996 Carrera 4S broken AWD
11-12-2010, 12:49 PM
#1
Cruisin'
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996 Carrera 4S broken AWD
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.
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.
11-12-2010, 01:15 PM
#3
Burning Brakes
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.
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.
11-12-2010, 01:57 PM
#4
Racer
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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.
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.
Edit: How can we Make Sure awd is working Properly
Last edited by V225; 11-12-2010 at 02:16 PM.
11-12-2010, 02:06 PM
#5
Rennlist Member
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.
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.
11-12-2010, 02:16 PM
#6
Weathergirl
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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.
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.
11-12-2010, 03:04 PM
#7
Race Director
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.
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11-12-2010, 06:28 PM
#9
Cruisin'
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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.
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.
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?
11-12-2010, 10:32 PM
#10
Burning Brakes
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?
11-13-2010, 06:00 AM
#11
Cruisin'
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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?
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?
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?
11-13-2010, 11:53 AM
#13
Three Wheelin'
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
DC4
11-13-2010, 05:31 PM
#14
Drifting
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
DC4
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.
11-13-2010, 05:40 PM
#15
Drifting
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
DC4
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.