Rear-end/Differential groaning noise during low-speed turns

Created 7-7-01
Updated 8-8-02

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dealer currently recommends their own BMW brand (formulated by Castrol exclusively for BMW) SAF-XLS which has now been superseded by SAF-XJ.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, API GL-5 spec. 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 90 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

gale, 92 735i

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