c4/c4s/turbo viscous coupling/transmission question

i have used search but am unable to find out exactly how the awd(?) system functions in our cars. please ignore this if my searches did not find the answers. - typical torque split 5% front/ 95% rear - ability to vector front/rear to 40% front/60% rear - viscous coupling (vc) in front driven from trans by shaft (picture please) - viscous coupling distributes power to front wheels so here is my quandry. my understanding (correct me here) the shaft to the viscous coupling is driven from the trans. when both rear wheels are spinning at the same speed, the shaft to the front vc provides torque at a min of 5%. both rear wheels power sliding can send 40% forward. but my car has an open rear differential. psm uses the brake to control wheelspin on a tire that does not have traction. so it does not shift torque from one rear wheel to the other, it simply tries to transfer it to the wheel that is not spinning using abs. but what hapens when the rear wheels, or more specifically a rear wheel begins to spin. the open rear diff opens up, abs pumps and tries to slow the spinning wheel, but no additional torque is sent to vc because only one rear wheel is spinning. so does this mean that when you are stuck in al mud, or ma snow that a rear tire spins, instead of sending torque forward to vc? am i missing something? thanks

i'm glad you asked this. I haven't found a good article that describes how the drivetrain works on the C4S.

A. Streather in the 996 essential companion attempts to answer that very question on pages258 and 259.

It is the INPUT to the rear diff'l, transmission OUTPUT, that also drives the forward drive shaft, one input to the VC. The VC does not "lock" until or unless the front diff'l "output" rotates at a different rate than the driveline. Simplist way to think about it is to mentally place a VC across, between, the two rear diff'l output halfshafts. One wheel spins faster than the other and the VC "tightens" the coupling, a for of LSD. That being said if my VC should ever show any signs of failing, failure, it will go into the trash can and my C4 with then be a C2.

Both the front and rear differentials are open. With an open diff, the sum of the two tire rotation rates is equal to twice the input drive shaft rotation rate. The viscous coupling is a variable binding between the front and rear drive shafts. If they are turning at the same speed, there is near zero force applied through the VC. As they turn at different speeds, the VC fluid heats up and expands and increases the connection between the front and rear drive shafts. Let's look at the example of a launch with one rear tire in the mud. The car is initially at rest. You apply gas to get the car moving. Most of the force initially goes to the rear diff. The rear diff then sends it to the tire that's easier to move, which is the one in the mud, so it rapidly goes up to speed, and that one rear tire will spin at 2X the input rate of the rear diff while the other rear tire remains still. The front tires are not moving, so the VC now sees a big difference between the front drive shaft and the rear drift shaft (half the speed of the one spinning tire), so it increases friction which will apply some force to the front tires. That's the theory anyway, in practice I've seen a lot of videos of C4's spinning the rear tires and the fronts not moving. I think that a lot of people have VC's that are shot and they don't realize it. Picture of a 997.1 attached but I believe it's the same.

So how can you tell it's shot? TIA

so the system works not by looking at individual tire slip, rather the difference between the front and rear drive shaft rotation. so when the rear drive shaft spins faster than the front (because one rear tire is spinning due to open diff) the vc sees a large drive shaft difference and tightens up sending more torque to the front drive shaft. i do understand how the open differentials work too.

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I don't know what the official test is, but I'll try some suggestions. VC's usually fail with too much binding, not too little. You can tell you have too much binding by doing a low speed turn at full lock on good tarmac - if you feel jerking, that's a bad VC. Too little binding is hard to tell by driving. The official VW test for a similar VC unit is to jack up the rear of the car (front on the ground) and apply some gas. If you don't drive off the jacks, the VC is shot. I'm not so sure doing this test is a good idea, but if VW says so... Like this : http://www.youtube.com/watch?v=zMqOe...eature=related

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Seems like a great way to do some serious harm to a car.