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YOUR RESEARCH996 Carrera 4S broken AWD
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Check 996 workshop manual, and here is how
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I checked the 996 Workshop Manual, and here is how to test the function of the VC:
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"The viscous coupling is filled with a precisely defined volume
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It's basically a torque converter filled with fancy silicone fluid.
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This is when the tech spotted the tranny leak and a new tranny was installed (courtesy of the car's CPO warranty).
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Because a new (replacement) transmission fitted I was not charged for the transmission/diff fluid change.
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Suggestion
I just had my 2002 Porsche C4S smoged at a station that did not have a 4 wheel dyno and they attempted to smog the car on a 2 wheel dyno and during the smog check a loud boom was heard, with a simutaenouls hearsh vibration of the car.
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Hi,
My 996 C4S 2003 mod. only pulls with the rear wheels. It has been confirmed that the viscous coupling (VC) is broken. My question is where the VC is located? I have heard that it sits inside the front differential. Is this correct?
The reason I am asking is because I have a spare front differential which I hopefully can use to fix the problem. I live in Norway, and here it is an advantage with AWD.
Could you share more details?
I hope you are aware that AWD is not the same as 4X4.
If, for example, you are spinning rear wheels ONLY, when you
are stuck in snow from stand still, that is just normal behavior.
You need to be moving at a certain speed for the front wheels
to be assigned some torque from the front differential.
you make it sound like that's a bad thing
Power distribution is 10% front, 90% rear and up to 40% front, 60% rear under maximum slip.
I wonder if people change out the fluid regularly (well, every 30k miles). I don't know if service schedule calls for it, but that's what I've been doing.
The viscous coupling is sealed. Fluid cannot be changed.
When a VC fails, it locks solid.
I'm always curious when someone posts about "losing" front wheel drive, like it's a transfer case or something.
I asked that my 03 996 Turbo's VC fluid be changed a while back and while I don't have the records handy to check I think this was done. I do know I was not told that the unit was sealed and the fluid could not be changed.
Before this I had the front diff fluid changed and when I took the car in for the VC fluid change I also asked the tranny/rear diff fluid be changed as well. This is when the tech spotted the tranny leak and a new tranny was installed (courtesy of the car's CPO warranty).
Because a new (replacement) transmission fitted I was not charged for the transmission/diff fluid change.
Sincerely,
Macster.
Thanks for all the replies.
I only tried it from stand still, hmm.. Do you have a idea of how fast you have to be moving for the front differential to get enough torque?
I do not know a precise velocity number, only the concept;
there is a change in viscosity in the multiplate unit, that needs heat to become
more solid and then couple... gee something sounds funny in the last sentence.
In colder weather, the transfer percentage maybe even lower,
until the transmission gets up to normal operating temperature.
As for changing the viscous liquid, I also though it was non serviceable, and
permanently sealed. There is no reference anywhere in the manual, or
anywhere, to change it after a certain mileage.
Macster,
could it be that they just changed the differential oil?
Today I got stuck in a fairly steep uphill of snow, and I could not notice the front wheels spinning at any time. So i definitely think the VC is broken.
I checked the 996 Workshop Manual, and here is how to test the function of the VC:
1. Connect Porsche System Tester 2 and select "Actual values" menu.
2. Drive front wheels of the vehicle onto the roller brake tester.
3. Drive front wheels via the rollers of the brake tester (max. 8 km/h) and measure traction of the front wheels after 20 seconds.
Please correct me if I am wrong, but doesn't this mean that it should at least work with speed exceeding 8 kmh/5 mph? This is supposed to be measured in room temperature, so in colder conditions you might need to try at a higher speed.
I do not have the equipment to test the VC the way it says in the workshop manual. But i would guess driving in a straight line in snow, with a certain amount of speed would do the same. What do you guys think?
I'm no expert on this, but I did work for a company for 13 years that produced silicone oils of various viscosities for various purposes. I understand the oil in the front diff is silicone - and if kept clean will never need replacement. I also understand that Porsche changed to the viscous coupling from the electronic multi-clutch units (like the 959 and 964) only after sophisticated machine tooling was invented in the 90s. So the concept should work something like a shear thickening fluid - when there is increasing slip as the front wheels spin at a different RPM than the drive shaft, the fluid gets thicker (more viscous like honey) and begins to transfer more of the torque between two rotating elements. Not sure how the cold affects all this - as it seems the colder it is, the higher the viscosity. But of course lab measurements are made at standard conditions - usually room temperature. Some food for thought...maybe someone can correct this theory or produce the actual design theory from Porsche.
DC4
The only way I know, off-hand, for testing a VC is to disable TC (pull the ABS pumpmotor fuse) have all four wheels off the ground, put a light 1x2 board to block the front wheels from turning, and run the rear wheels at 10-20MPH.
The 1x2 light duty wood should break within seconds.
Any level, even minor sustained level, of locking of the front drive to the rear drive on a tractive surface can quickly compromise components of the drive train. So the VC is typically designed to be slow to react to rear wheelspin/slip. With this being the case the use of TC braking and engine dethrottling will almost totally negate any positive aspect of the VC.
Some VC systems even use a gas "bubble" of a controlled size to delay the onset of coupling. The gas must be compressed to "zip" before the expansion of the VC fluid due to heating will have any efffect on coupling.
I have long thought these C4's using the VC could gain an advantage in autocross by driving them in a tight accelerating circle, "pre-stiffening" the VC, just before entering the course.
The new 997 C4's are now using the same Ford Escape, RX350, etc, electromechanical clutch with PWM to modulate the coupling level.
"...the fluid gets thicker..."
The VC case is hermetically sealed so as the fluid heats due to the shearing of the clutch plates it cannot expand in volume so it "thickens".
The higher the expansion ratio of the VC fluid with temperature rise the more rapidly will be the onset of coupling.
"...the fluid gets thicker..."
The VC case is hermetically sealed so as the fluid heats due to the shearing of the clutch plates it cannot expand in volume so it "thickens".
Sorry, I don't buy either argument, liquids get less viscous with increased temperature. And the idea of the gas bubble is in the VC is simply strange - can you point to an example? I took the time to look it up, and I'm correct - as the shearing of the fluid increases, the viscosity increases - the torque is increasingly transferred from disk to disk via the fluid. It has nothing to do with temperature. Pre stiffening the fluid also will not work, as it simply reverts back to the lower viscosity when the shear is removed (here is a quote from Wiki):
Traction control
Dilatant materials have certain industrial uses due to their shear thickening behavior. For example, some all wheel drive systems use a viscous coupling unit full of dilatant fluid to provide power transfer between front and rear wheels. On high traction road surfacing, the relative motion between primary and secondary drive wheels is the same, so the shear is low and little power is transferred. When the primary drive wheels start to slip, the shear increases, causing the fluid to thicken. As the fluid thickens, the torque transferred to the secondary drive wheels increases proportionally, until the maximum amount of power possible in the fully thickened state is transferred. See also: limited slip differential, some types of which operate on the same principle.
To the operator, this system is entirely passive, engaging all four wheels to drive when needed, and dropping back to two wheel drive once the need has passed. This system is generally used for on-road vehicles rather than off-road vehicles, since the maximum viscosity of the dilatant fluid limits the amount of torque that can be passed across the coupling.
"...fluids get less viscous with increased temperature..."
Yes, EXCEPT....
If you take an already "thick" fluid that has a HIGH volume expansion ratio with increasing temperature and "trap" it in a fixed volume hermetically sealed container and then begin heating it.
With the gas bubble you can delay the onset of increased coupling so there is little or no reaction to "ordinary" F/R drive line slippage, turning or even slightly different tire wear F/R. But once the delay is overcome the slope of the onset can be dramatic.
"...as the shearing of the fluid increases the viscosity increases.."
Yes, absolutely...!
The "shearing" HEATS the fluid, the fluid CAN NOT expand in volume, so the fluid's effective density, "viscosity", increases, becomes more "syruppy" and the coupling coefficient between the VC clutch plates increases.
Post #7 and #15, Reads authoritively, be your own judge.
http://www.can-amtalk.com/forums/ind...howtopic=13134
Of special interest, notice...
...they do have the virture of failing gracefully...
Were the rear wheels spinning at this time? The fronts can't turn unless the rears are turning, but it can be very hard to tell when the fronts are pulling; I rarely notice it on my C4.
God help the person that tries to change out their viscous coupling fluid. That stuff isn't meant to be changed like engine or differential oil. Hell, I'm not sure where you'd even buy the stuff.
Were you (able to) watch both...? Due to the diff'l one front wheel might turn but not the opposite.
But even with the VC having failed "gracefully" there should still be enough coupling to turn the front wheels off the ground with no resistance.
Drive line/shaft coupling...??
The test you quoted in post # 11 indicates it would take 20 seconds of sustained low speed "slippage" before the VC coupling coefficient is measurable, and that was with the front brake "sticktion" overcome otherwise.
Twenty seconds....??
Leads one to understand why Porsche has gone the way of adopting Ford's (Escape) ECU controlled electromechanical coupling clutch.
Hmm, I don't think that means what you think it does. The test procedure simply requires the tech to check at the 20-second point and isn't indicative of when the front wheels are activated.
I had a guy watch the front wheels from outside. Haven't considered that only one wheel could have turned and not the other one. Nevertheless, I am quite sure if the AWD were working properly it would made it up that hill.
I don't think it is BROKEN; like I mentioned in post #3, AWD and 4x4 are two different things.
Do you think this Porsche C4 in the link is also broken?:
http://www.youtube.com/user/JohnnyGu...25/iV8pB6OQCTI
This has nothing to do with the C4/C4S: "If you take an already "thick" fluid that has a HIGH volume expansion ratio with increasing temperature and "trap" it in a fixed volume hermetically sealed container and then begin heating it.
With the gas bubble you can delay the onset of increased coupling so there is little or no reaction to "ordinary" F/R drive line slippage, turning or even slightly different tire wear F/R. But once the delay is overcome the slope of the onset can be dramatic."
Can you find an example of the gas bubble theory employed in a vehicle? Lets stay with how the vc works in a C4/C4S. Here is another quote lifted from
http://www.356-911.com/post1974/2004...6carrera4.htm;
note there is nothing about bubbles or density change - it is not part of the equation:
Porsche 911 Carrera 4
Traction on Four Wheels
The viscous multiple-plate clutch is housed in the front axle drive unit,conveying drive power to the front axle and compensating for any difference in engine speed between the two axles. Like a multiple-plate lock, the clutch is made up of inner plates fastened to the hub and outer plates connected to the housing. Silicon fluid comes between the plates in a totally sealed compartment. As soon as there is a difference in speed, shear forces between the silicon fluid and the plates transmit torque and drive power to the front axle as required.
A further advantage, particularly in a sports car, is the low weight, all components of the four-wheel-drive system weighing a mere 55 kilos or 121 lb. This makes Porsche's dynamic four-wheel-drive system one of the lightest in the market.
and...another good link describing the device:
http://www.axlefacts.com/education/v...-coupling.aspx
testing the VC operation:
Using a heavy floor jack with wheels, put a block of wood on the jack and raise the rear of the van using the skid plate. Be careful, you can bend it. The wood block, if long enough, distributes the weight across enough of the skid plate to minimize that risk. Lift both back wheels off the ground 6" or so on a smooth level parking lot, with the jack's wheels parallel to the Syncro's. The rear of the van will be moving on the jack wheels, so you need to make sure there are no obstructions that could catch the jack wheels and cause the van to fall off the jack.
Now, put a 1x1 piece of wood in front of each front tire, 2x4 may work too. You need to block the front wheels like this to be able to test whether the VC is capable of absorbing the spinning of the rear wheels without locking up and causing the front wheels to engage and climb over the wood. If you can get the rear wheels to turn/spin in the air with the clutch fully disengaged, and without the van climbing over the wood blocking the front wheels (can't be too high...1-2"), then the VC is definitely good, or the fluid is not cooked. The van should climb over the blocks as soon as you increase the engine RPMs.
It may take several tries to get the van to do this. The VC is very sensitive, and it will want to engage as you let the clutch out. Some advise using the hand brake to help slow the spin of the rear wheels ... or to start them spinning slowly at first. It is really neat when you get it to work. Suddenly, the VC is working before your eyes in a very graphic way!
That is enough research for one day...
DC4
I think you would agree that it is highly desireable to have the coefficient of coupling increase at a non-linear rate, HIGH "attack" rate, as a function of the difference in rotation rate of the two sets of clutch plates. I think that you would also agree that the "shearing" results in at least some level of HEATING of the fluid. Now if you can accept that a VC fluid can be formulated, SPECIALTY FORMULATION, to have a very high expansion ratio with even a slight rise in temperature the we can come to an understanding.
But now we have a VC that might "stiffen", dramatically STIFFEN, even in a light throttle tight turn. Highly UNDESIREABLE, that, as it would result in premature drive line component failures due to over-stressing/HEATING.
So. Add a small air bubble that must be compressed to "ZILCH" before the VC fluid coupling coefficient can begin having a dramatic effect on engine torque distribution and you have an ideal REACTIVE F/R drive coupling system.
Allow me to add a real world scenario regarding the 'vicous coupling' -
I just had my 2002 Porsche C4S smoged at a station that did not have a 4 wheel dyno and they attempted to smog the car on a 2 wheel dyno and during the smog check a loud boom was heard, with a simutaenouls hearsh vibration of the car. That was the viscous coupling ecploding due to excessive heat becasue the front wheels were restricted from turing during the test.
The car is now at my Indi Porsche Shop being repaired.
Ummm...ok... you can't just write the above and not state the whole story... You're just being a tease. How about stating what happened afterwards? Did the station manager apologize, are they paying for the damages, etc. etc... tell us the story man!!!
Sorry didn't think all details were relevant to the topic. Yes, the station owner has accepted full liability and the car is presently at a Porsche Shop awaiting repairs (Hergesheimer Motorsports, in Lake Forest, CA.)
http://www.hergesheimer.com/
This just happened this week Monday, so the car has not been repaired or even looked at as of yet. The shop Technician was negligent in his duties and skipped the references which would have told him that he was going to smog an AWD vehicle, thus the cost to return the vehicle to original condition is being covered by the owner.
Directly from the Porsche Manual...
"The viscous coupling is filled with a precisely defined volume
of high-viscosity silicone oil and sealed. It is not possible
to top up oil or to check the oil level."
"If the inside and outside discs rotate at different speeds,
friction in the fluid transfers torque from the faster to the
slower discs. The discs are completely separated by the
oil and abrasion or wear therefor cannot occur."
