I have a '90 legacy AWD with the 5mt and i was wondering how i can tell if the center coupling is going bad (other than getting out of the car and looking if all 4 wheels spin). Of course this is based on my assumption that my car has the viscous coupling. Thanks for the help!!

There's really not a way to test it unless you took the rear part of the tranny apart.

 

Are you thinking you're not getting power distributed evenly between the front and rear?

 

If it's snowy up there, have someone stand outside the car, and you peel out in the snow and see if both the front and rear spin. Just a side note, you'll probably want someone on both sides, or have that one person watch both sides, because opposite sides may spin.

Thanks for the tip, looks like I'm going to have to recruit some friends and find a snowy parking lot, just to make sure my coupling is good of course.