Center viscous differential differences A/T - M/T

[wyattmccandless]

Hey guys, I just bought a 93 Legacy L with a 5 speed, and I am having trouble deciding if the center differential is good. I used to have a 91 Legacy L with the 4EAT, and in parking lots and times when turning sharp at low speeds, I could feel the rear end pushing the car, so I knew the AWD was doing it's job. With this 5 speed I feel nothing. I read somewhere that the center differential in the manual trans is actually different from the automatic...is that true? If so, what can I do to test it and make sure it is doing what it should? I know my A/T had a FWD fuse the disengaged the center diff, but I don't see one on the manual and have been told it doesn't have one. Any help would be greatly appreciated.

[vrg3]

They're completely different. The automatic doesn't have a traditional center differential at all -- it has a transfer clutch that can be modulated to control how much power is sent to the rear. That clutch (the "multiplate transfer clutch") is operated by ATF pressure which is regulated by a solenoid under control of the transmission control unit.

The manual has a standard differential with a viscous coupling in it. It's always functioning; it can't just not work. Torque is always going to the front and to the rear.

I suppose the viscous coupling could fail, though, which would prevent the limited-slip action from working correctly. Even when it's working right, though, it doesn't lock up the way the auto's AWD system can. I guess you could do a very basic test by raising just one side of the car (left or right), securely chocking the wheels on the ground, and seeing which direction the other free wheel turns. An open differential would make it turn the opposite way, but an LSD would make it turn the same way.

If you try this, please don't kill yourself by making your car drive itself off of the jackstands.

[wyattmccandless]

So what I am feeling while driving is probably normal? The center diff is probably working fine? I will try raising one side to see what happens. So if it is not electrically controlled, how does the torque split from front to rear happen? Does it just use the viscous coupling in place of where the clutch packs would be on a clutch-type LSD? If I understand right, the viscous coupler heats up with a difference in speed from one side of the diff to another, and as the liquid gets warmer, it get thicker, causing the extra friction that allows more torque to be transferred to the rear (in this case). So does this mean there is never a true 50:50 split?

[vrg3]

Yes, you understand correctly. I'm not sure if it's the heat exactly that makes the fluid thicken -- I believe some of it is just the nonlinear nature of the fluid -- but the basic gist is right.

I don't know how to answer the question of whether there is ever a 50:50 split or not... Under normal non-slipping conditions, the torque sent to the front and to the rear is equal, because that's what a differential does. But, yes, when one end is slipping, the other end gets more torque as the viscous coupling tries to make the speeds match.

[wyattmccandless]

Ok, that is good to know. I will do a little more diagnosing then and make sure it is working properly. Is there any opinion as to which of the two setups (4EAT or 5 spd) work better in adverse traction conditions? I know my automatic did great, but from what I have read, it also had the capability of locking into a 50:50 split. Does anyone feel that that is an advantage on slick roads?

[vrg3]

There doesn't seem to be a consensus on this one. My experience has been that I do better with a manual under slippery conditions, but I think that's largely because I'm better at driving a stick than an auto and also the manual offers more control.

Your front and rear differentials are both still open, remember, so it's still possible to experience a great deal of slip even with a limited-slip center diff.

In practice, though, I think it just doesn't matter that much most of the time.

Another board member, ciper, feels that the automatic's system is far superior, largely because it permits full lockup. You should be able to find some of his posts explaining in more detail.

[wyattmccandless]

I will check it out. Thank you very much! You have been most helpful.