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-   -   DIY: Seat Belt Buckle Switch - Trick the Airbag Light (http://www.318ti.org/forum/showthread.php?t=35103)

CirrusSR22 01-07-2012 08:04 PM

DIY: Seat Belt Buckle Switch - Trick the Airbag Light
 
If you have an Airbag/SRS red light on your dash and the code comes back to either (or both) the passenger or driver side seat belt buckle switch, here's how you can fix it for good. From the best I can tell this DIY will only apply to 318ti cars made in June 1997 and earlier (** or even earlier, see responses below** ). It looks like in July 1997 the 318ti got the updated "hall effect" belt buckle switch which was more reliable, so you probably don't have this issue. Also, the wiring is different in those cars. It'll cost you $2, 6" of electrical tape, and about 10 minutes. But, you will need an SRS code reader to reset the Airbag/SRS fault light once done.

Both selt belt buckle switches are designed to provide the SRS computer with either 100 ohms resistance if the seat belt is latched, or 400 ohms if the seat belt is unlatched. We are going to simply add a 100 ohm resistor into the system to make the airbag computer think the seat belts are always latched. This bypasses the faulty buckle switches in pre July 1997 models which triggers the airbag light. Mine would randomly come on every few months and I was getting both the driver and passenger side codes.

Supplies:
Radio Shack #271-1108. 5 pack, 100 ohm Resistors. 1/2 watt, 5% tolerance. $1.19
Electrical Tape
Needle Nose Pliers
Wire Cutter
SRS/Airbag code reader

The resistors:
http://i44.photobucket.com/albums/f2...8/srs/srs1.jpg

----------------------------------------------------

Disconnect the battery.

You do not need to remove the seat for this project, just slide the seats forward and tilt them up with the rear access lever. You'll be kneeling on the ground to access the work.

Start with the driver's side and locate the electrical connector for the seat belt buckle switch (see image #3). Unplug the connector. The female side of the connector is the "car side" and the male side is the "buckle side" (which we will no longer use). You will need to bend the resistor as shown and trim off about 1/4" of the leads as they come longer than needed.

Trimmed and bent into proper position.
http://i44.photobucket.com/albums/f2...8/srs/srs2.jpg

Insert the resistor into the outer female pins on the connector (#1 and #3) and seat them fully. They will slide in about 1/4". The leads on these resistors are just the right size to provide good friction and make a solid contact. On the driver's side you will notice there is a third, center wire in the #2 position. This leads to the seat belt warning on the instrument cluster, as well as the buckle chime. By leaving the buckle unplugged you will NOT get the buckle chime or warning lamp upon turning the igniton on. If want that feature, you will have to figure out another way to do this.

http://i44.photobucket.com/albums/f2...8/srs/srs3.jpg

Repeat on the passenger side. The passenger side has the exact same plug, just without a center, #2 wire installed. This is because there is no dash/chime warning when the passenger buckle is not latched. There is also a little black box under the passenger seat. Leave that alone, it's for the passenger seat occupancy sensor mat.

http://i44.photobucket.com/albums/f2...8/srs/srs4.jpg

I decided just to use a single piece of electrical tape to secure the resistor. No way it's going to fall out now, unless it's tugged.

http://i44.photobucket.com/albums/f2...8/srs/srs5.jpg

http://i44.photobucket.com/albums/f2...8/srs/srs6.jpg

I also taped the resistor end to the braided portion of the wiring harness. Keeps it further secured and away from rear passengers feet. You can also tape the now-loose buckle end of the connector out of the way.

http://i44.photobucket.com/albums/f2...8/srs/srs7.jpg

Reconnect the battery.

Use your code reader to reset the Airbag light. Turn the ignition on/off fully a few times to make sure the airbag system passes its test and the airbag light goes out. Take it for a test drive also.

I originally planned on clipping off the wiring connector and soldering the resistor in place. I decided against this once I felt how secure of a friction fit the resistor makes. I have doubts it will ever come loose or lose contact. This way is also fully reversible. And if it were to fall out - no big deal. You'd just get a Airbag warning light and then can try another method, like soldering.

And the standard safety warning.... This DIY guide is only educational, always take your car to a BMW dealer to fix any Airbag faults. :biggrin:

cooljess76 01-07-2012 10:12 PM

In the event of an accident, will the passenger seat latch retracting mechanism still retract and lock and will the passenger airbag still deploy? If so, thumbs up.

1996 328ti 01-07-2012 10:38 PM

Quote:

Originally Posted by cooljess76 (Post 317779)
In the event of an accident, will the passenger seat latch retracting mechanism still retract and lock and will the passenger airbag still deploy? If so, thumbs up.

It's supposed to.
The pressure mat and seat belt sensor is supposed to control the passenger airbag. If no one is sitting in the seat, then it doesn't deploy.
This way the SRS thinks someone is sitting in the seat so it should deploy regardless.
I don't know anyone who has tested it.

cooljess76 01-07-2012 11:10 PM

Quote:

Originally Posted by 1996 328ti (Post 317782)
It's supposed to.
The pressure mat and seat belt sensor is supposed to control the passenger airbag. If no one is sitting in the seat, then it doesn't deploy.
This way the SRS thinks someone is sitting in the seat so it should deploy regardless.
I don't know anyone who has tested it.

Cool. I'd rather the airbag deploy regardless if anyone is sitting there than not deploy if someone is sitting there. Nice writeup CirrusSR22!

1996 328ti 01-07-2012 11:38 PM

Quote:

Originally Posted by cooljess76 (Post 317785)
Cool. I'd rather the airbag deploy regardless if anyone is sitting there than not deploy if someone is sitting there. Nice writeup CirrusSR22!

The only one who doesn't want the airbag to deploy needlessly are insurance companies, IMHO.

CirrusSR22 01-08-2012 04:19 AM

1 Attachment(s)
Quote:

Originally Posted by cooljess76 (Post 317779)
In the event of an accident, will the passenger seat latch retracting mechanism still retract and lock and will the passenger airbag still deploy? If so, thumbs up.

I was looking into that too and it appears that E36s up to 3/97 build (mine) had a mechanical pretensioner. I'm not exactly sure how it works, but there is a cable coming out of the mechanical unit which I think triggers it.

I'd say if anyone has a 4/97 build and later, more research needs to be done on how the pyro charge activates. The Bentley wiring diagram seems to show the buckle switch and pyro charge circuits are seperate, but who knows.

I attached a BMW Service Bulletin which addresses these faulty buckles. In there they talk about mechanical and the newer pyrotechnic pretensioners.

CirrusSR22 01-08-2012 04:21 AM

Quote:

Originally Posted by cooljess76 (Post 317785)
Cool. I'd rather the airbag deploy regardless if anyone is sitting there than not deploy if someone is sitting there. Nice writeup CirrusSR22!

That was my theory. If I got in an accident where the airbags deployed this car will be junked. I don't care if the passenger airbag went off needlessly, as long as it goes off in any situation, passenger or not.

CirrusSR22 01-08-2012 04:26 AM

Does anyone know the specific reason for the seat belt buckle switches anyway? The only thing I've come across is that the system might deploy the airbags differently (timed differently / more or less power) depending on if someone has their seatbelt on or not. Sound reasonable?

pdxmotorhead 01-08-2012 06:54 AM

Seatbelt sensor tells the system someone is there, the mat and seat sensor are an attempt to make sure the passenger is the right size to use the airbag. SO called smart airbags.

Meh, I'd take them out if it wasn't such a freeking liability. Hate airbags. They are there to total cars not save lives. If your belt is properly on you dont even hit them. (In my experience....)

Dave

John Firestone 01-08-2012 02:42 PM

Quote:

Originally Posted by pdxmotorhead (Post 317813)
Seatbelt sensor tells the system someone is there, the mat and seat sensor are an attempt to make sure the passenger is the right size to use the airbag. SO called smart airbags.

That came later. The E36 still used "dumb", fixed charged airbags that explode the same way for are a child or an adult.

The American-market seat belt buckle switches determine the threshold and moment to fire the airbags. The issue is that the U.S. NHTSA requires the airbags to protect the occupants whether they are wearing seat belts or not. The airbag has to go off in milder crashes and go off earlier, if someone isn't restrained and slowed by a seat belt. Conversely, if they are, the airbag need not go off until a more severe crash and can fire later. The American seat belt buckle switch adjusts the firing, accordingly, to protect both belted and unbelted occupants.

European E36s don't have the troublesome SRS buckle switches since people are expected to wear their seat belts.

The passenger side occupancy mat only determines if someone is in the seat: to avoid firing the airbag when the seat is empty.

John Firestone 06-24-2012 11:45 AM

I'm sorry to bump an old thread, but I have a question whose consequences might be important.

Did anyone have a good look at the electrical connector the 100 ohm resistor plugs into? Does the connector use gold-plated contacts or tin-plated ones?

I ask because I found many of my SRS connectors use gold. Mixing those with tin-plated leads and contacts can result in eventual failure through fretting corrosion.

trusty110 11-08-2013 02:43 PM

my driver side seat belt connector has four wires... which do i put the 100 ohm resistor?
tks, bill

trusty110 11-16-2013 08:52 PM

i tried the resistor in every option and still the air bag light comes on. any other ideas?
tks, trusty110

NuovoTech 07-31-2017 05:41 PM

Hi - I have the same problem & have ordered the Radio Shack resistors. Any chance of my downloading the images from this olde thread? Thanx!


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