B100C: Driver Tensioning Retractor Squib

Circuit fault

What B100C means

An emergency tensioning retractor pulls the seat belt tight at the start of a crash so the occupant is already against the belt when the airbag arrives. It fires through a small igniter, and the restraint control unit monitors that igniter circuit for correct resistance. B100C means the measured value on the driver side unit is outside the range the module accepts.

The circuit reaches from the control unit under the console, through the floor and sill harness, and up to the retractor in the B pillar. Mercedes runs that wiring near the seat and door sill, where trim removal and water are both common. A retractor that already fired will also read out of range from the moment power is applied.

Common causes

• A driver tensioning retractor that already deployed and was never replaced
• The connector at R12/1 left unlatched after seat or B pillar trim service
• Harness damage along the sill or under the driver seat rail
• An open or shorted igniter inside the retractor assembly
• Water intrusion at the driver floor corroding the connector terminals
• Pin damage at the restraint control unit connector from a prior repair

Crash data or hardware fault?

The module measures this circuit live, which puts B100C with the circuit codes. It tracks the actual electrical condition of the retractor loop and clears when that loop reads normal. Crash data is a separate category entirely: a permanent record written when something fires, held in memory, and cleared only by a module reset.

Does our reset clear it?

Sending the module in will not change a faulty retractor ignition circuit. Replace a fired retractor or repair the harness on the vehicle, and confirm the resistance falls back in range. Our reset service is for modules that are locked with crash data after a deployment, which is a common companion to this code when the car has actually been hit.

Which makes throw it

B100C is Mercedes-Benz specific, and so is the R12/1 component designation it references. Mercedes uses that alphanumeric system throughout its wiring documentation, so the same designator will point you to the driver emergency tensioning retractor across their service material.

What to do next

Check whether the driver retractor has already fired, since that single answer decides whether this is a wiring job or a parts job. A fired unit shows a belt that will not retract normally and a retractor that may look heat marked. If it is intact, unplug and reseat the connector at the B pillar, then measure the loop from the module end. We stock genuine OEM Mercedes seat belts and can rebuild fired retractors for many applications. Related codes: B1008, B100E, B1057.