (latch contact information). In addition, the new system will incorporate a front passenger seat occupancy sensor (SBE), which will avoid unnecessary front passenger airbag deployment under certain circumstances.

This system works as a complementary system to the safety belts, and is NOT intended as a substitute for wearing a safety belt.

Affected Vehicles:
The system will be installed starting with 1994 model year E31, E34, and E36 with the following two exceptions:

^ E36 Convertible produced before 8/24/93 does NOT have the ZAE system.

^ E34 with M60 engine produced before 8/27/93 does NOT have the ZAE system.

When in doubt, a 1994/1995 MY vehicle can be positively identified as ZAE equipped by examining the SRS hardware information screen with the Service Tester/MoDiC, or by the absence of the orange impact sensors on the front fenders.

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.

Phasing in during 3/94 E31 and 6/94 E34/E36 production, the ZAE system utilizes contacts in the safety belt latch, and also monitors passenger seat occupancy (SBE system). This information can be 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 both a driver's and a front passenger's airbag, as well as a pyrotechnic safety belt buckle tensioner for driver and front passenger. Note that current (1994/1995) models do not employ pyrotechnic safety belt tensioners. Pyrotechnic safety belt tensioners are planned to be introduced with the E38, with the tensioner assembly mounted in the buckle side of the system. 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.

^ There will be a single hardware version for all BMW vehicles and models. Country-specific and model-specific information (e.g., deployment strategy, available equipment) must be coded into a new ZAE module after a replacement. This is accomplished with the MoDiC and Coding software version 10.0 or later. Note that models equipped with ZAE and deployment threshold/seat occupancy detection (phased in during 3/94 for E31, 6/94 for other models) must have these features coded in when replacing the ZAE module. All features are programmed by the central coding key; however, replacement "spare parts" ZAE control modules are delivered uncoded and must be coded before use.

Note:
The SRS is designed to operate only in certain frontal collisions, but the safety belt must always be worn if the driver and front passenger are 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 occupant restraint.

Physical Overview:
The ZAE control module is small and square, and use one main locking style connector integrated into the control module body. On E31 and E38 vehicles, the ZAE module is located between the front seats in the console behind the parking brake lever. On other models, the ZAE module is located under the middle of the rear seat bottom cushion.





The seat occupancy sensor (SBE) consists of a special sensor mat, along with a small control unit incorporated into the front passenger seat bottom cushion. The SBE system was phased in on all models starting during 3/94 production E31, and 6/94 E31/E36. The safety belt latch contacts and associated circuitry are incorporated into the female belt latch receptacles.

Electrical Overview:
Although the ZAE SRS does not employ external impact sensors, the system remains largely unchanged from previous SRS configurations.

Twisted-pair wire is used on all the igniter wires for the pyrotechnic devices. This improves the system's immunity to electromagnetic interference.

Other changes include the contact circuits on the safety belt latches. These 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.





The diagram illustrates the inputs and outputs to the ZAE module:

Note:
Pyrotechnic safety belt tensioners are not available on current models The ZAE module, however, has the capability to deploy such devices.

Starting during 3/94 E31 and 6/94 E34/E36 production, the ZAE system will feature a seat occupancy detector (SBE) that 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).

Communication with the ZAE module is accomplished with a single direction serial databus. 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 any SBE failure occurs, the ZAE assumes the seat is occupied (failsafe).





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. Refer to the end of this T.R.I. for information on placement of child restraint seats.





Internal Description:
The ZAE module is comprised of several subsystems. The purpose of each subsystem is described below, along with a diagram showing the organization of the subsystems in the ZAE module.

The ACCELERATION SENSOR consists of two piezoelectric accelerometers, set at 45 degree forward angles, that convert acceleration (in this case, deceleration) to voltage levels. Unlike the wheelhouse-mounted impact sensors used in the past, this device can not only detect an impact, it can also sense the magnitude and direction of the impact. The Acceleration Sensor is contained within the ZAE module, and replaces the external impact sensors.

The ANALOG/DIGITAL CONVERTER uses voltage levels from the Acceleration Sensor and converts them to digital numbers for use by the Processor.

The SAFING SENSOR is a mechanical impact sensor, with a low threshold. It is monitored in conjunction with the Acceleration Sensor. In order for deployment to occur, the Safing Sensor must detect an impact, and the Acceleration Sensor voltage must be higher than the deployment threshold. In addition, both sensors must "activate" virtually simultaneously, or a diagnostic trouble (fault) code will be set and no deployment will occur. This timing check makes certain that the signals from both the Acceleration Sensor and the Safing Sensor are valid.

The ENERGY subsystem simply stores enough electrical energy to deploy all SRS devices, even if a power failure of up to 100ms occurs. In addition, there is enough reserve power to ensure that a proper impact message is recorded in the non-volatile memory.

The IGNITION TRIGGER(S) act as a final stage transistor, directing the Energy supply to the igniter pellets when directed by the Processor. Activation time of the igniter pellets is held for a minimum of 10ms (to ensure deployment even if the impact sensing threshold is exceeded only briefly). The maximum activation time is 50ms, at which point the Ignition Trigger is reset. The Ignition Trigger is activated again if the impact threshold is again exceeded.

The PROCESSOR uses all ZAE inputs as well as information from the sensors to control the Ignition Trigger(s), and thereby controls the deployment of airbags and pyrotechnic belt tensioners. The Processor performs all self-tests, monitoring, and fault indication functions. In addition, the Processor has the ability to disable the Ignition Trigger(s) in the event of certain faults.

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 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/Service Tester to initiate testing, read out diagnostic trouble (fault) codes, read out ZAE identity, and program (MoDiC Coding 10.0) vehicle-specific information.





External Description:
The illustration shows the general appearance of the ZAE module and the single connector. As with past versions of the SRS control module, the connector includes tabs on some of the pin spaces. The tabs break shorting bridges on the female connector when the harness is plugged into the ZAE module. If disconnected, spring contacts in the female connector close, shorting ignition circuits of all the pyrotechnic devices. This reduces the chance of inadvertent deployment during service.

Operation Strategy:
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 seat occupancy detector (SBE), allows the ZAE module to employ a "tiered" deployment strategy in impact situations.

There are several advantages to this capability. First, in certain situations, unnecessary deployment of the front passenger airbag is avoided. In addition, pyrotechnic safety belt tensioners that are not in use (confirmed by latch contacts) are not deployed unless a sensor failure has occurred.

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 below. The table that follows shows the effect of the various possible inputs and impact scenarios for the front passenger seat.








Breakdown of Deployment Strategy: DRIVER Seat

Maintenance, Repair and Handling:
Since the control module conducts regular self-diagnosis and system checks, the only inspection necessary consists of a visual examination of external components (as indicated on the Service Maintenance Checklist), and a verification that the SRS control module memory has no stored diagnostic trouble codes (faults) in memory (as indicated by the SRS warning lamp).

The following safety precautions should be observed when repairing or troubleshooting on or around all types of SRS components:

^ The system can only be serviced by qualified personnel. The battery should always be disconnected and the negative battery terminal insulated before servicing. Any welding work also requires the battery be disconnected.

^ Do not allow the SRS airbag assembly to come in contact with degreasing chemicals or solvents.

^ Avoid temperatures above 200~F, even for brief periods.

^ Any airbag component which has been dropped from a height of three feet or more must under no circumstances be used again.

^ The gas generator and airbag assembly should only be checked electrically with the unit(s) installed. Never attempt to electrically check a "loose" airbag. Refer to S.I. 72 01 92 (3633) for proper electrical check procedures.

^ Storage of undeployed gas generator/airbag assemblies is subject to local regulations. The assemblies must be stored with the cover side up, so as to prevent the entire unit from being launched upwards and causing injury should the unit be deployed inadvertently.

^ The plug connection to the ignition pellet (before the contact ring) in the steering column should be disconnected before servicing the SRS assembly.

^ Install gas generators/airbag assemblies immediately. Do not leave loose airbag units accessible to untrained personnel. When scrapping used (replaced) airbag assemblies, follow local regulations. SRS airbag gas generators contain hazardous materials and must be deployed (converted to non-hazardous material) before disposal. The deployment of replaced SRS gas generators requires special tools and procedures. Consult a BMW repair manual (E32, Group 32) for the proper tools and procedures.

^ SRS airbag gas generators, as well as pyrotechnic safety belt tensioners (such as installed on some E30 convertible models) must be transported, shipped, and handled according to certain local and federal regulations. Consult S.I. bulletin 72 03 89 (1835) for further information.

^ Parts information concerning the ZAE SRS Control Module is located in the current release of BMW Parts Microfiche (Group 62). Front passenger SRS components are located in current Parts Microfiche (Group 51).

In severe frontal collision, the inflating passenger airbag could cause injury to a child riding in a child restraint seat in the front seat. Therefore, child restraint systems should not be placed in the front passenger seat of these vehicles. Children should always ride in the rear seat.