Side question - is there an x-drive or diff temp sensor on the e53?

I imagine there may be a limp mode if the heat builds up too much.

Cole:
... you carry a large canister of Tyre Weld which is an inflaitor with puncture repairing slime inside it.

I have heard they are in the market, how much are they costing (in Rands) and where one can get 'em?

Bought mine in Game in Jo'burg but its available from most motor spares outlets. I can't remember the price off hand but I think it was less than R200. If you do buy some make sure you get the bigger canister for 4x4 tyres. I have never had to use mine yet so I can't really vouch for it, but it has a good reputation...

Holts Tyreweld
Tyre Repair :: Repair & Maintenance :: Holts Auto

I agree with nom3rcy that that is what a differential is designed to do, but it may be helpful to separate the discussion into the various diffs, and the differences between pre/post Xdrive (up to 2003 vs 2004 onwards on the X5).

A differential allows two output shafts to spin at different speeds. It does that whenever you go around a corner, for example.

The front and rear axle differentials on an X5 are open, as BMW relies on brake application for slipping wheels. If you have a slight difference left to right in rolling diameter, then the diff is always turning; it is essentially the same as constantly driving around a corner. Limited distance and speed will manage the wear issues to get you back out of the bush if you have a smaller tire. You may not even be on paved roads, so the tire can slip more easily regardless. However, if there is too large a difference between the left and right wheel speeds then the DSC will think that one wheel is slipping, and apply a single brake via the ABS system. There is a cutoff point that BMW recommends for maximum tire difference, a few % difference as I recall, and it has to do with not engaging the ABS constantly, which overheats that brake. If you overinflate your now-undersized spare, you can approximate the same rolling diameter.

The centre differential is a slightly different issue. Up to 2003, there was a constant 38/62 torque split, and the differential could handle some rotational speed difference front/rear, similar to other diffs. Beyond that small %, the differential had to work harder. Now consider the 2004+ models: the differential is now open, with a set of wet clutches that are applied if slip is detected (ie, too much rotational speed difference between front and rear). This centre differential has a variable torque split applied through those clutches. Those clutches will wear faster if they are being constantly applied. The best solution is to simply keep your spare at a higher than normal inflation pressure.

The entire discussion around temporary donut tires is not really relevant. Firstly, space saver spares use a higher pressure to produce the same effective rolling diameter as the tire they are replacing. Secondly, speed limits on those tires are due to design and construction issues; they are not crappy tires but rather are simply designed for temporary use. Speed issues have more to do with the safety and handling of a vehicle with a much narrower tire footprint on a single wheel. Third, BMW doesn't generally use space saver tires on the X5; the spare is simply a standard 17" tire which has the same rolling diameter as the 18", 19", and 20" optional tires. Yes, I know that the 4.6 used a special 18" spare wheel to clear the brake calipers.

You could always try putting your spare on at home, and see if you get a light on the dash. Measure the height difference. Pump up the spare to the maximum static inflation pressure and see if it gets close to the same height, or if the light goes off.

There isn't a temperature sensor shown in the parts book.

The x-drive transfer case has feedback circuits that measure the functioning of the electrical actuator for the clutches, and the resulting wheel slip. Through that algorithm, there is a learning mode that essentially can tell when the fluid condition is out of spec for the wet clutches. That shows up as a fault code. I suspect that if any of the monitored functions in the transfer case went out of range (not temperature, but things that may be dependent on temperature) then a fault code would show up. That would likely start with a code for too large a speed variation front and rear, prior to any resultant damage.

Good info all around JCL :iagree:

JCL...Wow...excellent informative posts :thumbup:

:iagree: +1 JCL. i would like to call that the most intelligent post of the month.

Thanks JCL I guess using the undersized spare is a bit more of problem than I had imagined and its usefull to know that I need to be more careful if I ever have to use it...

You mentioned that you thought that the 265/65 x 17 wouldn't fit in the spare tyre well. Perhaps a 245/65 x 17 or a 235/70 x 17 would? Either would give you a taller tyre than standard 235/65 x 17 which would improve the spare's functionality. In rough figures compared to the 265/65, the 235/70 would reduce the rolling circumference difference to 2%, the 245/65 to 3%, both better results than the 5% difference you have now.
On surfaces with some slip, gravel, mud, etc, you won't have a problem with the 235/65 regardless. On high-traction surfaces with the 235/65 you just need to be a little mindful of speed and distance and treat it a bit like a temporary spare. JCL's higher-pressure tip is the way to go for sealed roads.