Shared Knowledge
"BMW Z3 - Seat Belt and Airbag Light"
7 May 2013
Summary: if airbag light comes on and you can relate it to seat moving back and forth, my theory is that wiring harness under the seat is too short for seat belt connection. Can be fixed do-it-yourself (DIY) with nothing more than cable ties.
Background:
1997 BMW Z3.
55,000 miles.
Airbag light in instrument display comes "on" after moving seat back and forth.
Can be reset with a Peake airbag reset tool but next time seat moved fully back and then forward, airbag light "on" again.
Tools required DIY repair:
A Peake airbag reset tool if you want to reset the airbag light yourself.
A ratchet with extension and couple of sockets (not sure now but 11 and 15 or 16?).
Cable straps.
Wire cutters to clip off cable strap ends.
Overview:
The seat belt assembly shown above, has a cable (3 wires) coming from it that connect to a wiring harness connector under the seat.
Although not shown above, there is also another cable coming from the area of the seat belt assembly that has 2 wires in it and is also connected to the wiring harness under the seat.
The actual pins in connectors are the size of sewing needles or very small. Don't like any connector pins to be this small.
Both cables connect to the warning harness under the seat and are "held-in-place" by being clipped into a connector holder as part of the seat.
While the cable coming from the seat belt assembly (3 wires) is extra long, actually too long, the wiring harness cable it connects to, in my opinion, about 2-3 inches too short.
As the seat moves back and forth, the wiring harness coming out of the floor is pulled back and then "pushed" forward under the seat.
Thus, as the seat moves back and forth, the wiring harness pulls on the seat belt connector, eventually loosing it to the point that one of the 3 wires does not make good contact and the airbag light comes "on".
How to fix? Do away with seat belt connector clip and cable strap seat belt connection right where wiring harness wires come out of wiring harness. This way, as the seat moves back and forth, any pull is on the wiring harness and not directly on the seat belt cable or connectors.
Removing seat:
To remove the seat:
- With seat all the way back, remove the 2 nuts holding the seat rails to the floor board in the front.
- Now move the seat forward and remove the 2 bolts holding the seat rails to the floor in the rear. You will probably need a ratchet with an extension to do this.
- Now disconnect the battery. Make sure you do this as do not want anything you do under seat to trip airbag!! Battery disconnect is always undo the negative or body chassis side of the battery.
- Now lift the side upwards slowly and twist it such that the bottom is towards you and on the lip of the door jam.
Above shows the two cables coming from the seat belt assembly. The 3 wire cable is the one in my hand and the other cable is shown connected to a yellow connector.
You can see what is the cable connector holder at the edge of the seat back and is white plastic.
Note how long the seat belt cable is and how short the wiring harness cable it connects to is from where it comes out of wiring harness wire bundle!!
The seat belt cables come routed through a hole in the seat cover shown above. What this does is crush both cables if the seat is moved all the way back!
I "fixed" seat belt cable routing by moving both cable out of the hole they where in and routed them along a groove in the seat bottom frame. I then taped them in place.
Note from image above, I have aborted using the white plastic connector holder.
At this point, I could have cut connectors off the 2 cables coming from the seat belt assembly and soldered them to the wiring harness but decided to keep connectors and simply cable strap both cables tight to the wiring harness where actual wires come out of the harness.
Again the purpose of cable strapping the seat belt cables right at the wiring harness is so as the seat moves back and forth, any pull or push is on the wiring harness and not any connector.
When I got all done with the seat belt harness strapping I put seat back into car and remounted seat rail bolts and nuts. Of course if you have electric seats, you will have to reconnect the battery to move the seat back and forth. Make sure the bolts and nuts are tight!!
Below shows how much wiring harness cable there is under the seat when the seat is all the way forward. Again the wiring harness is going to move back and forth but no longer are individual wires or cable coming out of it going to be tugged.
Now, you will have to use your Peake tool to reset the airbag light.
If above does not solve your problem, could be a connector pin is bad. I have my doubts that seat belt assembly could actually go bad electrically but I guess anything possible.