325ix Rear Differential Rebuild DIY

[The Dark Side of Will]

Well don't reuse them now...

[Motorsport1207]

Hey, reviving this post. Diving into rebuild my 4.10 ix diff. One question I have how do you know the viscous unit is good and not worn? Do you do a breakaway torque test? What value is good vs bad Thanks Ted Sent from my iPhone using Tapatalk

[AWD_E30]

I found my rear viscous coupler was bad was by jacking up the rear of the car, placing the car in gear (manual) or park (auto), and trying to turn one wheel forward by hand. If the viscous coupler is bad the other tire will spin in the opposite direction pretty easily. If the coupler is good the other tire will spin in opposite direction but with considerable force.

Side note: Jacking up only one rear tire will test rear and center coupler if the car is in neutral. This is not ideal because it's hard to tell which coupler is providing resistance. I accidentally did this when I bought my car and found out later the center diff was good but rear was bad. Good luck!

[Motorsport1207]

Hi, Thanks for this info, can this be bench tested similar to the standard LSD diffs where you lock up the pinion flange in a vice and use a torque wrench to turn the axel output shaft flanges and determine the breakaway torque (BT). So for a M20 at 168 HP 25% lock up would yield a BT of 42 ft lbs I presume this can be applied to the IX viscous coupling diff and they also would have the 25 % lock up Thanks Ted Sent from my iPhone using Tapatalk

[nando]

you can bench test it but not like a clutch pack diff. There's not a breakaway torque, the locking torque increases with the speed of rotation, which takes a special testing method to measure.

technically the VC can lock nearly 100%, of course there is a limit to the actual torque it can hold without overheating.

[AWD_E30]

Yep Nando is right. My center diff viscous coupler testing has shown there is actually a speed vs. Temp relationship (think 3D surface plot) that results in a different torques. In my case I was able to compare the bad diff vs good diff out of the car by turning left/right output flanges but this was not very scientific. My bad one was completely dead... No life left. No guarantee if the coupler was weak or on it's way out by using this method.

[Motorsport1207]

Ok, So before I disassembled this diff I tested it like a clutch pack diff, holding input flange and using a torque wrench to see the point it start to rotate. I measured 39 ft / lb torque. I presume this is good as this would be the similar result that AWD_30 observed. Sent from my iPhone using Tapatalk