Johnny_law

Motion will take the path of least resistance. In that video the motion of the left wheel was absorbed by the right. I don't think anything would move to the front until you stopped the motion of the right wheel.

However, even if the front wheel did spin under these conditions it may not really be transferring any useable torque. Therefore, I think the test with the rear wheels on ice and the front wheels on pavement will provide a better idea of the condition of the VC.

My observations are not based on experience, but academic thought, so take it for what it is worth $0.02.

BTW, thanks for showing the video.

Johnny

Johnny_law

This brings up another question. I wonder how many of these Turbos and CS4 have faulty VC? Does the silicon substance that is used breakdown with age, cycles or by just excessive forces.

Any info on this?

Johnny

DiegoR

You have a Porsche and you don't have a lift!? J/K

It should work the method you described, however if your car has the LSD and in good shape, you might need extra force to break down the lock of the LSD to be able to move the wheel.

Now, I'm not expert on these cars at all, so all these tests on the air/lift might be useless since these system could work totally different under load/motion, etc.

Now, if someone can confirm when the VC is gone, by doing the lift test as shown, shouldn't move the front tires when spinning the rear tires or vice versa, then we have a valid test?

Ridin Dirty

IMG_5787.MOV

Ridin Dirty

http://m.youtube.com/watch?v=no4xYyP...ure=c4-feed-u#

Johnny_law

Sure looks like it's shot.

Sorry you found out when you did.

Note to self, check CV the best you can before having engine out.

Can the CV be replaced with Transmission in car?

Johnny

Essexmetal

Spent 7 years with FF Developments. The company invented the viscous coupler / differential. For the simple explanation we gave people was a VC is just like stirring a gallon can of latex paint. If you stir it slowly the can does not spin, stir faster and the can starts to match the rotation of your stir stick. A VC works with the same principle of torque transfer. The 993 system is an on demand driveline not an all wheel drive one. Since there is no center differential the the only time drive torque is transferred to the axles is when there is a speed difference between the front and rear axles. If the front has grip and the rear wheels are spinning the "paint stirring action" transfers torque to the front.

The internal construction of a VC is a simple stack of plates with notches. These plates shear the fluid which is how the torque is transferred. Besides the number of slots the units are also tuned with the silicon fluids viscosity. The biggest failure point is when the seal fails and the fluid leaks out in the trans oil. Heat usually expands the internal plates and locks the unit up and not a failure mode. The other failure mode, and it is a gradual one is the wearing of the plate grooves edges which softens the unit and makes them less responsive but "stir" fast enough and they still transfers torque.

Rick

Ridin Dirty

Thanks for the explanation and the latex paint analogy. unfortunately I think it further supports my dead VC hypothesis. Based on my video above in the snow the rear wheels are spinning wildly and the fronts are just...flaccid. That depicts a rather dramatic difference in wheel spin (zero up front versus spinning in back).

So VC dead right? I would have never known this except for the HIGHLY unusual situation of absolutely having to drive in deep snow b/c the my plow on my wrangler is also broken and we have this unusual spell of frigid weather not allowing the snow to melt. I was just driving it home from an engine out new clutch install. I would have never known....it typically never even sees rain.

LexVan

Maybe the prior owner converted to rear wheel drive????

Johnny_law

Very interesting Rick, thank you for that. My question would be, what response time can be expected from a system like this. I would expect it to be almost instantaneous to really be functional.

Also how can the system be tested? Any ideas?

Johnny

PiB993

This is how the 325ix guys test it - with a funny roller contraption.


But really some snow or antifreeze on the driveway plus throttle and spotter seems easier. Is there an even easier way to test?

Next time I'm at my mech I'll ask him. I fear this may be a more common failure than expected due to dumb smog tests and tow trucks.

Robert993TT

Many cars with viscus coupling technology are having issues with transfering the power to the front/back wheels depending on what type of car that is. Just google it. A couple of Volvo, VW Golf models used it. They are not to be compared to real mechanical Torsen differentials as used by Audi or other better systems developed at later date. There are some videos with 996 4S/Turbo on the youtube with owners filming their AWD or lack of it in snow. I don´t think its something wrong with those cars and I cant say that its something wrong with this one. Let someone have a look at the car that knows them.

th20100

you can look at that video

you will see that front wheel are running together with rear until speed is quite low

when he is going reverse, then front looks not helping ?

Essexmetal

Johnny, To test is as some mentioned earlier in this post pretty easy. Have the car in the air. Turn a rear wheel still hopefully connected to the other one by the limited slip. Have the fronts restrained, perfect would be both sine it is an open differential. You should feel a defined torque transfer. If not the VC is toast. You could pull the plug on the trans. If the seal failed your trans oils will probably be black and nasty. No harm since the silicon is a lube but it will trash the function of your LSD.

There is some hysteresis designed into the VC so that the car can turn sharp corners without the two axles fighting themselves. But the very nature of why I think Porsche legal team wanted some drive on to the front, pulling the car out of an oversteering condition, the VC needs to be pretty responsive. For the purpose of this discussion lets say by one full revolution you are at full torque transfer. Ther eis much more to it than that but I don't type very fast.

As noted above, the VC is made up of a stack of slotted plates. one hooked to the trans, the next one to the output. They alternate with the number and diameter contributing to torque capacity. Think of a 5mm thick plate with 5mm wide slots 50mm long around the diameter. When cut there is a burr formed as the cutter exits. These burrs were removed but care was exercised to keep the corner perfectly sharp. The corner offers the best "grip" on the silicon fluid. As a VC wears these corners round over tha the unit looses some crispness. Still works just more lag.

budge96

One possibly overlooked item maybe the physical coupler that attaches the transmission to the torque
tube ..if that fails or is somehow disconnected the same result ! No front drive..
Not saying your clutch replacement guys are at fault possibly they didn't even remove the trans but
hey at this point it's a weird coincidence, Bert