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Old 06-01-2010, 11:32 AM   #1
scottjal
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Join Date: Apr 2008
Location: Nashua NH
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2008 WRX hatch
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Default How do I diagnose 2008 5MT viscous coupling?

With one tire in the air and the trans in neutral I was sort of assuming that I should not be able to turn the tire by hand or even by a wrench given the torque the engine can put on the wheel? Right? What about cold verses hot?

I read a bunch of posts about the viscous coupling binding and locking up in turns and I definitely don't have that, I am thinking mine is wide open and offering no resistance. Last winter there were some instances where I had lots of rear wheel spin and nothing really pulling from the front. Was never stuck with wheel spin or towed or tracked or anything that I can think that would kill it.
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Old 06-01-2010, 01:41 PM   #2
wrxsubaru
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The stock viscous unit on WRXs is pretty weak. I believe it is a 2kg/100 rpm, which means the center diff will transfer around 7 ft pounds of torque for every 100 rpm difference between the rear and front axle. I am not sure what is required to cause the center diff to lock the diff or if it is possible. To get the most torque transfer from the diff you have to be cause a large speed difference between the rear and front axles. If you’re spinning your rear tires at 10 MPH and not moving the diff should be transferring around 65 ft pounds of torque to the front. My experience with a 2k/100 center diff is that they are very weak and can barley pull you around unless you’re trying to beat the hell out of it. The viscous units will degrade over time and use so unless it’s a new I would bet that the amount of torque transfer is a fair amount less than 2k/100 rpm. I have been able to back up a hill while on three wheels but I had the tire in the air spinning at probably 20 + MPH to do it and that was with a 220,000 miles 1st gen legacy. It’s kind of backwards from most other cars but viscous units will transfer more power the more you spin the tires.

Last edited by wrxsubaru; 06-01-2010 at 10:31 PM.
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Old 06-01-2010, 03:15 PM   #3
scottjal
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Thanks, I just thought there would be a lot more initial bite than that. This link helped me understand a bit too.
http://www.mrtrally.com.au/performan...s_coupling.htm

On another thought I guess these cars don't have any sort of active traction control either then? In my land rover if I had as much wheel spin as I had in the winter it would be clamping the brakes down on spinning wheels, same with your one wheel up situation.
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Old 06-01-2010, 07:27 PM   #4
sniper1rfa
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Yes, the 08+ have traction and stability control. The 08+ WRX does not have any type of LSD in the rear. Only the center diff is limited-slip.
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