A viscous coupling is employed on the integrale/155Q4.
It is used to link the back wheels to the front wheels so that when
one set of wheels starts to slip, torque will be transferred to the
other set that are hopefully gripping The viscous coupling has two
sets of plates inside a sealed housing that is filled with a thick
silicone based fluid . One set of plates is connected to each output
Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because of a loss of traction, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels in this case the wheels that are not slipping.
When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping, and because the two sets of plates in a Viscous coupling are not connected to each other, but merely in close proximity in the same fluid, the plates can move relative to each other through the fluid. The torque that a viscous coupling transfers is a function of the relative speed of the plates, the faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the plates are only moving slowely against each other so the amount of torque transferred during a turn is small.
This however highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.
The VC does not share the same oil as the gearbox, it has it's own special silicone fluid, It is said that after a number of years that the fluid in the VC can loose some of its viscosity and the car will not handle as it should, the fluid can be replaced by a higher rated fluid (300NM), Bara Motorsports can do this in the UK for £250 + VAT and postage.
Contact Bara on the following:- 01527 880011 or email him on:- firstname.lastname@example.org for more info.
The front diff and Viscous coupler off of the car
The Viscous coupler with end cover removed
The main components of the viscous coupling
On an integrale, the VC is housed in the drum shaped housing on the offside of the transmission casing, behind the upper sump and has a driveshaft coming out of it it. The VC can be removed from the car without taking any major parts of the transmission off, here is how it is done: First off jack up the front of the car and support safely, drain off the transmission oil into a suitable container, this is done by loosening off the drain plug in the bottom of the gearbox. You can also remove the transfer pipe to remove the oil this actually drains more of the oil off if you are doing an oil change, it needs a new copper sealing washer on reasembly. Remove the offside front wheel and possibly the plastic wheel arch liner (the car in the pic doesn't have any so I don't know if they get in the way or not). While the oil is draining, loosen off the suspension swinging arm, I just took off the front and rear mountings, the ARB link and it swung away, you need to do this to gain clearance for the output shaft when it all slides out. You can actually get away without loosening off the suspension, but by reaching around the back of the shaft (inside the VC housing) and removing the circlip which fixes the two together, this is ok for removal (maybe if you are pushed for time), but on putting it back on you will really need to undo the suspension as it is a real bugger to get to and not worth the risk of the circlip not engaging fully. Now remove the offside driveshaft at the end where it is attached to the output shaft which is coming out of the VC, make sure the hexagon headed bolts are free from dirt internally before trying to undo them, you will curse if you round one off with a badly fitting tool! Also make sure that you don't snag the rubber boot which surrounds the CV joint, it is easily punctured if you catch or nip it with a sharp tool. Remove the shaft end and push it out of the way towards the left, maybe hold it up with some string.
The Viscous coupling on the car
Gearbox oil drain plug
Gearbox oil transfer pipe and banjo connector
Circlip inside VC housing
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