Thought I should post my findings here as it is the same topic.

A little background first, I am up for 2 new front tyres and have been reading that a difference in rolling diameter of more than like 4mm can cause the transfer case to crap it’s self. That being said the spare appears to be slightly different rolling diameter 275/40 r 20 (728mm) on the fronts vs 155/90 r 18 (736mm) for the spare, 8mm difference (not accounting for manufacture variations). So, I looked at disabling the transfer case in the event I have a flat and have to change wheels, to prevent damage to the TC.

Basically there are two fuses going to the TC control module fuse 92 & 125. Pouring through diagrams on Rheingold, these two fuses go only to the TC control and are the only two that go to there so, pulling these fuses should deactivate the TC control unit and therefore not give a signal to the TC actuator. They don’t seem to be linked to other things like the dashboard or indicators as seems to be the case in other BMW’s. Error codes are stored and shown relating to DSC and xdrive/transfer case failure that can be cleared.

I find it hard to believe BMW would supply a spare tyre that would destroy the car if used, but I’m not willing to bet $1000+ on it. Needless to say I have a tyre repair kit in the boot and a little 12v compressor on the way.

Does anyone have any experience, thoughts or input on this?

A disclaimer that this will also disable safety features and from my understanding will be a rwd only car with an open diff. I am only looking at this for an emergency means of getting home without destroying the transfer case and nothing else. I do not support the idea of doing this so you can do skids(as I have seen some posts about) , if you want a rwd car buy a rwd car, and be responsible, do it on a track or skid pan.


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