R129 seat belt locking

[E55BOF]

The driver's side seat belt in my 1994 R129 SL500 will not pull out from the seat unless the ignition is on, when it functions normally. There's obviously some sort of interlock. Should I be worried, or is this normal?

 

[Doodle]

I'd be worried, I don't recall anything of that nature when I took mine apart.

There is a sensor wire that triggers the warning if you're not belted in, but that's it. It doesn't stop you starting the car like a true interlock would, and an interlock should never stop you unlocking the belt.

 

[E55BOF]

Thanks for that. It's totally consistent; unlocks with the ignition on, not with the ignition off. Remarkably consistent coincidence if it's a coincidence...

I think I may have found the answer, though. This came from a US site, relating to a 1990 SL; mine is 1994, so also pre-facelift.

"Likely either the seat belt take-up reel is jammed or the lock for the belt at the reel is not releasing. When the belt is not buckled a bowden cable from the buckle releases the lock; and when the belt is buckled, an electrical signal from the buckle to the roll bar controller causes the latter to energize a solenoid on the lock to release it.

The first thing I would do is buckle the passenger seatbelt with the ignition key on and see if the belt moves freely. If it does then I would take the passenger belt and insert it into the driver seat buckle and see if the driver's belt now moves. If it does, then the bowden cable is probably jammed at the buckle; if the belt does not move, then the take-up reel is probably jammed. If this is the case, you might attempt to repair it or purchase a replacement."

So there is a lock on the reel. Apart from the belt needing to be buckled to send an electrical signal, that fits my symptoms. Possibly it's been bodged by rigging it so there's an electrical signal whenever the ignition is on? I'd not be too worried if that were the case; the inertia locking works fine. It would also imply that the roll bar would automatically rise if the car rolled over with nobody in it; that wouldn't worry me much either!

It's too late and cold to try inserting the passenger belt tongue in the driver's side buckle tonight. but I'll try tomorrow.

 

[Doodle]

I think they've misunderstood how the locking system works - the bowden cable is connected to the seat belt pretensioner. That's driven by a one-time pyrotechnic charge, not a solenoid motor.

I'm not sure if/how the tensioner system is connected to the rollover bar, as the circumstances under which each will fire is different and the rollover bar isn't a one-time system (hydraulic rams and a bloody big spring)

 

[E55BOF]

Tried it this morning. Inserting the passenger belt in the driver's side (ignition off) frees off the reel, as does switching on the ignition. The passenger belt pulls out at all times. I conclude that inserting a belt into the buckle sends a signal to the reel, either electrically or mechanically.

Maybe an inspection will give me a clue. I'll take a look; it's not high on my To Do list, but it's there, and I'll get round to it.

 

[Doodle]

I think this might explain it.

R129: Why seatbelt won't release - PeachParts Mercedes ShopForum

 

[E55BOF]

That will be the Bowden cable referred to, but there's obviously an electrical release for the locking mechanism too. Now that I see what's wrong, it can move even further down my To Do list...

Thanks you, Drew.

Dennis

 

[E55BOF]

And it goes on... I took it for its first half-decent round trip last Friday (40 miles each way), and on the way back the seat belt warning light came on, accompanied by some beeping, and stayed on. I checked the cockpit electrics to see if anything else had come out in sympathy, and lo! and behold, the electric windows had ceased working. I can't see the faults (fuse and switch respectively, I expect) are likely to be connected, but I'm beginning to suspect this car only wants to be a Garage Queen...

 

[Doodle]

The seat belt warning also kicks in if the backrest isn't properly locked.

 

[E55BOF]

...the seat belt warning light came on, accompanied by some beeping, and stayed on (and) the electric windows had ceased working. I can't see the faults (fuse and switch respectively, I expect) are likely to be connected...

WRONG!!! Fuse 6 had blown in the main fuse box, so I replaced it, and both the windows and the seat belt light are back to normal.

Every sorted fault gives me a greater understanding of the car, and that fuse is listed as protecting 'relay left safety package, power soft top control unit, backup lamp and horn'. I didn't think to check the horn or the reversing light, but I will next time the fuse blows.

I'm wondering whether the fault is most likely to lie in that known problem area, the roof control module (which also controls the windows). Can anyone confirm, will a blink code reader be useful in diagnosing a fault in that? I think it should be, using Pin 21 in the 38-pin socket, but electronics is not my forte...

 

[retep8]

i have a similar problem 1994 SL500 seat belt locked how to tell if it has the pyrotechnic
attachement also how to remove

 

[Doodle]

It will have the pyrotechnic charge.

See these videos on youtube for removal:
https://www.youtube.com/watch?v=UnT0pQHil6I
https://www.youtube.com/watch?v=rESXckAIldA