Thread: Viscous Coupling

Heat is what locks up the VC. So when there is a difference between front and rear driveshaft speeds, the silicone fluid in the VC heats up and the centre diff locks.

Very simple and effective.

I have had to reverse down a slope in my P38, have not had dramas with both fronts locking up, steered my way down.

Best method involves driving down, only using the brakes as little as you can. You will then keep your steering. Easy done in an auto, not as comfortable in a manual!

Cheers
Keithy

The Classic has traction control?
Im pretty sure my P38 doesnt have traction control in reverse.
The later model D3/L322 onwards TC I should imagine is better than the earlier TC.
From observing vehicles it certainly seems to be the case off road.
I dont think the VC will respond and lock fast enough in the situation you describe, so a manual CDL is most likely better.
Hill descent control is what you are looking for I would think.
Its a pretty tall order asking (at extremes) for steerage to be maintained in reverse.
Safest thing to do is use a winch.

Originally Posted by RR P38

The Classic has traction control?
Im pretty sure my P38 doesnt have traction control in reverse.
The later model D3/L322 onwards TC I should imagine is better than the earlier TC.
From observing vehicles it certainly seems to be the case off road.
I dont think the VC will respond and lock fast enough in the situation you describe, so a manual CDL is most likely better.
Hill descent control is what you are looking for I would think.
Its a pretty tall order asking (at extremes) for steerage to be maintained in reverse.
Safest thing to do is use a winch.

Yes 92 or 93 was the first year of traction control (rear axle only). My 93 has it. It's quite harsh compared to modern traction control but it does work.

It is my experience that the VC doesn't lock. It's just a very stiff coupling with some give in it. It is this give that allows wheel speed differences when turning etc.
When I take mine in for it's 6 monthly WOF (warrant of fitness, it's a roadworthy check) they test the brakes on rollers one axle at a time. If the VC was going to lock, you'd expect it to do so there.

I haven't had my 93 in a steep reversing situation yet. Haven't owned it long enough to chase the bugs out and fully trust it.

I've a tc 93 as well, and the vc will lock, and stay locked when in reverse (Caladonia river valley proved that just before it was closed off). As Dougal says it's not as good as a locking diff though. I tend to think 95 percent.

But you need to make sure it's locked. If you drive up a hill and get cross axled it's not going to lock as the front and rear output shaft are turning at the same speed, so if you back down from this, you've at best got a centre lsd.

But, on steep terrain say you lift a front wheel, both the rear will be on the ground, thus it'll lock then.

They will also lock up in reverse too, but dissapointingly you can't do front diggs with a vc transfer case.
One thing also, you cannot over torque them.
I've had the etc working in reverse on my classic and p38.

Originally Posted by RR P38

The Classic has traction control?
Im pretty sure my P38 doesnt have traction control in reverse.
The later model D3/L322 onwards TC I should imagine is better than the earlier TC.
From observing vehicles it certainly seems to be the case off road.
I dont think the VC will respond and lock fast enough in the situation you describe, so a manual CDL is most likely better.
Hill descent control is what you are looking for I would think.
Its a pretty tall order asking (at extremes) for steerage to be maintained in reverse.
Safest thing to do is use a winch.

First thanks all for the replies. Reason I'm asking the above as I'm researching buying a late model classic.

First question did classics have traction control on rear wheels only right up to 1995.

Secondly I had a 2001 D2 with TC only and no CDL ask anyone how dangerous a D2 is with no CDL on a steep slope and after the TC has done its thing both front wheels lock up result impossible to back down a hill without losing control even a meter is impossible.

I'm trying to understand if the same dangerous situation would arise in a late model classic with TC with VC and not a manually operated CDL like in earlier model Classics.

Originally Posted by ak

First thanks all for the replies. Reason I'm asking the above as I'm researching buying a late model classic.

First question did classics have traction control on rear wheels only right up to 1995.

Four wheel traction control happened many years later in the P38 and D2. Classics only got the rear axle.

Originally Posted by ak

Secondly I had a 2001 D2 with TC only and no CDL ask anyone how dangerous a D2 is with no CDL on a steep slope and after the TC has done its thing both front wheels lock up result impossible to back down a hill without losing control even a meter is impossible.

I'm trying to understand if the same dangerous situation would arise in a late model classic with TC with VC and not a manually operated CDL like in earlier model Classics.

They can't do that. A VC never works like the open centre diff your D2 had. It can only allow small speed differences. If you were to lock the fronts with VC, the rears could only turn very slowly.

I'm not sure what Benji is talking about. There is nothing in a VC that can lock, it can only provide more or less torque transfer and can only transfer torque with a speed difference. Being cross-axled doesn't really matter what you have for a centre diff (or lack thereof).

The silicon based fluid is what does the 'locking'.

Trust me on this, I've mucked around with this vc stuff a lot over the past few years. The etc isn't strong enough either to lock the vc.

Originally Posted by benji

The silicon based fluid is what does the 'locking'.

Trust me on this, I've mucked around with this vc stuff a lot over the past few years. The etc isn't strong enough either to lock the vc.

I still don't understand what you mean.

The silicone fluid in the VC can't lock. It can get thicker when it gets hotter. But that's still not locking. It's just getting stiffer.

The ETC only works by braking individual rear wheels. It has nothing to do with the VC.