Verifying AWD functionality

[gwroclaw]

I got lax the other day when I brought my 99 C4 to the NJ DMV inspection station. I've been there with this car a half dozen times before and they always knew not to try to put it on the dyno when testing emissions. I was in the waiting area on my cell phone when I noticed they had it on the dyno and just started the rear wheels turning. They had the left front tire chocked. I quickly ran out there and after a half a minute of waving my arms and yelling and hollering I got them to stop the test.

Judging from the wheel speed I don't think it ever got above 15mph. It was all over in less than a minute. I was surprised that the car didn't try to crawl up the chocked left front tire.

I know that all the power to the front differential travels through a viscous multi-disc clutch. There are no electronic controls on it of any kind. The 996 AWD system has no center differential. The viscous center clutch allows for some slippage when making sharp turns. Any difference in speed between the front and rear axles causes the plates in the clutch to shear through the viscous silicone oil in between the plates of the clutch causing some torque to be transferred to the front axle. How can you test the amount of torque transfer? The Porsche service manuals I have don't mention a way to test this (My manuals are current through revision 75)and the service manger at my local dealer Paul Miller Porsche)was of no help.

I thought I could come up with my own informal test. I jacked the the rear of the car off of the ground on my 2 ton roller jack. I figured with only the rear tires off of the ground. If they started spinning freely there would be a large enough speed difference between the front and rear axles causing torque transfer to the front and the front tires would try to pull the car on the roller jack forward. No such thing happened. I then jacked the front tires up off the ground and they did start spinning with the rears (still off the ground). So there is some torque transfer - enough to overcome the friction of the front drive train. It just seems like there is too little torque transfer.

Is it possible that the short time on the dynamometer caused the silicone fluid in the viscous clutch to heat up so much that the fluid broke down and is too thin now to effectively transfer torque to the front axle. I believe my idea of how the AWD system works is valid. It is a very simple system.

The Porsche 911 Carrera service information manual says never use two roller test stands when performance testing on a dyno, only use a 4 roller test stand.

I don't notice any difference in how the car drives but I've only driven it on dry roads. Mechanically everthing seems normalwith the front differential.

When I did my own informal test I would have thought that a speed difference of 10mph between front (statioanry) and and rear axles(10mph) would tranfer enough torque through the clutch to the front axle to try to move the car forward with the front tires. How can I verify that the AWD is still working properly (i.e. proper torque transfer through viscous clutch to front axle)?

__________________

Greg Wroclawski

99 C4

[JeTexas]

Try this thread. http://www.renntech.org/forums/index.php?s...amp;#entry58986

It looks like the wheels have to slip for an extended amount of time before it hooks up any real power to them.

And you could always try a burn out and see which wheels leave marks on the pavement ...

I got lax the other day when I brought my 99 C4 to the NJ DMV inspection station. I've been there with this car a half dozen times before and they always knew not to try to put it on the dyno when testing emissions. I was in the waiting area on my cell phone when I noticed they had it on the dyno and just started the rear wheels turning. They had the left front tire chocked. I quickly ran out there and after a half a minute of waving my arms and yelling and hollering I got them to stop the test.

Judging from the wheel speed I don't think it ever got above 15mph. It was all over in less than a minute. I was surprised that the car didn't try to crawl up the chocked left front tire.

I know that all the power to the front differential travels through a viscous multi-disc clutch. There are no electronic controls on it of any kind. The 996 AWD system has no center differential. The viscous center clutch allows for some slippage when making sharp turns. Any difference in speed between the front and rear axles causes the plates in the clutch to shear through the viscous silicone oil in between the plates of the clutch causing some torque to be transferred to the front axle. How can you test the amount of torque transfer? The Porsche service manuals I have don't mention a way to test this (My manuals are current through revision 75)and the service manger at my local dealer Paul Miller Porsche)was of no help.

I thought I could come up with my own informal test. I jacked the the rear of the car off of the ground on my 2 ton roller jack. I figured with only the rear tires off of the ground. If they started spinning freely there would be a large enough speed difference between the front and rear axles causing torque transfer to the front and the front tires would try to pull the car on the roller jack forward. No such thing happened. I then jacked the front tires up off the ground and they did start spinning with the rears (still off the ground). So there is some torque transfer - enough to overcome the friction of the front drive train. It just seems like there is too little torque transfer.

Is it possible that the short time on the dynamometer caused the silicone fluid in the viscous clutch to heat up so much that the fluid broke down and is too thin now to effectively transfer torque to the front axle. I believe my idea of how the AWD system works is valid. It is a very simple system.

The Porsche 911 Carrera service information manual says never use two roller test stands when performance testing on a dyno, only use a 4 roller test stand.

I don't notice any difference in how the car drives but I've only driven it on dry roads. Mechanically everthing seems normalwith the front differential.

When I did my own informal test I would have thought that a speed difference of 10mph between front (statioanry) and and rear axles(10mph) would tranfer enough torque through the clutch to the front axle to try to move the car forward with the front tires. How can I verify that the AWD is still working properly (i.e. proper torque transfer through viscous clutch to front axle)?

__________________

Greg Wroclawski

99 C4

[judgejon]

take a look at the other thread going on re "understanding the aws system." interesting questions... hope there are some answers out there.

[Malakas]

Well - I've been thinking a lot about the AWD system recently (see my other thread)... and based on what I have learned... essentially, you should be able to jack the car up - put it in gear and engage the rear wheels. This should even at baseline rotate the fronts and the rears (95/5 split). There is no mechanical diff, but as the rears rotate (slip) the fronts should pick up. Obviously disable the PSM or the computer might freak.

[wwest]

Well - I've been thinking a lot about the AWD system recently (see my other thread)... and based on what I have learned... essentially, you should be able to jack the car up - put it in gear and engage the rear wheels. This should even at baseline rotate the fronts and the rears (95/5 split). There is no mechanical diff, but as the rears rotate (slip) the fronts should pick up.

Obviously disable the PSM or the computer might freak.

I suspect it would be the TC, Traction Control, Trac, mode of PSM that would "freak" and the only result of that would be dethrottling of the engine along with braking of (both..??) spinning wheel(s).

Will disabling PSM also disable TC/Trac...??

Probably not, since it is the PSM's TC/Trac sub-mode that is used to "virtualize" a rear LSD.

So, with PSM/TC/Trac is the VC worthless, can/will it ever come into "play".

[Xeattle]

Well - I've been thinking a lot about the AWD system recently (see my other thread)... and based on what I have learned... essentially, you should be able to jack the car up - put it in gear and engage the rear wheels. This should even at baseline rotate the fronts and the rears (95/5 split). There is no mechanical diff, but as the rears rotate (slip) the fronts should pick up.

Obviously disable the PSM or the computer might freak.

I suspect it would be the TC, Traction Control, Trac, mode of PSM that would "freak" and the only result of that would be dethrottling of the engine along with braking of (both..??) spinning wheel(s).

Will disabling PSM also disable TC/Trac...??

Probably not, since it is the PSM's TC/Trac sub-mode that is used to "virtualize" a rear LSD.

So, with PSM/TC/Trac is the VC worthless, can/will it ever come into "play".

Does anyone have the part number for the center viscous clutch and how hard is it to replace it?

If they start to wear out around 70K I think I'm due for a new unit as my C4 had no front drive this morning when I got stuck on a 20 degree slope with 1-2 inches of fresh snow...

I haven't found any DIY articles on replacing this so I'm trying to figure out how much this is going to cost or if I can replace the part myself.

Edited March 8, 2009 by Xeattle