[LondonThing]

OK, so: a bit of a tune-up on the 300TE has awoken the 4matic, just in time to earn it's keep during London's only snowy week this year.

The behaviour of the 4matic is clearly not right: when sitting at idle, in drive with the brake on, a medium-frequency vibration goes through the whole car. Go into neutral and it stops; move the 4matic cutout in the engine bay, and it stops. The vibration is harsher in reverse than in drive.

I think this is the 4matic unit slipping; it has less friction than the brakes or the torque converter, so when sitting still with the driveshafts held by the brake, the gearbox is rotating the driveshaft to the transfer case, against the grip of the clutch inside the 4matic.

Now, the $5000 question is: is the 4matic clutch pushed closed by pressure of the hydraulic oil in the system, and could this symptom therefore be fixed by changing the hydraulic oil? I haven't seen any instructions on lifetime of the oil in the system and therefore I suspect mine has been around since 1988. If so, I'm gonna drain it and refill it. If on the other hand the 4Matic is "normally closed", and oil pressure in the hydraulics opens the clutch, then it has to be the friction surface that's dead, and I'm just looking at a recon transfer case ($$$$$$!).

I have some hope that it's about the oil, because the behaviour gets worse as the car warms up, which I wouldn't associate with a friction surface problem.

Or have I misunderstood the way 4matic works?