[4ePikanini]

One or two comments:

While both are gear oils and are available in many different viscosity grades, the difference between GL-4 and GL-5 oils is that the latter are designed for situations where a large percentage of sliding occurs at the contact interfaces under high contact pressures (such as in hypoid or amboid gear sets commonly found in differentials' crown wheel and pinion sets).

These days there are GL-5 oils that are compatible with yellow metals and can be used in the gearbox, but just make sure on the specific oil before using it.

The viscous coupler in the transfer case is a sealed unit and therefore does not depend on the lubricant used in the transfer case for correct operation (unlike the friction plates in the limited slip section of the rear diff in Pajeros so equipped).

This is news to me. Have you got any reading material that I can have a look at? I'd like to know more about it.

[4ePikanini]

aha. It basically works the same as a viscous fan but it has gearing associated with it. With different gearing the torque split can be changed from 50/50 ( 1:1 ratio input:output) to a more biased front or rear depending on the input/output ratio.

Very clever mitsu designers.

[Gerrit Loubser]

Marius, yes, the viscous coupler is indeed very similar to a viscous fan in operation: The device is a coupler that connects two shafts. It consists of a sealed housing, containing a dilatant (i.e. shear-thickening) fluid and two sets of plates, one set connected to each of the shafts. The dilatant fluid is sheared if there is a difference in rotational speed between the two sets of plates and this shear increases the viscosity of the dilatant fluid, which generates torque that progressively resists the relative motion more and more as the rotational speed difference increases (although the relationship between shaft speed difference and coupling torque is not linear). The beauty of the system is that:
* There is no wearing friction material
* The coupling reacts to speed differences, and in an LSD application it still functions even if there is no resistance to rotation at one of the output shafts (i.e. it can combat spin-out)

One disadvantage is that these couplers can seize solid if they overheat and the plates warp.

The gearing that you mention is not really part of the viscous coupler. In the Paj Super Select setup, the gears are part of the center differential. In some 4x4 drivetrains (e.g. VW T3 Syncro Kombi), there is no center diff and the viscous coupler is required to mimic the function of a lockable center diff (i.e. allowing some relative speed between the front and rear ppropshafts, but locking them together when one or the other end spins out). The Pajero does have a proper center diff and this is also mechanically lockable, but it is also equipped with a viscous coupler that can help control spin-out when in 4H mode on dirt/wet tar or such.

In the original Super Select system (as fitted to Gen 2 Paj and some Tritons), the center diff is a conventional bevel gear diff (similar to a typical axle diff). These devices split torque equally between the output shafts, so when not locked it will send 50% of the drive torque to the front axle and 50% to the rear axle, if equal grip is available at each end, but if the grip is not equal, 100% of the torque can be transmitted via either end.

In the Super Select 2 system (as fitted to Gen 3 and Gen 4 Paj), the center diff is a planetary gear differential. These devices are able to split torque in unequal proportions to their output shafts, depending on their geometry. The diff used in the Super Select 2 transfer box directs 67% of the drive torque to the rear axle and 33% of the torque to the front axle, when unlocked and with equal grip available at each end. This generates a slight rear-wheel-drive handling balance and helps to fight understeer on the limit. When this diff is locked mechanically and both ends have equal grip, the torque split will be 50/50, but if the grip is not equal, 100% of the torque can be transmitted via either end.

If tyres of very different sizes are used on the front and rear axles of a vehicle with a viscous coupler between the axles and the vehicle is run in four wheel drive mode, the viscous coupler is constantly working, trying to limit the relative rotational motion induced between the front and rear propshafts by the difference in rolling radius. This can lead to overheating and seizing of the coupler.

One of the most amazing devices is the hybrid-Torsen LSD. This device was available in the rear axle of some Gen 3 Pajeros (e.g. Aussie market ones). It uses a Torsen gear-type LSD in conjunction with a viscous coupler. The Torsen LSD has the advantages of not having wearing friction plates and being able to generate very high lock-up rations, but has the major disadvantage that the lock-up ratio is linearly related to the torque transmitted, so if a wheel lifts on the axle, for example, the Torsen acts just like an open diff. However, with a viscous coupler also connected between the output shafts to generate torque resisting spin-out, the axle will always be able to transmit torque and the Torsen will be able to transfer torque to the output shaft with grip. This is a fully mechanical and very rapidly acting solution that is also totally automatic and quite effective.

[4ePikanini]

I can see it my minds eye but you have a great ability of putting tech into understandable words. Thanks Gerrit!

I knew the effect of the VCU in the pajero but never knew the mechanism. Never considered it a sealed unit. Now I know!