How does 996 all wheel drive system work?

[GeneseeMtn]

I just got a 996 C4 & it has been snowing here in CO ever since. Going up & down my driveway, the "!" light is on constantly because of the snow and ice. It's not dangerous, a little fun in fact b/c there is no danger of going off the road. When the fish-tale switches from one side to the other, I hear a mechanical sound from under the car. What's the noise? Can anyone explain how the all-wheel drive works? Is there a driving technique that controls traction better than another?

Thanks,

Geneseemtn

H1

C4

Chainsaw

[wwest]

The 996 C4 uses a VC, Viscous clutch, in "series" with the driveline to the front diff'l. It is my understanding that the VC is always "stiff" enough to couple about 15% of the engine drive torque to the front diff'l but the coupling level will increase to as much as 45% if the rear tires slip/spin for an extended period. The VC does not react instantaneously to rear wheel slippage.

Additionally you have a "virtual" rear LSD, Limited Slip Differential in that the rear brakes will be applied, differentially, to the wheel/tire with the most slip/spin.

On top of all that you have traction control which will apply braking to BOTH rear wheels if they both slip/spin. I haven't encountered such a condition but I would imagine that the engine would be dethrottled within a few hundred milliseconds if the rear wheelspin does not abate or the driver does not react and lift the throttle on his/her own.

[kfostad]

All C4's, at least here in Europe, have the PSM system which is a bit more sophisticated than the standard traction control system. The sounds you are hearing is the PSM system working, actually applying the brakes trying to keep the car pointing in the same direction as the front wheels. It's perfectly normal.

Here in Norway where I live we have lots of snow during the winter, and I often find the PSM system a bit too intrusive while driving on snow. Much more fun if you switch it off :D

Regards

Kristian Fostad

[BjornH]

"...It is my understanding that the VC is always "stiff" enough to couple about 15% of the engine drive torque to the front diff'l but the coupling level will increase to as much as 45% if the rear tires slip/spin for an extended period."

The 996 C4 has a 5/95 power distribution by default. This can increase up to a maximum 40/60 if the rears spin enough.

Kristian,

I agree! For spirited driving on snow etc the PSM needs to be turned of, otherwise it will be like driving with one hand tied behind your back. With somebody else mananging the throttle! You will soon find yourself under steering of the road. (Slip angles in the snow tend to be a little high for the system´s taste!)

For transportation and normal driving I always keep the system on. I find the PSM system is very well calibrated and impessive over all. It allows you to slide the car around quite a bit as long as you´re not too abrupt and "panicky" in your movements. I even keep it on on summer track-meets sometimes without losing much speed.

(Trying to keep the lights turned off while going as fast as you can at the track is a good way to practice precision and a fluent driving style)

[GeneseeMtn]

We got a HUGE snow storm today & I got stuck several times in the C4 today on the way home...so I need to bring this thread back to life:

1. After continual slipping up a 1/2 mile incline (I kept going 2-3 mph, but with wheel slippage) using PSM & the warning light on most of the way, with me keeping on it with a steady throttle to keep the momentum up a snowy/icy hill, the PSM turned itself OFF, the ABS light came on. I got stuck a few seconds later. Is this normal? Is there a way to keep PSM on during a prolonged duration of slipping/sliding to get grip to climb a hill?

2. Shortly after the ABS/PSM OFF lights came on, I got stuck & the engine died. After I started it back up, I got stuck (on another genlte incline) & I got passed by a subaru with all wheel drive. When I was stuck (not with snow, but mostly slipping on ice), I seemed to only have rear wheel drive - the rears would spin, but the fronts wouldn't spin. I am wondering if I need to "reset" the viscous clutch so I have all wheel drive again? With the subaru passing me, it seemed pretty clear I didn't have all wheel drive or I wouldn't have been stuck. The subaru stopped & was able to start moving again on the same slippery road I was on, so I'm pretty sure there was no power to my front wheels.

Any advice/technical data is appreciated. Thanks in advance.

[wwest]

We got a HUGE snow storm today & I got stuck several times in the C4 today on the way home...so I need to bring this thread back to life:

1. After continual slipping up a 1/2 mile incline (I kept going 2-3 mph, but with wheel slippage) using PSM & the warning light on most of the way, with me keeping on it with a steady throttle to keep the momentum up a snowy/icy hill, the PSM turned itself OFF, the ABS light came on. I got stuck a few seconds later. Is this normal? Is there a way to keep PSM on during a prolonged duration of slipping/sliding to get grip to climb a hill?

2. Shortly after the ABS/PSM OFF lights came on, I got stuck & the engine died. After I started it back up, I got stuck (on another genlte incline) & I got passed by a subaru with all wheel drive. When I was stuck (not with snow, but mostly slipping on ice), I seemed to only have rear wheel drive - the rears would spin, but the fronts wouldn't spin. I am wondering if I need to "reset" the viscous clutch so I have all wheel drive again? With the subaru passing me, it seemed pretty clear I didn't have all wheel drive or I wouldn't have been stuck. The subaru stopped & was able to start moving again on the same slippery road I was on, so I'm pretty sure there was no power to my front wheels.

Any advice/technical data is appreciated. Thanks in advance.

1. All of the PSM auxiliary functions that apply the brakes use pressurized brake fluid supplied by the ABS pumpmotor assembly. This pumpmotor is a relative small fractional HP 12 volt DC motor and typically cannot be operated continuously or semi-continuously without overheating. So most of these systems have a timeout for the ABS pumpmotor to prevent its failure due to overheating while reserving some level of operating capacity for actual ABS. The one in my 2001 AWD RX300 will come back online about a mile or so down the road if I recycle by restarting the engine.

2. There is no "reset" of the VC. The VC will not come into "play", dramatically increase the coupling coefficient, typically, absent an extended period of rear wheelspin. Which, by the way, the traction control system will act to prevent.

Sorry, that's just the nature of the "beast". In my experience driving a Porsche in the snow, C4 or not, is a matter of mastering the art. Starting off in 2nd or 3rd gear, carefully feathering the throttle to prevent wheel slippage, etc.

My 2001 AWD RX300 runs on nice quiet and comfortable riding summer tires, Bridgestone Turanzas, all year around. When winter approaches the tire chains are always onboard and are quickly installed, all four, at the first indication of need. My C4 stays at home in a nice warm garage where it belongs.

As you have just discovered the Porsche C4 is not an adequate (not even close , actually) AWD setup for severe winter roadbed conditions.

PS: You don't say if your C4 is a manual transmission or not, if it isn't it should always, most definitely, IMO, stay in the garage in those conditions.

Edited December 29, 2006 by wwest

[GeneseeMtn]

If I am stopped on a hill on ice & the rear wheels spin constantly without the front wheels pulling, is something broken on my car or is this a normal response?

[PeterK]

If I am stopped on a hill on ice & the rear wheels spin constantly without the front wheels pulling, is something broken on my car or is this a normal response?

I think it is a "normal" responce. As I understand the 4S system is not anywhere as advanced as AWD on either WRX or EVO. It is also not designed for "heavy" slippery driving. It is good in the rain and adds some perfomance, but it doesn't make a car a good ice driver.

[wwest]

If I am stopped on a hill on ice & the rear wheels spin constantly without the front wheels pulling, is something broken on my car or is this a normal response?

I think it is a "normal" responce. As I understand the 4S system is not anywhere as advanced as AWD on either WRX or EVO. It is also not designed for "heavy" slippery driving. It is good in the rain and adds some perfomance, but it doesn't make a car a good ice driver.

Once the ABS pumpmotor is timed out that would be a normal response only if the VC is non-functional for some reason. In the alternative how sure or you that the front wheels weren't "driving" (also "slipping" on the same slippery surface) and torque limited due to the continuous slipping of the rear tires?

[alexcwt]

The viscous coupling is just as it sounds. It uses fluid to transfer power to the front. Kind of like a torque converter. The only difference is that the fluid would thicken as temperature increases when there is slipping between the two turbines. In cold weather, this almost doesn't thicken. 5% of the torque is only about 15 ft lbs at peak. Even at the full 40%, your talking 100 ft lbs.

What kind of tires are you using? That plays a much bigger part than any electronic nanny or awesome drive train you have. If you don't have snow tires on, your not going anywhere.

Subaru AWD uses a transfer case. Very much like a normal differential. It's actually very backwards like a truck. With that said, it's a lot more "useful" in getting you going in the snow. WRX and EVO just added clutch packs to the diffs and use electronics to control the engagement. The 4WD in the P-cars are not designed for this purpose. It's to assist in cornering. That's why they drive like a RWD car. The 997 C4S is actually quicker around the track than a CS. They do share the same AWD system with the 996.

The 997 turbos does change this to a system similar to the WRX and EVO which allows the engineers to control things electronically.

There is nothing wrong with your system. Get some good snow tires. Narrower the better. You'll be able to get around.

Edited January 1, 2007 by alexcwt

[wwest]

The viscous coupling is just as it sounds. It uses fluid to transfer power to the front. Kind of like a torque converter. The only difference is that the fluid would thicken as temperature increases when there is slipping between the two turbines. In cold weather, this almost doesn't thicken. 5% of the torque is only about 15 ft lbs at peak. Even at the full 40%, your talking 100 ft lbs.

What kind of tires are you using? That plays a much bigger part than any electronic nanny or awesome drive train you have. If you don't have snow tires on, your not going anywhere.

Subaru AWD uses a transfer case. Very much like a normal differential. It's actually very backwards like a truck. With that said, it's a lot more "useful" in getting you going in the snow. WRX and EVO just added clutch packs to the diffs and use electronics to control the engagement. The 4WD in the P-cars are not designed for this purpose. It's to assist in cornering. That's why they drive like a RWD car. The 997 C4S is actually quicker around the track than a CS. They do share the same AWD system with the 996.

The 997 turbos does change this to a system similar to the WRX and EVO which allows the engineers to control things electronically.

There is nothing wrong with your system. Get some good snow tires. Narrower the better. You'll be able to get around.

IMMHO cold weather, climate, will have little effect on the rate at which the viscous fluid heats and "thickens" (actually becomes compressed due to the inability for the HEATED volume to expand in the fixed volume hermetically sealed VC case) to increase the coefficient of coupling to the front.

There is NO snow tire of any type that would have helped you move forward on an incline on a surface as slippery as you describe.

If you MUST drive your C4 in those conditions then tire studs or tire chains will be the only reasonable solution.

My 2001 AWD RX300 runs on nice quiet and comfortable summer tires, Bridgestone Turanzas, all year around. There is ALWAYS one set of easily installed tire chains in the spare tire "well" and durng the winter months I always have a second set on board. If I had a need or a good reason to have the C4 out in that kind of weather I would keep my standard tires and rely on tire chains if/when the need arises.

[GeneseeMtn]

What kind of tires are you using? That plays a much bigger part than any electronic nanny or awesome drive train you have. If you don't have snow tires on, your not going anywhere.

I am running Pirelli P-zero tires.

If there was torque to the front wheels, they weren't moving/spinning.

The rear wheels were constantly spinning.

Sounds like the car responded as it was designed and that the AWD is really for cornering, not for traction on a slippery road. I have a 2nd vehicle that is good in snow, so the C4 may stay garaged when the storms roll through. This last blizzard was the worst we've had in years.

Thanks for the information.

I may try BLIZZAK tires when the Pirelli's wear out.

[wwest]

What kind of tires are you using? That plays a much bigger part than any electronic nanny or awesome drive train you have. If you don't have snow tires on, your not going anywhere.

I am running Pirelli P-zero tires.

If there was torque to the front wheels, they weren't moving/spinning.

The rear wheels were constantly spinning.

Sounds like the car responded as it was designed and that the AWD is really for cornering, not for traction on a slippery road. I have a 2nd vehicle that is good in snow, so the C4 may stay garaged when the storms roll through. This last blizzard was the worst we've had in years.

Thanks for the information.

I may try BLIZZAK tires when the Pirelli's wear out.

Tacky question, but from the driver's seat how do you know the front tires weren't turning...??

And don't forget that since there is no "virtual" LSD for/at the front (open diff'l) the right front tire might have been spinning wildly while the driver's side was stationary.

Also,with the rears spinning, LOTS of noise, you may not have been able to discern the much lesser noise from the front with so little torque/traction.

Edited January 2, 2007 by wwest

[GeneseeMtn]

Tacky question, but from the driver's seat how do you know the front tires weren't turning...??

A passerby with a shovel stopped to help me clear a dry spot for traction & he was looking at the tires (back spinning, front not spinning).

[su20]

What kind of tires are you using? That plays a much bigger part than any electronic nanny or awesome drive train you have. If you don't have snow tires on, your not going anywhere.

I may try BLIZZAK tires when the Pirelli's wear out.

I wouldn't recommend that... Unless you're only driving in extreme cold, your Blizzaks will wear out extremely quickly. Winter tires, in addition to having tread patterns optimized to snow and ice, are made of compounds that remain compliant in cold temps. When it's warm, they wear like silly putty. Summer tires are optimized for warm temps, which is one of the big reasons they suck in snow - AWD or not. They're like rocks.

I think you'd find winter tires would make your car very useful in snow and cold. But you should either find a trusted shop to swap your tires each winter, or get a second set of wheels to keep the winters on.

Or drive something else when it snows. :)

[dpolson]

996 C4 viscous coupling center differential failure!

I have two 2002 996 c4's with the same mileage (about 50,000) and live in Ouray, CO. One of the two does not pull with the front wheels, like yours, and thus is much more prone to getting stuck. I put both cars up on the lift to compare. Locking the transmission in gear (or park in the case of the tiptronic), I tried manually rotating the front wheels (both in the same direction) and on the car that pulls well, the wheels were somewhat difficult to turn whereas on the car that pulls badly, they turned quite easily. Conclusion: the viscous coupling on the latter car is shot and not providing any significant coupling. I called Prestige and discussed with the tech there and she said she has seen this before and had to replace the viscous coupling. They are going to do so on ours this week. I also read a VW discussion site which said these viscous couplings commonly wear out. Their opinion was that the VW synco viscous couplings (whcih are very similar to ours) have a life span of 60-90K miles. By the way, I use Blizzaks in the winter and other tires in the summer, and recommend the Blizzaks very highly for winter use. I can plow snow with the front spoiler going up a 20% grade! (If the viscous coupling is working, of course!) ;)

[wwest]

996 C4 viscous coupling center differential failure!

I have two 2002 996 c4's with the same mileage (about 50,000) and live in Ouray, CO. One of the two does not pull with the front wheels, like yours, and thus is much more prone to getting stuck. I put both cars up on the lift to compare. Locking the transmission in gear (or park in the case of the tiptronic), I tried manually rotating the front wheels (both in the same direction) and on the car that pulls well, the wheels were somewhat difficult to turn whereas on the car that pulls badly, they turned quite easily. Conclusion: the viscous coupling on the latter car is shot and not providing any significant coupling. I called Prestige and discussed with the tech there and she said she has seen this before and had to replace the viscous coupling. They are going to do so on ours this week. I also read a VW discussion site which said these viscous couplings commonly wear out. Their opinion was that the VW synco viscous couplings (whcih are very similar to ours) have a life span of 60-90K miles. By the way, I use Blizzaks in the winter and other tires in the summer, and recommend the Blizzaks very highly for winter use. I can plow snow with the front spoiler going up a 20% grade! (If the viscous coupling is working, of course!) ;)

Sorry, I'm not so sure your test of the VC could be considered valid.

Are you sure the "rolling" resistance of the front tires with the "good" VC wasn't the result of brake pad friction...??

Viscous clutches or couplings are "reactive" in nature, they do not provide much of a level of increased coupling co-efficient until teh fluid is heated via stirring by the two sets of opposing clutch plates.

And which car turned more freely, the Tiptronic...??

When we dyno tested my '01 AWD RX300 for F/R torque distribution we found it initially to be 95/5 F/R. It was only after an extended period, 5-10 seconds, of simulated front wheelspin/slip (dyno braking of the rear wheels above the braking level at the front) that we were able to determine that the VC was actually functional, the F/R torque ratio went up to ~75/25.

Initally, for a good long time, braking the rear wheels had virtually no engine loading effect.

There are two primary ways the factory can "adjust" the "attack" rate of a VC, the base fluid formulation and the ability to add a volume of gas, gas "bubble", inside the hermetically sealer VC "canister". The latter method is used to delay the attack rate entirely. The gas volume must be compressed to "zilch" before the rising viscosity of the fluid (due to heating) begins to create additional coupling between the two VC clutch plates.

Once the gas is fully compressed the fluid volume can no longer increase so its viscosity begins to rise.

Is it possible that a different VC "formulation" is used on the Tiptronic vs the manual..???

And I'm quite sure, CERTAIN sure, that you cannot put enough energy into the VC by hand turning the front wheels to bring the coupling co-efficient into effect.

Edited April 8, 2008 by wwest

[wwest]

Might the TC, Traction Control, firmware on the Tiptronic model be quicker reacting than with the manual transmission. They are LOTS of very frustrated F/AWD SUV owners out there that are experiencing just how easily these vehicles get STUCK due to the TC system's QUICK, INSTANT, reaction. F/AWD TC systems react to even the slightest level of wheelspin/slip resulting from too much engine torque being applied for roadbed conditions. With F/AWD TC the brakes are applied to the slipping wheel(s) and the engine is dethrottled.

Some of the newer RWD and R/AWD TC systems have a TC sub mode wherein some extended period of wheelspin/slip is allowed due to INTENTIONAL application of engine torque being applied. Say to get UNSTUCK by using a bit of wheelspin for rocking the vehicle back and forth.

It is my understanding that while PSM can be turned off on our Porsches that does not include TC functionality, virtual rear LSD or otherwise. I haven't "forced" the issue with mine so I don't know for sure.

Edited April 8, 2008 by wwest

[Xeattle]

I don't know if we're seeing the front axle providing drive because it is in the air but from a stopped low traction situation I would hope you would see some front wheel rotation if the rear diff is spinning.

[Krombacher]

there are a few videos on the 996 4wd system - which don't look very impressive.

watch how the front tires of don't spin:

porsche 996 turbo -

996 c4s -

another 996 c4s -

Re: the delay associated with the system in the c4 (due to the viscous clutch (VC) fluid), there are plenty of videos showing there really is very little delay (below). I think either:

a) Porsche's choice for their 996 Viscous Clutch was designed to transfer very little torque, or

b) there are a lot of worn out VC's in 996's out there.

Here's a few videos of people testing the functionality of their AWD cars' viscous clutches. When their VC's were not worn out, they worked very well.

older AWD bmws in parking lot on rollers -

older AWD bmws in parking lot on a fork truck -

older AWD bmw on ice -

AWD volvo wagon on a hoist -

AWD volvo wagon climbing a very steep hill (watch the rear tires) -

[Krombacher]

After further review, I think Porsche intentionally chose a relatively weak viscous coupler (compared to the BMWs and Volvos above), to avoid understeer. The stronger the link is between the front and rear driving wheels (i.e. the stronger that viscous coupler is), the harder it is to turn (esp. on pavement, will increase understeer). Considering Porsche's customers probably find street/track performance more important than going through deep snow and ice, compared to volvo and bmw owners, and since 911's are decent in the snow with their engine on top of the driving wheels, and p-car owners probably have a 2nd car for nasty winter days, they probably didn't want to trade off too much increase understeer for winter traction.

Of course, when they went to the electronically controlled centre differential in the 997.2 (no-more viscous coupler), they could increase winter traction without harming understeer (because they could actively control the "strength" of the link between the F & R tires). I think the very nature of the simple viscous coupler design makes the designer make huge tradeoffs between avoiding understeer on pavement vs. increased winter traction.

Wouldn't an actively controllable viscous coupler replacement upgrade be welcomed for all 996 and 997.1 AWD 911's? Maybe a stronger AWD in just first & reverse gear, and rear wheel drive for 2nd gear and up? What do you think?

[Y2K911]

In the 997, Porsche should have gone from the VC to an Haldex AWD such as the Audi TT or VW 4-motion or Volvo XC70.

[PorscheServiceThen529Plan]

I put Vredestein Wintrak Extremes on my 2001 C4 Cab. Very Pleased. Does daily driver commute round trip to boston all winter and this was a nasty winter. Shoots up pretty steep hill that I live on in heavy snow no sweat. Thinking about putting a plow on it.

[Krombacher]

In the 997, Porsche should have gone from the VC to an Haldex AWD such as the Audi TT or VW 4-motion or Volvo XC70.

You're right, and the Haldex system would have been the right technology, a good match for the 911s layout... Which is now used by cars like the Bugatti Veyron, etc..

It would require inputs from steering angle sensor, a gear selection sensor , parking brake switch, brake pressure sensor, or at least a brake light switch.

[Krombacher]

I put Vredestein Wintrak Extremes on my 2001 C4 Cab. Very Pleased. Does daily driver commute round trip to boston all winter and this was a nasty winter. Shoots up pretty steep hill that I live on in heavy snow no sweat. Thinking about putting a plow on it.

Could you shoot a video with your smart phone and show us the front wheel spinning on snow or ice (or anyone else)?