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. 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 this (My manuals are current through revision 75)and the dealer was of no help.

I thought I could come up with my own 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 speed difference between the front 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).

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 concept 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 would have though that a speed difference of 10mph between front and and rear axles 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?

 

That's a really good question. From my own experience on ice I know that from a standstill it can take a certain amount of time of rear wheel slipping before any torque is transfered to the front wheels. It's not instantaneous, but is that time greater or less than 30 seconds? I don't know.

Can't the dealer examine the car for damage to the differential? Know of any shops with an AWD dyno? Try posting this question over on renntech.org, if anybody would know, it would be Loren.

How is the DMV dealing with the potential damage? Are they offering to repair anything or pay for any testing?

 

Four wheel dyno.

 

Four wheel dyno, dealer visit....or the cheap way would be take it out the next time it rains and really give it the "beans" (obviously pick a safe location).

 

5k RPMs, sidestep clutch, count black marks.

 

5 lb bag of sand
broom to sweep up the road later
and a piece of low traffic road

You place sand on a road from the back of the front tires to back of the rear tires. Then from a stop. Let the clutch out around 3500 rpm. The rear wheels will have no traction and will instantly slip and send power to the front.
The car will feel like its pulling and its like you only have half the power. Very weird sensation.

 

http://www.autozine.org/technical_sc...tion_4wd_2.htm

I think the only method to test the AWD would be to put the rears in the air and see if the front diff will propel the car. I think you are proabably fine IMO as I have seen many of times, TT's spinning the rears on a launch without the fronts doing anything.

 

He did that though. Sounds like he might have a problem.....

 

Doh, yes he did.

 

When we dyno tested my '01 AWD RX300 it took maybe ~30 seconds of dyno "braking", equal braking, of all four wheels with the engine at 4,000 RPM and in gear before the VC "stiffened" up enough that the torque distribution F/R rose to 75/25 from the normal 95/5.

There is a wide range, a very wide range, of viscous fluid formulations that can be used to determine the "attack" rate of the fluid. There is also the possibility of the use of a gas "bubble" of a controlled volume being added to the fluid to delay the onset of a VC that otherwise might have a very STEEP attack rate.

Prior to the dyno testing of my Rx I did my own "shade tree" mechanic testing.

With all four wheels off the ground I put 1X2" light pine wood strips throught the rear wheel openings to block them from turning. With the RX I could accelerate the driveline, front wheels spinning, to a substantially high level and all I would get was some minor stressing of the 1X2s at the rear. Reverse the procedure, block the front wheels from turning, and just a very slight application of throttle and at least one of the two 1X2's would snap immediately.

Proof: My '01 AWD RX300 is primarily FWD only.

In addition to all that, without disabling the TC, Traction Control system on my '01 AWD RX300 TC would intervene, brake the spinning wheel(s) and simultaneously dethrottle the engine. I suspect that is why the VC was dropped entirely from future Toyota/lexus F/AWD products, RX330 and RX350 included, TC would/will always activate long before the VC could/would come into "play".

It appears to me that may well be the case with the VC in my '01 911/996 C4. VC is there, but can, may never come into "play" absent disabling of PSM's TC mode.

 

wwest - love your test, that is something I would do.

I thought I read a thread awhile ago about someone getting stuck in snow. They said there fronts were not moving at all on light acceleration and were wondering why the fronts were not "trying" to pull the car out of the snow.

Someone stated you have to mash the throttle to get them to "work".

If you read the link, it states that the higher the differentiation between the fronts and rear, the higher the push or flashing to the coupling.

That is why I posted the link.

**Safety first** Until the OP is aggressive with the throttle, it will not propel the car by the fronts only with this system.

 

http://www.autozine.org/technical_sc...tion_4wd_2.htm

I suppose a 4 wheel dyno is your best bet to make sure things are ok. Im sure they are though - you'd get some kind of warning light if they busted the viscous clutch... Porsche seems to be over the top on the whole warning light thing - the owner's manual should be titled "Warning signals that may go off while driving your car".

 

Most viscous clutches are simple hermetically sealed "canisters" with no non-destructive way to check the fluid or multi-plate clutch disc condition. The way my '01 AWD RX300 acts these days, ~70,000 miles, it would be my guess that the fluid has now permanently congealed into the consistently of 90 weight gear old.

But maybe that's not altogether a bad thing.

The only way I could think of on-car, in-car, testing would be to apply front braking with the engine delivering a fairly high level of "drive" and have torque (windup) sensors in the front driveline.