BMW: 1989 BMW 325 ix problem with awd, bmw 325 ix, viscous coupling

Question
Josh, I was hoping you might be able to help with a problem with my sons BMW.We try to do all the work ourselves on our cars.About 2 weeks ago we had some snow and we noticed the awd wasn't working,[rear wheels spinning only]I haven't had a chance to take a look at it till last night and another problem came up a couple days prior to that.This was a noise and it turned out to be the plastic 'cover' that covers the front of the transfer case where the front driveshaft goes in.It seems to have come loose and is now spinning freely sliding back and forth on the front driveshaft making the noise.I don't see how this cover is supposed to be secured and would assume the 2 problems are related. What I have done so far....I topped off the front differential gear oil,it was a little low.I tried to find the fill for the transfer case but couldnt find it,I see the drain plug but not where to fill it.I then thought maybe it just used the tranny fluid in the transmission which is full.The front driveshaft seems to have a lot of play in and out of the transfer case.I jacked the car up so all 4 wheels are off the ground with the transmission in neutral.If I spin the front wheels by hand the driveshaft spins and the back wheels don't.If I spin the rear wheels by hand [which seem to be much more effort required] they both spin and so does the front driveshaft but the front wheels do not spin at all.One other thing that may be a related issue is when the car is in Park it will inch backward on an incline if the parking brake is not engaged.The shifter cable is adjusted and the transmission IS actually in Park...I have confirmed this.What do you recommend? Any help would be appreciated.Thanks, Bill

Answer
Bill,
 The "ix" is a rare E30, and I personally have never owned one.  I do know that they use a Viscous coupling in the t-case to transfer power to the front wheels.  This works much like a fan clutch, and also like a fan clutch... they wear out.  There is no "direct" connection between the front and rear driveshafts, it runs through this viscous coupling.  This viscous coupling contains a thick silicone solution that will break down over time and not transfer power to the front driveshaft.  Unfortunately replacing this requires dropping the t-case and opening it up replace this.  The viscouse coupling is also a VERY pricey part as well.
 The following website is exactly what the Dealership would use to find the parts you need.  It also contains very good diagrams of every part of the car.
 www.realoem.com
 Click on "click here"
 Select "3' E30"
        "coupe"
        "325ix"
        "USA"
        "year and build date"
        "and select "auto" for tranny.
        Then click "Browse Parts"
  Once opened scroll down to "27 Transfer box"
        Under sub group click "transfer box"
           Then select the second illustration
            and #32 is the viscous coupling (called  a "clutch"

 Other websites to check out:
 www.e30tech.com
 www.bmwe30.net
 There is a guy who knows everything about the 325ix there, as I dont know much about the t-case and front axle on them.  
 
 Hope this can help some...
 Josh