1999 C4 - Viscous Coupling problem - front wheels no spin

Got stuck in snow, and the front wheels will not spin no matter what I try. I tried switching PSM off - still no front wheel spin I tried spinning in place for 30+ seconds (normally a stupid thing to do when stuck, but I heard it somewhere to try it) - still no front wheel spin Forward or reverse - there is nothing I can do to get the front wheels to try to get me out of the snow. I think the viscous coupling differential unit must have a problem. I called the dealer (500Km away), and they said that this unit is a "non-service" part - it does not require maintenance - which suprised me. I was thinking maybe I could get some new fluid in that thing and restore the missing AWD function that I seem to be suffering from. I have to say that I have not noticed the lack of AWD previously, so I have no idea how long this has been going on. I have driven 5 Saskatchewan winters and always found that my car can handle snow really well - but no longer - the front wheels will not engage with any power. Any ideas or help appreciated!

wow.... no error codes? No PSM lights..... usually for the modded turbo guys.... they burn out their VC because of the extra hp. Essentially that is what you have (extra hp) when you are in the snow.

Saskabush, great choice of screen name... If the front wheels dont engage while you are stopped, dead or stuck. the viscous coupling is normal. The way the C4 awd system works (as i understand it) you need some forward momentum before trackin get transferred to the front wheels. Since the car was completely stopped or stuck the system will not transfer any traction to the front wheels. and hence you feel the system is no longer working. The system will provide a percentage of the power to the front as a ratio to the rear wheels. If the rear wheels are getting for example 70% and the front 30% but the back wheels arent moving the car, that you get 30% of zero.. and thats the normal way it was designed. Try your car in a snowed filled parking lot and as you spin around you will observe the front wheels will be getting power and traction, you could even get a freind to stand outside to vaildate... Good luck

Mean while he is stuck in the snow cussing at his internet phone. Good luck with the problem. I guess this means no backing off and letting the slid play out.

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Haha, no, I am fine. I was coming home from swimming Friday night and was having some fun as I usually do up to the cul-de-sac - side sllipping drifting, and was planning on drifting right up to the driveway, but I didn't realize until I got stuck, that my neighbor tried to do me a favor with his snowblower. He snowblowed the driveway off, but a lot of extra snow got dumped in front of the driveway in the cul-de-sac and it was way deeper than I thought. I stopped dead right there. Luckily my neighbor was around and I got pushed out in a minute or two, but we both noticed that the car wasn't pulling me out from the front. I have seen other viscous coupling systems pull with all 4 wheel, so I still think mine is toasted. But I do relate to the previous post about it working better once you are in motion. I sure do feel it pull me out when I am bogging down from speed, and this saves my bacon many times I am sure. But yep, if you lose your momentum, you are in trouble. Don't back off, give'er more beans. It's always the way.

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Correct - no error codes. I thought there should be one too. I read another thread, and it's possible the VC is non functional. But actually I do think the fronts pull when I am drifting in parking lots with snow - I do that a lot when nobody is looking, usually with PSM off. I still don't know much for sure here. I'll have to get an observer to tell me if he sees 4 wheels spinning hard when I go in the parking lot to know for sure. Either that or I get a 4 wheel dyno test to prove it. I am interested to know, because I can be a big fool sometimes, and it would be funny if I never ever did have AWD for all these years! It's possible for sure. I got the car in 2005, and maybe the original owner messed it up and that info was never reported to me when I bought it. The car has been great for 5 years for me. I bet that VC is wearing out somehow or something - when I find out the truth, I'll post here. In the meantime, if anyone else can relate or has questions, let me know.

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What you say is very interesting. Does the car have an accelerometer to know if it's moving? I mean if the rear wheels are spinning (maybe 40Kph in place, and I am at 5000 RPM in first gear), I would expect the front wheels to be turning too. I am not aware of how the car would sense the inertia - but yeah, maybe that's how the PSM works eh? Hmmmmmm Now you have me thinking that the Porsche engineers just have no idea about getting a 911 out of a snowbank - probably very likely too. haha. If they have the means to measure the momentum, they should clue in that I am stuck in snow and want to get unstuck eh? It's weak design if that's how it's supposed to function when stuck in snow, in my humble opinion. I have seen very little information about the extent of the AWD. Maybe I'll look for it tonight as I am stranded at home! My wife has the Volvo cross country all day and night running around doing errands it seems - lol.

According to Adrian Streather's " Porsche 996: The Essential Companion " (pg. 257) "One major disadvantage of any viscous coupling system is that the vehicle actually has to be moving or both rear wheels spinning before drive can be transferred forward. The advantage that the 964 Carrera 4 still has over its later rivals is that there is always a minimum of 31% front wheel drive available. This was excellent if stuck in heavy snow or in sand at the beach. A viscous coupling cannot provide this." . As such I would guess that your issue is related to the viscous coupling not being able to lock up to transfer traction. This may be compounded by the fact that the viscous coupling is a set of interleaved plates that rotate in a bath of viscous silicone fluid that is extremely heat sensitive. Heat is required to get the fluid to become viscous, so it would appear that the cold actually reduces the effectiveness of the coupling. According to Streather (Pg. 259) "In the standard (read original 993) viscous coupling the fluid hardens at 160C. When both sets of plates are spinning at the same speed and the friction level insufficient to bring the the fluid to hardening temperature, it will remain in a liquid state and 100% of the drive is directed to the rear wheels and nothing to the front wheels.... The ZF design (read new for 996) ensured that the friction generated within the coupling under normal driving conditions is sufficient to exceed 160C, causing the silicon fluid to harden around both sets of plates to provide a small percentage (5%) of permanent front drive. In reality this takes approximately 45 seconds to develop after driving off. " My guess is that the coupling fluid does not get hard enough to engage the front wheel under the circumstances of being stuck in a snow bank.

If the prop shaft is spinning, and the front wheels aren't, even the weak-*** VC that Porsche specced for our "AWD" cars will lock up (to the extent it is able) in less than one full revolution . Please don't read Porsche marketing materials--or books sourced from them--and attempt to diagnose problems. Put it on a lift and do the test in the service manual.

Not as simple as a fuze out. Darn!

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"...as I understand it.." You have serious level of miss-understanding. The viscous clutch case is hemetically, SOLIDLY, sealed and consists internally of two sets of closely interleaved/intermeshed clutch plates and filled with the VC fluid. One set of clutch plates driven via an input shaft and the other attached to the VC case itself. The VC fluid is specifically formulated for a RAPID rise in volume with rising fluid temperature. One input from the front driveline and the other from the rear. Spin the rear wheels with the front wheels stationary and the fluid gets HEATED via being stirred, FURIOUSLY stirred, and since the VOLUME cannot change the fluid effectively "thickens" increasing the coupling coefficient between the two sets of clutch plates. Completely passive, no driver input required, and COMPLETELY reactive, no increase in F/R coupling coefficient UNLESS disparate/different driveline rotation is present. Sounds as if the VC is failed.

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"...fluid hardens at 160C..." Maybe the fluid hardens "enough" at 160C to reach the ~45% coupling coefficient specified but I do not think of the VC coupling as being on or off in this manner. It is my understanding that the VC fluid "thickens" somewhat linearly as its temperature rises. And it occurs to me that on a cold winter day the VC would be less, perhaps a LOT less, reactive. Not a good aspect, but there it is. Porsche's aren't designed for wintertime driving, not even C4's, maybe.

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"..less than one full revolution.." No, NOT...!! Multiple rotations, disparate rotations, consuming many seconds on the clock, possibly moreso on a cold day in wintertime.

There are aftermarket shops out there that can repair or rebuild your VC. Not only that, but they can also use a different VC fluid formulation that will increase the rate of coupling coefficient rise with differing wheelspin. But be careful with this as a too high or too consistently high F/R coupling coefficient might unduly stress other components in the drive train, the gearbox mainly. Increased tire scrubbing would also result, increasing the tread wear rate. The Ford Escape and Escape hybrid F/awd versions are currently enduring an extraordinarily high rate of PTO failures for this very reason, too much F/R drive coupling.

From my testing and reading both rear wheels need to be rotating for the VC to transfer more the 5% power to the front. The early C4 do not have a limited-slip diff nor do they have a virtual-limited slip so when you're stuck you'll only see one rear tire spinning at any point in time. From what I've read the PSM system wouldn't work with a limited slip at the time. When you're actually moving both rear wheels are rotating and if you loose traction while in-motion up to 40% of the torque will be transferred via the VC to the front diff. Once you're stuck and not moving I've found you're dead in the water as far as front drive goes. I think the Porsche theory was if the system behaved like a Subaru or Volvo the 911 would have too much understeer and not behave like a RWD car. On the street/track in the rain or dry if the the rear end looses traction while in motion, the transfer of power to the front will function. The 2010 C4 and Turbo 911s have a newer electronic all-wheel drive system which is able to send power to the front under different scenarios such as slow speed or stopped conditions since it doesn't solely rely on a static VC coupler which has been used on both 996 and late-model 997 C4/Turbo 911s. Snow tires seem to be the only thing we can do to help our C4s in the winter...I wish there was a modification we could do to help the C4 in a stopped state..."Snow Mode" or something I'm not even sure there are any worthwhile limited-slip diff options for an early 996 C4 w/ PSM.

Please pay special attention to AWD system limitations, reference to speed.. The following is an excerp from AutoZine Technical Viscous-Coupling differential Viscous Coupling center LSD is commonly used in many simple 4WD systems. One of the earliest examples was Volkswagen's Syncro system. Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together. In normal condition, front and rear axles run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyre slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented. Characteristic of Viscous Coupling center differential Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD. Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage. The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero. To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD. It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.

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The following confirms my understanding of the C4 AWD system limitations, please refer to the underlined section below. Viscous Coupling center LSD is commonly used in many simple 4WD systems. One of the earliest examples was Volkswagen's Syncro system. Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together. In normal condition, front and rear axles run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyre slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented. Characteristic of Viscous Coupling center differential Note that Viscous-Coupling LSD is a speed-sensing device : under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD. Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage. The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero. To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD. It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars. Advantage: Cheap and compact Disadvantage: Part-time 4WD only. Normally feels like 2WD. Who use it ? VW Syncro, Lamborghini Diablo VT, Porsche 993/996 Carrera 4 and Turbo, Volvo 850 AWD etc.

The Porsche C4 transmission output drive shaft is ALWAYS connected to the rear diff'l, SOLIDLY connected. ANY torque coupled to the front diff'l via the VC is deducted/subtracted from the rear drive. The best torque distribution you might possibly attain would be 50/50. "..visco them together.." See post #11 for a breakdown of the operational aspects of "visco".

When I had my 996TT, I noticed the front wheels wouldn't even spin on ice. I did some research at the time and basically came to the conclusion these VCs wear out and depending on how your car's been driven and mileage, you very likely have nothing but front end ballast after about 50k miles!

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I used to have a C4 and also Audi, MB 4matic, Range Rover and other 4X4 SUVs but I now drive a RWD and I can offer these observations: It's possible that the VC is in need of repair e.g. if someone strapped it to a 2WD dyno and ran it for a while with the front wheels strapped down. It's also possible that it's working as intended. It may be that there is torque transferred to the front axles, but just not enough to overcome friction to spin the front wheels. The Porsche VC AWD is a road going system and is not really designed for offroad or to get the car out if it is buried. It does not have the traction of a 4X4 system, or even Quattro. The whole point was that Porsche wanted an AWD system that drove like RWD. IMHO Porsche used the VC center diff for a few reasons: cheap, mechanically simple, lightweight, maintenance free. The principle of the VC is like using an electric fan to blow a windmill. It's actually not the best way to maximize traction and nobody would use it on the Rubicon Trail. There is torque transfer, but there is no solid physical connection between the front and rear axles. This VC cannot be locked. Combine this with the open rear diff and low ride height and it's easy to see how the car can easily get stranded. It should be no problem to confirm the front wheels are getting some power. With the steering wheel turned hard over, either: a) at low speed sliding around a slippery parking lot (autocrossers know what I mean) or b) pulling out of a driveway when the front wheels have traction but the rear wheels do not, there should be enough feedback through the steering wheel and the motion of the car to confirm that the front wheels are pulling the car. It's not instantaneous, it can take a slight delay for the fronts to pull while it "senses" the rears lose traction, but there should definitely be more of a 4 wheel drift sensation compared to the oversteering pushing sensation of a RWD. Does that make any sense?

Since PSM will cut/reduce the throttle in a rear slip condition, how will the VC ever hardens? Does that mean that with PSM on all a C4 gets on the front wheels are the 5% torque defined by the difference between front and rear tire diameter? (rear tires being smaller in diameter resulting to higher rear wheel revolutions than front, resulting to higher revolutions of the shaft from trans to vc than front wheels to vc, revolution difference of the shafts hardening the viscous liquid to the level of about 5% torque to the front)

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I think the PSM will only cut the throttle if it senses rotation of the car to help keep the car from going into a spin. The only time I've ever noticed the throttle being cut on me is when I make a hard u-turn. Rick 99 996C4 87 944S

OK I think we've spun of our axle... talk about a lot of opinions and no real solutions. That said I'm with wwest, the wheel sensors detect the difference in wheel rotation and will use the vis. dif. to compensate if the rears are spinning and the fronts are not. So whether your in motion or just digging yourself in your 4x4 system should kick in. In other words, your vis. dif. is busted... One of the main reasons why this happens is because owners change their tires without paying attention to the very specific overall wheel+tire diameter related to the 4 wheels drive system. Stock there is 1mm difference between the front and rear wheels (section width) on the 996 C4S/C4 if you where to put a tire or wheel sizes where the overall height/diameter is of by more than 3mm then effectively your front and rear wheels are running at different rotations per km and this will screwup your vis. dif. GUARANTIED and without giving you any fault indications! So if that's the case it's an easy fix to make sure it doesn't happen again (right size tires), but a costly one to fix (replace) your vis dif, I know that totally sucks!. Check out this site and you'll see what I mean. http://www.wheelsmaster.com/rt_specs.jsp Good luck ! Cheers, Sid.

Any sealed bearings in that there viscous coupling?

Sealed as in being inside the VC housing, yes, 2.

Anyone ever remove and replace their viscous drive line on a 996 C4? Any photos and can you drive around with it removed? I think it is pretty easy to remove but I'm not sure until I pull all the underbelly plates off to have a look. If the viscous drive is worn out I don't see any harm in leaving it removed until a new unit can be ordered. I think this part is about $1000 but I'm not certain. I want to pull the part number off the unit this weekend so I can be sure to order the correct one. Is the 997 viscous driveline any different? Can we run a 997 viscous on a 996 or are the parts one and the same.