E46 M3 Airbag Light won't come on

[TouchingVirus]

Cheers, I thought as much. I continued on and have disassembled the instrument cluster down to the PCB. I have multimetered IKE X11175 Pin 21 (Airbag) to ground and see there's 10v on it. The trace runs straight from the pin to a transistor, from the transistor to a resistor and into the left side of the LED. I've got 10V at the left of the LED to GND (Pin 1). Funny though, I've 10V at the right of the LED too, there's a 1001 resistor joining the two sections together. I'm thoroughly confused.

I'm going to swap a known-good LED to the airbag tomorrow evening after work.

Meanwhile, here's a diagram.

2013-12-09-00.22.26.jpg

The red line is the trace from Pin 21. I've 10.8V all along it. I multimetered all yellow spots to Pin 1 (GND), all gave me ~10V. I was not expecting voltage on the other side of the LED, the further right yellow dot which is negative. There is no voltage across the LED itself (yellow to yellow). This confuses me.

Perhaps stranger still is that the convertible roof diagnostic light in the purple box lights up fine, has voltage across the LED etc but it shares the space with the airbag negative too :huh:

[TouchingVirus]

Ok, curiosity got the better of me and I swapped the seat belt LED (known to be working) with the airbag LED (believed to be faulty). The seat belt LED still works fine, the airbag light still doesn't light up. So it's not a simple blown LED, though I noticed I have a blown LED on my RPM cluster of 4 lights, no big deal tbh, 3 still work grand.

The mystery continues and I'm fast running out of ideas. I could use a spare MRS4 for testing purposes

[TouchingVirus]

So I'm back on this buzz of trying to fix the light. I finally got a hold of another car/instrument cluster to test with. When I put my cluster into the other car the airbag light works without an issue. When I put their cluster into my car the airbag light fails to illuminate. So there's no issue with my cluster, the problem is deeper. Contrary to previous thoughts on this I don't believe the IKE lights up the airbag light by default, I believe the light is lit by the MRS4 unit as soon as it's powered on until it performs its tests. So either wire 21 on the IKE's X11175 is broken/shorted or the MRS4 unit is playing up. I'm inclined to believe it's the MRS4 unit at this stage so I'm going to cut it out and get a spare one somewhere. Anybody know how to code it?

[rebel.ranter]

MRS4 is easy to code. Have you had anyone read the error codes from the car?

[TouchingVirus]

MRS4 is easy to code. Have you had anyone read the error codes from the car?

I read them myself wit the BMTechnic cable & INPA. HealyC read them himself too, both times the error was the same:

MRS4
81 Fault lamp (AWL)
Error frequency: 1
Error currently present
Short to ground / (Wiring) break
Error not sporadic