850R diff

[mhill53]

I understand the R uses some kind of viscous coupling and shaft as an lsd? How good are these as mine seems a little noisy? With the snow about mine seems to spin the wheels but if one wheel grips the diff appears to make a funny noise like a coiled spring spinnning on a plate? Has anybody else had the same issues? I've had proper plate type diffs in cars before and they seem to work much better?

[Flatout Phil]

I have done a bit of reading on viscous couplings as my R has one. Now according to a Wikipedia article ( http://en.wikipedia.org/wiki/Limited_slip_differential ), and interestingly, as VLSD wear out, they fail to a non-LSD state as follows:
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. They do not stand up well to abuse. In particular, any sustained load which overheats the silicone results in sudden permanent loss of the differential effect.[5] They do have the virtue of failing gracefully, reverting to semi-open differential behaviour. Typically a visco-differential that has covered 60,000 miles (97,000 km) or more will be functioning largely as an open differential; this is a known weakness of the original Mazda MX-5 (a.k.a. Miata) sports car. The silicone oil is factory sealed in a separate chamber from the gear oil surrounding the rest of the differential. This is not serviceable and when the differential's behaviour deteriorates, the VLSD centre is replaced.
Quite how you know it is shagged is a moot point - mine has 90k so should be well past its sell-by date.
It is importnat not to get the VLSD confused with the operation of the TRACS - this makes an ABS type noise (but gentler) - for myself, I can't say I notice the VLSD making any noise. What I do know is that my R puts its power down better than my T5 with less torque steer and so on.
Anyhow, anyone else wish to illuminate me I would be delighted to know more.

[mhill53]

Cheers mate,ive been trying to do a bit of research myself.i really need to try it again with the tracs on and off for a fair comparison.thanks again.

[Dangerous Dave]

The coiled spring noise is the TRACS operating.

Here is a pic of the viscous coupling on the driveshaft (taken from ebay advert).

http://i150.photobucket.com/albums/s101/bomb192uk/M59gearbox.jpg

My AWD has a viscous coupling to the rear wheel, and currently at 130000 miles it is still working.

The weak point is the splines on the output shaft on the gearbox. Mine had worn down, and the connecting sleeve was grinding on them.