Viscous Centre Diff To Open Centre Diff
 

Hi,

I have bad torque bind on my 2005 GT Legacy 5 speed manual.

Replacement viscous couplings are so expensive not to mention the labour on top to fit it.

As I have just spent loads on an engine rebuild, replacement gearbox, new clutch and new flywheel etc. I feel rather aggrieved at the thought of having to spend a load more cash on a new centre viscous coupling (part number 38913AA102).

If the diff had completely failed (broken / stripped teeth) I guess I wouldn't feel quite so bad but the fact is that the differential is essentially fine; it's 'just' the viscous coupling that is binding. Hence I feel aggrieved at the thought of having to buy a completely new centre differential and viscous coupling assembly.

If the viscous couplings failed to an 'open' differential rather than failing to a 'locked' differential then probably 99% of owners would never realise anything had gone wrong!



Question:

• Not all Legacy's have a viscous centre coupling; i.e. lower powered non-turbo versions have 'open' centre differentials I beleive. Do these cars have an equivalent centre differential but without the viscous coupling part? If so could one of these 'open' differentials be swapped for a viscous centre differential?

I still have my 'old' gearbox which has the same viscous coupling in as the one which is binding in my recently replaced gearbox. My old gearbox has done 100k miles and my recently replaced gearbox has done about 50k miles.



Question:

• Is it possible to take these viscous couplings apart and drain the silicone fluid and put back together to effectively have an 'open' differential? I know there are some downsides such as less traction but it still going to be better than a 2 wheel drive car and 99% of the time I probably wouldn't even notice the difference?

If this is possible I could save myself a lot of money!

Any thoughts welcome!

Telephonewire.

I'm pretty sure you're not really sure what you're talking about. If your car is a manual you have a viscus coupling. Every manual 5 speed has it. Automatics have transfer clutches. No you can't do what you're saying. An open diff doesn't mean there isn't a viscus transfering power to the rear. An open diff just meands there isn't an LSD in the front diff.

a new center diff is around $520+labor. not that bad considering aftermarket units are 2-4 times that.





All late model 5mt subaru's use a VLSD in the center including the non-turbo ones.

The VLSD diffs are sealed units. you can not "drain the fluid out." you can swap in the full diff in your "old trans" in your the one that is binding if its still good.


I'm a little skeptical of the fact that your center diff has gone bad. If you haven't had someone verify that it is your center diff, you should get a second opinion. Could just be a bad axle. If it is in fact your center diff, then it must be replaced. No getting around it. you could get one of the fatermarket ones, but of course they are alot more $$$$.

Actually, it would be a completely FWD car. The center differential allows the front and rear axles to rotate at different rates. An open differential simply lets them spin at different rates, but does not provide any power transfer. An open center diff = FWD.

You could probably hack the car to FWD by removing the rear drive shaft and sealing the coupler. I think someone on the first or second gen forums did this mod.

^wrong.

An open center diff is not FWD. its AWD with no limited slip TQ transfer. with an open center, power would just be sent to the path of least resistance, but it would still be either the front or the rear. if you were to remove the drive shaft on a true open center diff, the car would not move.

Ah, right. I stand corrected. In fact it would be worse than a FWD car, you could get stuck by having either the front or back tires on ice.

Ok - so there isn't a swappable 'open differential' alternative available.

Ok - so that option isn't avavilable either!

My old transmission has done 100k and having read various articles about them the viscous couplings often only last about this long so I am slightly reluctant to swap it as I may have the same problem again in a very short time!

Fairly certain it is the centre viscous coupling:

Test 1
Turning the stering wheel, whilst manoeuvring at low speeds, feels like gently applying the brakes; i.e. the transmission is binding.

Test 2
Front weels off the ground (rear wheels on the ground), with the transmission in nutral, I can't rotate both front wheels in the same direction at the same time.

Thanks for your advice "whitetiger" - looks like I will just have to 'bite the bullet!'

Thinking about why it may have failed - I did get a puncture about 3 or 4 months ago where I had to put the 'space saver' spare wheel on and drive ~30 miles. As the space saver is a smaller diameter this will have caused the outputs of the centre diff to be spinning at different rates and heat up the fluid which ultimately could have cause the failure I now have.

If this is the case why do Subaru put space saving wheels in their cars? ...because if they are ever used they are likely to result in centre viscous coupling failure! Do they warn about this in the owners manual (unfortunately I don't have an owners manual to check).

Center diffs typically fail from owner abuse and neglect. Poor tire pressures, doughnuts, hard launches, high mileage fluid changes, etc. The bottom line is: overheating and aggressive fluid breakdown lead to increased wear on the components.

Which axle did you put the spare on? If it was one of the rears, that very well could have caused serious issues as you mentioned.

From my recollection, with the car in the air and in netural, the front wheels will spin opposite directions. You shouldn't be able to move the front with the rear wheels on the ground. I'd double check this, but I believe this is a correct statement.

Best test is to take a highway drive for about 20 minutes to get the fluid to temperature, then go into a parking lot and drive in figure 8s at low speed, near full-lock. If the wheels hop/drag or the car is very difficult to turn, you have a bad center diff.

The braking sensation may actually be your brakes. Check if one of your calipers is sticking.

I'd recommend replacing the center diff with the factory unit. A brand new one can be had for around $450 from Heuberger (a forum vendor/sponsor). If you are mechanically inclined, you can do this in your driveway/garage with one set of ramps and some basic tools. It does not require a transmission removal. Here is a link of photos to help you determine if this is something worth doing.

I can also help you out with a new OEM unit and can send you a digital copy of the section of the manual that covers replacement.

+1 to what Whitetiger and compsurge have said. I suspect a different failure mechanism though, since the typical failure mode - "worn out" fluid - causes the diff to act open rather than lock-up.

AWD Fuse
 

Maybe this is just in the 5EAT, but isn't there an AWD fuse you're supposed to pull when you put on the spare tire (switching the vehicle to FWD only)?

If this is true for the 5MT, I think it would be a good temporary solution. Maybe I'm thinking of a different vehicle though.

No

The 5MT is a fully mechanical AWD system.

Its not in the 5EAT either. The FWD fuse is only found in the 4EAT, which is the 2.5i.

For 4th gen Legacies its best to put the donut on a front tire, as the open differential will help reduce the rate difference seen by the center diff.

This is normal behavior. If you were able to rotate both front tires in the same direction, it would mean that the center differential was acting as an open differential.

Spinning both front wheels in the same direction would rotate the propshaft between the center differential and front differential. Because the center differential is preventing the front prop shaft from rotating independently, the front wheels can't both be rotated in the same direction.

Hi,am having a similar problem.I drive a 5MT Legacy.Recently the car has been binding when taking slow turns and going forward especially after driving for a while.I managed to change the center diff but the problem is still there.I have pulled out propeller shaft and the binding is not any more could it be the rear diff?