Seat belt tensioner

[prsquash]

Hi guys, I got a M35x with 245K kms on it, roughly 150K miles, and under quick breaking, not even really hard, it recently started tighten the seat belts like I was going to impact something, like pre-airbag deploy sequencing or something like that. Any ideas?

[Ilya]

Welcome to the forum.

That's normal...not sure if your idea of 'not really hard' is the same as that is subjective per person, but it's definitely normal. At what % do you think it's doing it?

[prsquash]

about 90 scared the crap out of my mother

[prsquash]

and this is definately not normal. I've put over 100K miles on it and it never came on. I've done some faceoff type of stopping and it never came on.

[bret383]

Your brake pads are probably worn down, Same thing happened to me. The caliper was stuck and it prematurely wore the pads down on one wheel. Even regular stops would make the tensioner grab.

[Ilya]

90% brake application should trigger them. That's quite a lot. Normal breaking is probably 30-40%.

[EdBwoy]

Your brake pads are probably worn down, Same thing happened to me. The caliper was stuck and it prematurely wore the pads down on one wheel. Even regular stops would make the tensioner grab.

Makes sense. There is a stroke sensor for the brake pedal, so factors that make your foot travel further forward will trick the SRS system as to the urgency/emergency of the brake application.

At min, I'd just check for brake pad wear.

[prsquash]

Thanks for the info guys. As for the pads, I've had them down to metal on metal, I know, not smart, but I was changing rotors anyways, and this system might of come on once. Uneven wear or stuck caliper may be a ^possibility but am at odds to understand how the system would know, considering the only sensor in the brake system I know of are the anti-lock sensors that get fouled sometimes from an array of debris from the breaks, rust or a combo of them. So, what would be the start point?

[EdBwoy]

So right now are your brakes good - replaced pads & rotors?

If yes, and you've had the brake pads that low, you might have had air intrusion. Bleeding the brakes would be my next move. Good to do as well, since contaminated brake fluid loses its hydraulic properties = squishier brakes.

Edit: you're correct, the car doesn't know exactly that your pads are wearing unevenly or your calipers are stuck. We can only use other symptoms that your brake system is not in tip top shape, such as this condition you're experiencing, to troubleshoot the causes. The stroke sensor I referred to is at the brake pedal.

[prsquash]

Thanks again. It is definately the stroke sensor that is kicking in the tensioner. If I preload the brakes system, the tensioner does not kick in at all. 'Pumpimg up' stops the peddle from traveling too far down therefore not tripping sensor. Might have looked further into the idiot light that came on awhile back. Thanks for the help and I'll post what it turns out to be

[EdBwoy]

OP, any updates?

Anyway, am taking this car apart and I got a chance to take pictures of what's down there. The stroke sensor is spring loaded and follows (not permanently attached) the black bar attached to the brake pedal.

[bgreen]

Your brake pads are probably worn down, Same thing happened to me. The caliper was stuck and it prematurely wore the pads down on one wheel. Even regular stops would make the tensioner grab.

when this happened..did you notice a change in how the brake pedal felt? i'm having a similar issue where my brake pedal has gone soft and idk why but it causes the seat belt to tighten