Passenger Pre Tensioner Air Bag Light - Fault Code 987B SBSL

[EViS]

My e85 has thrown an airbag light in the last couple of weeks. A scan with ISTA+ shows up fault code 987B - SBSL Coding, firing circuit, belt tensioner, rear left, at variance. The code won't clear so definitely needs attention.

What should I be looking for next? Trapped wires leading to the pre-tensioner? Any secific pins which could be corroded on the plug leading to the tensioner? Or could this also be a faulty SBSL module? If so, where exactly is the module located?

[colb]

Could be the tensioner on the belt buckle, dirty or loose connection, check those out and for damaged wiring to the unit.
Video here shows how to test a pretensioner
https://www.youtube.com/watch?v=4ZHpPVZKQ6k&t=45s
If it checks out ok then turn attention to the satellite module hidden behind the trim by the B post just behind the passenger seat, the modules can get wet and be a cause of the fault.
Your light will not clear until the fault is rectified.

[EViS]

I'll get on this, thank you @colb. Probably a daft question, how do I inspect the tensioner on the belt buckle? What exactly am I looking for? Or is this precisely what the multimeter would be testing on the pre-tensioner?

[colb]

Check the wiring is undamaged and the plug for it is free of green crusties, clean it with electrical contact spray if it is green. The unit contains a pyro charge that fires when a collission is detected, when it fires it pulls the seat buckle back to hold you tight in the seat. Just inspect it and do the resistance check as described and see what result you get. It cant be dismantled so stick to visual inspection and the resistance test. When one of these units fires the plastic coating over the stalk cabel will be visually bunched up as a result of the cable being drawn in. Its scrap if its been fired and has to be replaced.

[EViS]

Check the wiring is undamaged and the plug for it is free of green crusties, clean it with electrical contact spray if it is green. The unit contains a pyro charge that fires when a collission is detected, when it fires it pulls the seat buckle back to hold you tight in the seat. Just inspect it and do the resistance check as described and see what result you get. It cant be dismantled so stick to visual inspection and the resistance test. When one of these units fires the plastic coating over the stalk cabel will be visually bunched up as a result of the cable being drawn in. Its scrap if its been fired and has to be replaced.

I removed the passenger seat this afternoon and tested the pre-tensioner which read 2.3 ohm. Supposedly between 2 and 3 ohms is within spec, could you confirm? For the test, I removed one of the pins from the socket housing, otherwise they are both shorted together.

Visually, the pre-tensioner and cables look fine. So, I suppose this would move me onto the satellite module. I assume this is the module as pictured?

Can these be repaired or am I looking for a 'new' module? I have read that the serial number needs to match with a replacement module otherwise it will need programming. Can I do this myself within ISTA+? Otherwise, which of the various serial numbers on the module label should I be looking to match (is it the top number - 65. 77 - 6928571 01)?

Airbag Module 001.jpg (185.32 KiB) Viewed 547 times

Airbag Module 002.jpg (149.54 KiB) Viewed 547 times

[enuff_zed]

You can replace like for like, or use any of the retrospectively exchangeable options.
Whichever you use, you will need to programme it with your car's VIN and update the 'system time'.

Capture.PNG (28.58 KiB) Viewed 539 times

[colb]

Congrats you found the left module, any evidece of water intrusion, thats the usual cause of it to fail.
Number is the first one 65776928571
Real oem parts site shows its no longer available so you need to look for a used replacement with either the same part number or one of those listed as exchangable retrospectively link to oem page here
https://www.realoem.com/bmw/enUS/partxref?q=776928571
Any module you replace it with will require coding to your car it needs your Vin number entered into it so it can talk to the other modules in the car. As long as it has the doner cars Vin in it it will set a new code for incorrect Vin number and the airbag light will not go out despite clerearing the code(s).
Not sure if ISTA+ will do the required coding, NCS Expert and DIS can code it and the more expensive Tablet Diagnostics like Autel and Launch can code these modules, far easier than the old NCS Expert and DIS software on a Laptop.
Getting hold of a working used module is a lottery, some come from crashed cars and are locked up and uncodable.
Just noticed enuff beat me to it with the oem link for modules he also makes a good point about starting the system time on the module after the Vin coding has been completed, again this can be done with the mentioned scanners/software.

[EViS]

Thank you for the replies, both . I've created a wanted thread in the event someone on here has a working unit to sell and can vouch for it being in working order.

The SBSL unit itself does not seem to show signs of water intrusion, however the carpet was absolute drenched on the underside (i.e. pool of water) from when I discovered that the cabin air filter water deflector was letting rainwater into the cabin, which I have now fixed. I can't see how water could have made it's way up to the module, unless the humidity of the wet carpet could still cause damage?

Before I venture down the replacement module route, is there anything else I can test? Assuming the 2.3 ohm reading I got on the pre-tensioner is confirmation that it is not at fault?

[EViS]

Is there any reason why I should not temporarily swap the driver's pretensioner with the passenger side to confirm that the same error remains on the SBSL and therefore be absolutely sure that the passenger pre-tensioner is not at fault?

[colb]

Modules are not interchangable they are either left or right

[EViS]

Modules are not interchangable they are either left or right

I meant the pre-tensioner itself, not swapping the SBSL module to the driver's side. I figured I could remove the entirety of the driver's seat and plug it into the passenger side loom and SBSL. I can't see how the car would know the difference between pretensioners... but maybe I'm overlooking something?

[colb]

Yes you could plug in the other tensioner and see if that fixes the code don't think the buckle tensioners are handed just the satellite modules

[enuff_zed]

Modules are not interchangable they are either left or right

I meant the pre-tensioner itself, not swapping the SBSL module to the driver's side. I figured I could remove the entirety of the driver's seat and plug it into the passenger side loom and SBSL. I can't see how the car would know the difference between pretensioners... but maybe I'm overlooking something?

If you do that then won't you have a missing part on the drivers side? Would that throw up another code and confuse the issue?

[colb]

Most likely the missing or disconnected will set another code for that side whilst you test the pretensioner on the opposite side

[EViS]

That would be my expectation, but would that not then confirm that the passenger pre-tensioner is either definitely functioning correctly or not? i.e. the pre-tensioner fault will either move across or remain in-situ? Obviously I'd clear all the codes before and after.

I'm just trying to find a way to be 100% certain that I require a new SBSL, especially as BMW cannot even give a date for when they can get one in stock and there do not seem to be any used modules available from my searches.