A viscous coupler is a drum inside the front differential is the heart of the on demand 4 wheel drive. It has a set of discs splined to the inner shaft of the drum, which are separated by .009″ 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, and the silicon fluid locks up 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. Viscous coupler are more or less “aggressive”, depending on the exact amount of silicon fluid in the drum, and the exact spacing of the discs.
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 1100 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 one failed coupler, and the transmission fluid was obviously and badly contaminated with the silicon fluid. We are not sure why we hear from customers that their 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.