A viscous coupler is a drum inside the front differential. It is the heart of the on-demand 4 wheel drive. There is a set of discs splined to the inner shaft of the drum, which are separated by .010″ to .012″ from a set of discs splined to the outer shell of the drum. All discs have slots cut in them, and turn at the same speed while going straight forward. The drum is partially filled with a silicon fluid. When a wheel slips, there is a speed difference between the inner and outer splined discs. As the discs shear through the silicon fluid, the fluid temperature near the discs rises. As the fluid temperature nears 100°C, the silicon fluid viscosity changes from thin to super Jello. The two sets of plates lock together almost instantly, transmitting power to the front wheels. This happens within 1/10th of a second, or about 1/4 turn of a slipping tire, although many variables affect that equation. Yet the viscous coupler still allows very slow differences in speed between the sets of plates, which allows cornering.
Viscous couplers are more or less “aggressive”, depending on the exact amount of silicon fluid in the drum, and the exact spacing of the discs. The slots that are cut into each plate have a sharp edge on one side of each plate, and the exact height of this sharp edge causes differences in the aggressive nature of the coupler. Some dedicated off-road enthusiasts may elect to have the plate spacing reduced to increase the grabbing force of the coupler. There are also different recipes for the silicon fluid, although non-stock blends are the realm of extreme off roaders only, and will ruin your drivetrain if used on pavement. We advise customers to stick with stock couplers for replacement.
How To Test a Viscous Coupler
Few mechanics know how to test the viscous coupler, most feel that if one cannot feel drivetrain drag when making a U turn, then the coupler is bad. Not True. The real test involves placing a 4X4 board in front of the front tires, then jacking up one rear wheel with as rolling type floor jack. Start the engine, close the door, and let out the clutch slowly. The raised rear wheel spins and the front tires lean against the 4X4. Slowly raise the rpms and note when the front wheels climb over the 4X4. STOPPPPPP!!! immediately. A very aggressive viscous coupler will climb over the 4X4 at 1000 rpm, and we like to call that one the “vicious coupler”. As long as the front wheels climb over before 1500 rpms, the coupler is still fine for winter or 4WD drive trail usage. Karmakanix has only ever seen a handful of failed couplers. If a coupler leaks really badly, the transmission fluid can be obviously and badly contaminated with the silicon fluid. We suspect that sometimes an unnecessary repair was performed when we hear from customers that their viscous coupler went bad and got replaced.
Note that a solid coupler is available to replace the front viscous coupler. And a vacuum operated Decoupler is available to disconnect the front drive system at the front of the transmission. We only advise that one considers this system if you are going to do BadAss 4 wheeling. The solid coupler CANNOT be used on pavement or even gravel roads with having the Decoupler to disconnect the 4WD function. Else severe gear bind occurs when turning, as the rear and front wheels HAVE to turn at different speeds to go around any corner. At Karmakanix, we feel that the 4WD is advantageous even when driving on solid pavement, and of course it is a distinct advantage in rain, snow or gravel. You cannot drive with a locked up front to rear drive system, something is going to either going to wear very rapidly or break right now. Why disable a good thing? Somebody designed the Syncro 4WD for more than just big mud and rivers.
Syncro customers should know that the front differential takes different fluid than the transmission, as it has no synchromeshes. We use Amsoil gear oil rated GL 5. You would just not believe how many Syncro front differentials we check that have never been checked before. It is a testimony to VW that this differential does not go bad more often.