Multi - Restraint System (MRS II)

Multi-Restraint System (MRS II)







The MRS II passive safety system is standard equipment on all E46 models. The system is a carry over from the MRS II system introduced on 98 Model Year E38/E39s.

The MRS II system for the E46 consists of the following:
- MRS II Control Module
- Driver and passenger's front air bags
- Driver and passenger's side air bags (thorax)
- Driver and passenger's Head Protection System (ITS)
- Front pyrotechnic seat belt tensioners
- Rear passenger's side air bags (thorax)
- Battery Safety Terminal (BST)
- Hall effect seat belt switches
- Front passenger's seat sensor (SBE)
- Driver and passenger's side impact sensors

Head Protection System (ITS)












The ITS consists of a hermetically sealed rubber tube that is encapsulated by a cross woven tubular nylon material. Each ITS is mounted from the "A" pillars to the roof panel just behind the "B" pillars by a nylon strap and mounting bolt.

When deployed, the inflation charge causes the diameter of the ITS to expand and its length to shorten. As the length shortens, it is forced out of its stowed position of the roof lining and it extends over the glass to protect the driver or passenger's head.

The ITS remains inflated for approximately seven to eight seconds to extend the protection time should the vehicle encounter additional side impacts during the crash.

MRS II Triggering Logic







FRONTAL IMPACT: - Deployment of the front air bags and/or seat belt tensioners is dependent on the severity of the impact as detected by the internally mounted crash sensor of the MRS II control module.

Additionally, the MRS II looks at the input from the SBE for deployment of the passenger 5 front air bag.

Deployment of the seat belt tensioners is also dependent on the presence of a signal from the seat belt switches.

SIDE IMPACT: - Deployment of the Side impact air bags and head protection (tubular structures) is dependent on the severity of the impact as detected by the MRS II control module and the side impact sensors. Generally only the impacted side will be triggered.

The tubular ITS structures and rear side air bags will always be deployed with the front and side air bags.

Battery Safety Terminal (BST)







The BST is an encapsulated pyrotechnic device. It is designed to prevent short circuits from occurring in the battery power supply to the engine compartment during impacts or collisions.

The ignitor capsule of the BST will be triggered, by the MRS II control module, with any front air bag activation.

The BST may be triggered with a side or rear impact, depending on the severity of the collision as detected by the MRS II control module.

BST Activation

















Once activated, The ignitor generates a gas charge which is directed down the internal discharge tube.

This causes the tapered end of the B+ cable contact to dislodge from its seated position which immediately opens the circuit.

The force of the charge continues to push the cable contact away from its seated contact. The spring tabs of the housing are compressed as the contact pushes to the end of its travel.

When the contact hits the end stop of the housing it bounces back against the spring tabs causing the contact to stop without returning to the seated position.

MRS II Diagnosis And Services Procedures













Diagnosis is possible with the MODIC or DIS tester, the DIS allows symptom troubleshooting, test modules, and ETM coverage of the ITS and BST.

Installation of a replacement MRS II Control Module requires ZCS coding also using the DIS or MoDiC.

Typical with any airbag, it is to be replaced as a unit after deployment. This is the same for ITS assemblies and the BST.

Follow the precautionary measures outlined in the repair manual section of TIS, and always disconnect the battery prior to any repairs.

All airbag units including the ITS assemblies are part number specific by model. Always use the EPC to verify the correct part number for any system part prior to ordering.

Once a BST has been activated the metal crimp of the B+ wire is pushed out of the BST housing and exposed. This provides a quick visual check of it's circuit status. A BST replacement splice kit is available from parts, this eliminates replacing the entire B+ cable. The splice kit comes complete with instructions and wire cutting/striping measurements. Follow instructions completely to prevent future voltage drop problems in this crucial circuit.