Multiple Restraint System (MRS)
Multiple Restraint System (MRS)
Introduction
The Central Airbag system (ZAE) is being modified to include the addition of side impact (THORAX) airbags. With the addition of the side airbags, the system's designation changes from ZAE to the Multiple Restraint System - MRS.
The MRS system is being introduced on the 1997 E39 and 1996 E38s as of 3/96 production. The following components are new to the central airbag system. Combined with the existing familiar components makes up the MRS system:
- MRS CONTROL MODULE - A new 50 pin control module, with integrated side impact electronics.
- DRIVER'S SIDE IMPACT AIRBAG ASSEMBLY - Mounted behind left front door panel trim cover. Same design as other airbag assemblies
- PASSENGER'S SIDE IMPACT AIRBAG ASSEMBLY - Mounted behind right front door panel trim cover
- LEFT/RIGHT SIDE IMPACT CRASH SENSORS - Mounted on the left and right footwell cross member braces
Side Airbag Assembly
The side airbag is designed to protect the upper part of the body (chest or thorax) in the event of a side impact. The assembly consists of a gas generator, ignition pill and folded airbag. The assembly is bolted directly to the door and the trim cover incorporates a break away airbag cover.
The side airbag has a total volume of 14 liters and is divided into two chambers for optimum inflation speed. The maximum inflation time for the side airbag is 10 ms. This inflation time is required due to the proximity of the door to the driver or passenger in the event of a side impact.
Disassembly/assembly procedures for the door panel trim cover is the same for doors with and without side airbags.
Side Impact Crash Sensors
Electronic (piezoelectric) sensors are used for side impact detection. They are mounted at the left and right front footwell cross members. The sensors receive power (KL R) and produce a pulse width modulated (PWM) input signal to the MRS control module for side impact detection. The pulse width of the signal decreases in proportion to the severity of the impact.
MRS Trigger Logic
The deployment sequence for the driver's and front passenger's airbags remains unchanged from the ZAE system. Frontal impact is detected by the internally mounted electronic crash sensor. A mechanical switch is incorporated in the module as plausibility protection for deployment. The sensor and switch must both recognize an impact above the threshold before either bag will trigger.
The design of the logic allows individual components to be triggered independently as needed.
The deployment thresholds for the front airbags are programmed in the MRS control module. The driver's airbag will not deploy with a minor impact where the seatbelt is sufficient protection for the driver. If the seatbelt is not latched, the airbag will deploy to provide as much protection as possible.
The passenger's front airbag is also dependent on seatbelt use for deployment. In addition, the MRS control module looks at the input from the SBE to determine deployment. If the passenger's seat is unoccupied, the passenger's airbag will not deploy unless a severe impact occurs.
The deployment of the seatbelt tensioners is dependent on the presence of a signal from the seatbelt switch contacts.
Side Impact Airbags
The triggering of the side impact airbags depends on the degree of lateral acceleration as detected by the crash sensors mounted in the left/right footwells.
The internal control module crash sensor acts as a plausibility check for the deployment of the side impact airbags. Both sensors must detect that the trigger threshold has been exceeded before the side impact airbags will deploy. The control module also looks at the signal from the SBE for deployment of the passenger's side impact airbag.
New MRS Features
Rear Impact Detection: The MRS control module will detect a rear collision and trigger the front seatbelt tensioners if the threshold for activation is surpassed.
ZKE Crash Sensor: The crash sensor in the MRS control module takes over the function of crash detection for the ZKE system. The previously used crash sensor for the ZKE is deleted. In the event of an impact, a signal is sent from the MRS control module to the GM III and the doors will unlock and the interior lights and four way flashers will switch on.
Diagnosis And Coding: Replacement MRS control modules must be coded when installed in the vehicle. Diagnosis is carried out with the DIS tester. The program is fault symptom driven as was the ZAE system.