318ti.org forum

Go Back   318ti.org forum > Technical, Maintenance and Modifications > Knowledge Base

Notices

Knowledge Base How to articles and things explained.

.
» Recent Threads
Happy Birthday ...
Today 05:01 AM
Last post by anassa
Today 08:23 PM
2 Comments, 113 Views
Which diff on 2.8 engine?
09-21-2017 01:06 AM
Last post by anassa
Today 08:16 PM
11 Comments, 474 Views
96, M Sport, 5 speed,...
09-09-2017 07:24 PM
Last post by cjm1219
Today 05:14 PM
13 Comments, 1,121 Views
Any active part-outs...
08-30-2017 09:35 PM
Last post by marleymon
Yesterday 01:22 PM
5 Comments, 715 Views
318ti Turbo by MK...
09-18-2017 03:02 PM
Last post by Nakamiki
Yesterday 09:23 AM
12 Comments, 651 Views
Hello from Wisconsin
09-21-2017 04:06 AM
Last post by MikeF
Yesterday 02:57 AM
5 Comments, 358 Views
Feeler: 98 318Ti Black M...
09-09-2017 03:57 AM
Last post by E30-M3
09-21-2017 11:33 PM
5 Comments, 733 Views
Euro glass headlights
09-24-2014 10:27 PM
Last post by apexspeedtech
09-21-2017 05:25 PM
172 Comments, 85,795 Views
Fixing yellow headlights...
09-19-2017 10:50 PM
Last post by Endat
09-21-2017 04:54 AM
5 Comments, 430 Views
97 with DASC
09-05-2017 03:12 PM
Last post by schiavonec
09-21-2017 03:37 AM
3 Comments, 596 Views
Comment Share/Bookmark
 
Tutorial Tools Display Modes
DIY: Seat Belt Buckle Switch - Trick the Airbag Light
DIY: Seat Belt Buckle Switch - Trick the Airbag Light
Published by CirrusSR22
01-07-2012
Default 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:


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

• 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.


• 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.



• 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.



• 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.





• 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.



• 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.
Tutorial Tools

  #1  
By cooljess76 on 01-07-2012, 10:12 PM
Default

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.
Reply With Quote
  #2  
By 1996 328ti on 01-07-2012, 10:38 PM
Default

Quote:
Originally Posted by cooljess76 View Post
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.
Reply With Quote
  #3  
By cooljess76 on 01-07-2012, 11:10 PM
Default

Quote:
Originally Posted by 1996 328ti View Post
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!
Reply With Quote
  #4  
By 1996 328ti on 01-07-2012, 11:38 PM
Default

Quote:
Originally Posted by cooljess76 View Post
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.
Reply With Quote
  #5  
By CirrusSR22 on 01-08-2012, 04:19 AM
Default

Quote:
Originally Posted by cooljess76 View Post
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.
Attached Files
File Type: pdf SRSLightBeltBuckle.pdf (113.3 KB, 130 views)
Last edited by CirrusSR22; 01-08-2012 at 04:46 AM..
Reply With Quote
  #6  
By CirrusSR22 on 01-08-2012, 04:21 AM
Default

Quote:
Originally Posted by cooljess76 View Post
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.
Reply With Quote
  #7  
By CirrusSR22 on 01-08-2012, 04:26 AM
Default

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?
Reply With Quote
  #8  
By pdxmotorhead on 01-08-2012, 06:54 AM
Default

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
Reply With Quote
  #9  
By John Firestone on 01-08-2012, 02:42 PM
Default

Quote:
Originally Posted by pdxmotorhead View Post
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.
Last edited by John Firestone; 01-09-2012 at 09:42 AM.. Reason: Improved wording (I hope)
Reply With Quote
  #10  
By John Firestone on 06-24-2012, 11:45 AM
Default

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.
Last edited by John Firestone; 06-24-2012 at 12:01 PM..
Reply With Quote
  #11  
By trusty110 on 11-08-2013, 02:43 PM
Default

my driver side seat belt connector has four wires... which do i put the 100 ohm resistor?
tks, bill
Reply With Quote
  #12  
By trusty110 on 11-16-2013, 08:52 PM
Default

i tried the resistor in every option and still the air bag light comes on. any other ideas?
tks, trusty110
Reply With Quote
  #13  
By NuovoTech on 07-31-2017, 05:41 PM
Question

Hi - I have the same problem & have ordered the Radio Shack resistors. Any chance of my downloading the images from this olde thread? Thanx!
Last edited by NuovoTech; 07-31-2017 at 05:44 PM..
Reply With Quote
Comment

Bookmarks


Currently Active Users Viewing This Tutorial: 1 (0 members and 1 guests)
 
Tutorial Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

Similar Threads
Tutorial Tutorial Starter Category Comments Last Post
DIY Leather seat repair and refinishing... wolferj-RIP Knowledge Base 27 09-15-2013 01:54 AM
Seat belt "issue" with aftermarket seats.. elfhearse Interior and ICE 11 02-15-2009 08:38 AM
Seat Swap = Airbag light Maverix Misc 14 12-09-2008 12:44 AM
Seat Belt Buckle fault code zoner Engine 2 11-12-2008 02:29 AM
Airbag light Q. Seat belt buckle. septimus Interior and ICE 0 11-02-2006 07:13 AM


All times are GMT +1. The time now is 08:51 PM.


.
Powered by site supporters
vBulletin Version 3.8.8
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright ©1999 - 2017, 318ti.org

Tutorial powered by GARS 2.1.8m ©2005-2006
© vBadvanced CMPS v3.2.2
[page compression: 94.73 k/125.55 k (24.55%)]

318ti.org does not warrant or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information or products discussed.