In another thread, I've been talking about trying to diagnose the root cause of a bearing-like noise. I now have the front diff off the car and the tailshaft separated from the viscous coupler.

My question is: If the viscous coupler has failed, what would be its behavior? Is it going to transfer 0% power or 100% power?

Right now, if I spin the two outputs of the front diff together, I can't keep the input of the coupler from spinning at the same speed too. In other words, it feels like 100% of the power is being transferred. Of course, this is just with my bare hands, so I don't know if this is a realistic test. I just assumed that given that the coupler should only transfer 10% or so at low speeds, I should have been able to slow down the coupler's motion with my hands. I'm hoping that the torque required to cause the expected slippage is a lot higher than my hands can handle.

This obviously makes me even more nervous about my mismatched tire diameters (the way the car was sold to me).

 

my car was making the EXACT same noise that yours was, i was convinced it was the bearings until i saw your video. I really thought i ruined my front diff, because i was running the wrong size tires on the rear. I too unhooked the driveshaft to see if it was coming from the front diff. I still had the noise, so i decided to replace the front wheel bearings, and that took care of all of the noise. Thank god it was not the diff! are you certain that it is NOT the bearings?

 

Oh. I know for sure that the wheel bearing is definitely toast. If you follow that old thread to its end, I just posted another video to prove that. I was just not convinced that the diff was completely ok.

Also, when you think about it, disconnecting the driveshaft does little to isolate the issue. Most of the diff will still rotate along with the wheels. If indeed the viscous coupler had failed (doubt it would make a bearing-like grinding noise if it did), the center shaft might not spin... and if the issue was only there, then yes, the sound might change/disappear.

I actually just swung by Barrier Porsche and spoke to a tech. He confirmed that the viscous coupler will fail open - i.e. 0% power transmission. The fluid apparently overheats and leaks out - which is an awesome failsafe. If it locked into a closed position, the drivetrain would be a complete mess.

I still plan to try and replace some bearings on my diff since it is out. It isn't as quiet as the replacement one that I bought (used). It isn't loud enough to be the source of the noise in the video, but might as well try.

 

Not to sure about the coupling failing open. I have cut one open before and it is just a whole lot of thin steel plates (looks like spring steel). The oil stinks really bad, I mean really bad.. There was nothing that added any force to help it clamp up and act like a clutch it was just plates stack up and sealed in a case. I am guessing that when the plates get hot from slipping they distort and that helps to lock it up. In my car the front drive would let the front wheels spin and make it very hard to control. I can not see the coupling itself making noise if it failed unless it needs to slip because of wheel spin.

 

It's a pretty standard viscous coupler. The fluid is a silicone based fluid. What happens is that when the input plates start spinning, the friction between them and the fluid heats the fluid up. As the fluid heats up, it gets more viscous and in turn begins to transmit power from the input plates to the output plates.

It is conceivable that if the fluid "cooks", the coupler will stay locked and/or prematurely engage. Other cars like old VW vans seem to have this happen. This is bad when your tires are not matched 100% since it will strain the entire drivetrain tremendously. The front half will want to maintain a certain rpm and the rear half another rpm, and the coupler is not absorbing the difference.

The Porsche tech was basically saying that if the fluid cooks, it just leaks out of the coupler. Leaving it in a completely open state. That means you need a new coupler, but the rest of the drivetrain stays stress free.