Recently the red SRS (Supplementary Restraint System) light appeared on the dashboard and refused to go off. Suspecting something wrong with the airbag (because I had fiddled around with it quite a bit when taking the steering wheel off during another adventure) I connected the Star diagnosis to see what was going on. This is what I got:
Note the failure (- F -) for “06 ETR ignition circuit Front Passenger“. I had found the source of the trouble. But what was this gibberish? What the heck is an ETR ignition circuit? Something to eject the front passenger out of his seat?
Some research later, this is what I discovered: basically ETR stands for Emergency Tensioning Retractor, otherwise known as a seat belt pretensioner. When a collision is detected that is serious enough to trigger the airbag, it also triggers the seat belt pretensioner. This is essentially a mini-explosive reaction (similar to what takes place during airbag deployment) that occurs at the base of the seat belt that reels in in the seat belt, making it tighter. This ensures that you will be more safely secured to the seat during an accident. It is quite separate from the mechanism that simply prevents the seat belt from reeling out during high inertial forces.
So it appears that the circuit to trigger the pretensioner has failed. First step I need to figure out where this contraption is. After studying the MB manuals, it appears to be at the base of what is referred to as the “B-Pillar” of the car (http://en.wikipedia.org/wiki/Pillar_(automobile)) which is the pillar between the front and rear passenger window. Some dismantling later reveals this:
Note the silver cylinder. That’s where the little explosion takes place that will cause the belt strap to be reeled in. I check the resistance of this device and indeed the resistance is too low (< 1 ohm). This means the part has to be replaced. It turns out that this part cannot be replaced individually; the entire left seat belt has to be replaced at a cost of RM 1000. Ok, so easy decision – simply disable the sensor by inserting a 3 ohm resistor, and that gets rid of the annoying SRS light on the dashboard, while at the same time ensuring that I will be notified of failures in other SRS components. But for now, I can live with a non-functioning seat belt pretensioner.