[cdolph07]

Hello folks, first time post here, trying to get some help with my 2000 Subie Legacy Brighton wagon, it is a 2.5, Manual trans. with about 175k.

Overall a wonderful car which has treated me well, however it has developed the dreaded "torque bind" clunking, in tight parking lot turns, when the car is hot after highway driving etc.. When the car is cold it turns totally normally. After some internet research, this has led me to believe that the center differential viscous coupling is bad.

I know for a fact that there is one mismatched rear tire, which I had put on due to an unfixable flat, I was fortunate enough to find a used tire of the same brand, model, and size, The used tire guys thought it should be ok, I did not have the cash for the set of 4, so went with the used one. I did not measure the tires, stupidly.

Today I was looking a bit more and came across this info online which seemed hopeful - http://www.northursalia.com/techdocs/pdf/transmission/bind.pdf

So my main question is, did I destroy the viscous coupling completely or is there a chance that replacing with a fresh set of tires may get rid of the binding?


Any help is greatly appreciated. Thanks!

Cyrus
[email protected]

 

[plain OM]

Unfortunately, I don't think a certain answer is possible.

The fluid in the viscous coupling reacts to heat. In operation, the heat is generated when the plates of the viscous coupling turn at different speeds and "shear" the fluid in between. This causes the fluid to become "gelled" and act more or less like a glue between the plates. This is what is needed to slow the spinning wheels and allow the gripping wheels to move the car.

Normally, the fluid remains sufficiently liquid even when the transmission, in which it is located, warms up due to use. But the fluid can fail; it can become permanently "gelled", or it can become more sensitive to heat, meaning it will gel due to the heating of the transmission alone.

In your case the odd size tire could be a factor. What we don't know is whether the coupling fluid is gelling abnormally because of normal transmission heat build up, or if it's the odd tire that is causing the coupling fluid to heat up due to the constant, difference in wheel speeds. All we do know is that, apparently, the fluid isn't permanently gelled.

You could try a set of tires that are the same make, model, size and have the same circumference (see attached). If that works, then you've been lucky. If not, then the coupling has to be replaced in order to return the car to proper AWD operation.

Also see:

http://www.subaruoutback.org/forums...umference-spec-confirmed-subaru-techtips.html

Please keep us informed . . .

 

[canubaru]

measure the tires, all 4 of them. buy the tape measure, jack up each tire, and write down what you learn. (write it down or you will forget, trust me.)

if there is a tire size error, that is likely the cause. and it fits with the symptoms. the longer you drive the worse the problem gets. when cold it seems to work all right. (the larger the size difference the faster the binding will start.)

the bad news, even if you find a size error, you cant be sure about the viscous coupling damage until you have all 4 tires with in 1/4 inch.

but if there is no tire size error, then it is definitely the VC.

regardless of the cause of the problem, VC or tires, you are going to need matching tires, unless you are going to scrap / sell the car. if the price of 4 tires is out of your budget, just to diagnose the problem, you can install any used tire that is the correct circumference. this will not be a long term solution, because different tires wear at different rates, but it will let you eliminate the tires as the cause and test the VC.

of course, i'm not exactly sure how much, a tire circumference will change from unmounted to fully inflated on the wheel. ?????

 

[plain OM]

i'm not exactly sure how much, a tire circumference will change from unmounted to fully inflated on the wheel. ?????

Lots. Far more than 1/4-inch.

In addition, two different tires (brand, model) with the same measured circumference when mounted and inflated but not loaded, will not necessarily have the same rolling circumference. Each tire will react to loading differently based on it's construction.

 

[idosubaru]

when the car is hot after highway driving etc.. When the car is cold it turns totally normally.

you have a chance of getting away with it in the short term with new tires. but it's highly possible to have issues in the near term.

all you need is tires that are the same size and close to the same tread depth. if you're currently there - then yes your center VLSD is hosed and new tires are not going to fix anything.

as bad as you're abusing it - it's likely that if the damage doesn't show now, it will sometime soon. but it's definitely advantageous to get tires the same size and close to the same depth on there immediately.

if it finally fails completely, it'll lock and you can convert it to FWD or RWD and run whatever tires you want and not have to worry about fixing it. LOL

 

[cdolph07]

Legacy Center differential

Thanks everyone for the responses. One of the tires definitely has way more tread depth than the other 3, I got an unfixable flat and could not afford all 4 tires, so I went to a used tire place which installed a used tire of the same make and model as what was on there, although it had more tread depth, At the time I got it installed, I thought the tread pattern was what mattered, not the circumference.

The tires on there definitely need to be changed anyway so I figure I will start with that.