This morning, as I was driving in the very quiet underground parking of the mall with my windows down; everytime I made a turn, I heard a painful groaning sound from the rear of my vehicle.
I had recently checked the differntial fluid level and it was fine. After some internet research I realized that although the color of the fluid seemed fine, the especial lubricating additive must have worn out. Therefore, I went out and purchased two Litres of Fully Synthetic Castrol gear oil.
I came home and replaced the fluid without any problems.
Guess what, the sound is gone. I did the parking lot test and no more sound. Now running smoother and no more painful sound.
I highly recommend doing this and saving yourself some money in the long term.
Here's a good reference: Rear-end/Differential groaning noise during low-speed turns
There's a simple solution for this one: it's caused by a stick-slip friction instability, or differing static & dynamic friction coeficients in the clutch plates which induces a harmonic resonnance in the clutch pack, a.k.a. chatter, similar to letting off slightly on the brakes when stopped on a hill. What has happened is either the special limited slip differential (LSD) additive is depleted or was not added after the last fluid change. BG & numerous other brands offer a conditioner formulated to provide good stick-slip friction characteristics and controlled adhesiveness. The adhesive, friction modifier, and lubricity additives eliminate stick-slip and chatter under thin-film, boundary lubrication conditions. This allows smooth, uniform motion at design travel rates. Solution: either change the fluid or drain the differential enough to make room to add the entire 5 or 6 oz. tube of the additive. Ref. BG-MGC #325, available at some BMW dealers. It is only sold thru auto repair facilities. It's compatible with all differential fluids whether conventional hypoid 90 whale oil base, or synthetic.
Bentley's recommends Castrol SAF-X0 for non-LSD and BMW's "Synthetic Final Drive Oil" for LSD. The dealer currently recommends their own BMW brand (formulated by Castrol exclusively for BMW) SAF-XLS which has now been superseded by SAF-XJ. If you choose not to use the BMW fluid, get a premium quality full synthetic gear oil such as Amsoil or Redline. Most synthetics do not require the separate fricition conditioner additive for LSD. Click here for more info on BMW's fluid spec.
E32 fill capacity is 1.9 liters or 2.0 quarts, when completely dry. A drain & fill will take about 1.5 quarts or slightly less.
Note: When draining the fluid, be sure to remove the fill plug first so that in the event it is frozen stuck, you're not left with a drained differential & no means to fill it up again. The fill plug is jammed up against the carrier bracket & will have either a 10mm or 14mm hex socket head plug, depending on year. A 90ş allen wrench may not fit in the tight space. You can cut off a short stub of an allen wrench & use the same size box-end or open-end wrench on it. or use a bolt & vise-grips as per below.
A few jpeg's:
- Drain & fill plugs
- D.I.Y. drain plug wrench
- Ring & pinion gear ratio tag, the "S" prefix indicates limited slip.
- Speed sensor sending unit
courtesy of : http://www.nmia.com/~dgnrg/page_16.htm