B100E: Passenger Tensioning Retractor Squib

Circuit fault

What B100E means

R12/2 is the front passenger emergency tensioning retractor in Mercedes component numbering. Its igniter fires the pretensioning charge that snugs the passenger belt at the start of a crash. The restraint control unit keeps a continuous watch on that igniter circuit, and B100E is what it stores when the resistance it measures will not fall inside the acceptable window.

Diagnosis follows the wire. The path includes the module pin, the harness across the floor and up the passenger B pillar, the connector at the retractor, and the igniter winding. Any one of those four can throw the reading off. Knowing whether the fault reads high or low helps, because opens and shorts point at different kinds of damage.

Common causes

• A passenger tensioning retractor that fired in a previous collision
• An unlatched or partially seated connector at R12/2 after trim removal
• Chafed harness where the wiring crosses the passenger sill or seat track
• An igniter that has failed open or shorted inside the retractor
• Corrosion from water entering at the passenger door seal or cowl drain
• Damaged terminals at the restraint control unit connector

Crash data or hardware fault?

Live measurement is what defines this family, and B100E is measured continuously. The code appears when the loop is out of range and it goes away when the loop is right. That is the opposite of crash data, which is written a single time at deployment and stays in the module until it is reset, no matter what the wiring reads afterward.

Does our reset clear it?

A reset does not repair a squib circuit. The module will re measure the passenger retractor loop on the next power up and set the code again if the fault is still there. Replace the retractor or fix the wiring first. Where our service fits is after a crash, once the parts are in and the module still holds deployment data that a scan tool cannot erase.

Which makes throw it

Mercedes-Benz is the make behind this code and behind the R12/2 designation. The lettering scheme is Mercedes specific, so treat the component identifier as a lookup key in their wiring diagrams rather than a universal label.

What to do next

Look at the passenger belt first and see whether it retracts and pays out normally, because a fired retractor announces itself and saves you from chasing wiring that is fine. With an intact unit, pull the B pillar trim, unplug the retractor connector, and inspect both halves for corrosion and spread pins. Measure the loop at the module end to split harness from component. We can supply genuine OEM Mercedes passenger seat belts when the retractor is the answer. Related codes: B100C, B1014, B1642.