I would totally agree with that - unless you also had the issue of a tyre that was mismatched for some reason and you had to drive for a while before you could get a properly matching tyre. 4wd would damage the drive-train in that situation and 2wd would not.
But I would rather as you suggested - automatically revert to 4wd not 2wd.
Cheers,
Skot
on could make sure proper vacuum is getting the decoupler vacuum connection.
..before digging deeper.
though one would think ..it the light on the dash says the right thing ..
the parts that move inside the decoupler housing to operate the light ...that should say it's working externally and moving that switch-affecting part inside.
At least it's not to hard to take the decoupler off the front of the trans.
I was surprised ..on the last one I installed anyway ....pretty sure it's like this :
the default ( like if vacuum isn't working ) position is 2WD ..thought that was weird.
I'd want the default 'swithching-mechanism-not-working' to be AWD.
so if it breaks ..you're back to AWD as original.
In my opinion many german automotive things are designed to work right only when they ARE right ..
I it's going to fail ..you'd want the decoupling ability to fail, automatically reverting to AWD.
Safety issue even.
Like get out of really bad boonies issue.
As Arne said, if the decoupler works the tail shaft will turn. I would also expect even a failed VC would drag enough to turn the front wheels when your up on a stands.
Does the light come on reliably when switching on/off? This would indicate the selector shaft is moving as it should and the vacuum unit and switch are also working. You could also take the switch out and confirm that is working and see the selector shaft moves also.
If the selector shaft is moving fine, then it indicates something is wrong further inside.
Silly question maybe, but I assume neither front wheel turns?
Richard