As in subject. I am little confused, read, talked to services and still no one concensus about how to tell if Visco LSD is ok ( taken from a car - on table) .

I mean : I have a open diff (100% its open regular sierra 7) which when turning imput shaft - both flanges for driveshafts turn in one direction .
Have second 100% open 7 sierra diff, which when turning imput - from propshaft shaft, both flanges turns in different direction.
The same behavior of both on cars with wheels up.

Now, bought used VLSD from a person that claims it was working (doing donuts etc) in his Sierra years ago, then he took it out of the car, was lying on the shelf for years. I bought, send to service which do building and servicing rally cars etc so he has experience with plate etc diffs.
He says this visco is faulty because on the table it behaves like open diff.
But - is it not how it should be? without load it behaves as open 100%, only when speed difference (well torque maybe better) fluid getting thicker and makes friction between plates?
But he says yes, but some little preload should be there .
Here is a video from service below. My question - is this how VISCO LSD behaves on table out of the car? :

https://youtube.com/shorts/4BKYNbh6h5c?feature=shar

I would expect it to be hard to tell, if there's some wear because I'd expect them to be fairly well locked together with no load on the outputs but if there's wear or a fluid issue they'll naturally tend toward acting like an open. I know they lock under load with speed difference but at near zero load you're just looking at a balance of friction forces really.

I will have mine on a lift next week if it helps.

[Edited on 24/2/23 by coyoteboy]

Sierra Viscous Test

[Edited on 25/2/23 by minitici]

Thank You!

So it seems that on my video there is no torqe at all if one can with no effort to turn with finger those gears :-/

quote:

---

Originally posted by minitici

Sierra Viscous Test

[Edited on 25/2/23 by minitici]

---

Yes You describe how it works when driving.
I supouse on road/track there are much difference left to rear wheel speed allows for visco fluid to "stiffen" and produce friction between plates.

But how it is with this procedure on cold diff and only half turn in one second as in this procedure? Mine diff for sure is not close to 70 or even 40Nm preload

I am confused. Person who sold me diff with visco (trust him knows a lot about Sierras bougjt from him some parts gearbox etc all as described ) claims that in his Sierra it was working and locking wheels years ago (donuts etc) . Then put diff out of this Sierra , diff was laying on the shelf for years.
Maybe its is worn but on heavy load (much left to right wheel speed difference) visco-oil is still able to produce friction between plates?

But even if so I will never be sure. Tempting to go for torsen blackline and forget.
.