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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.
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