Just put in brand new awd shaft.. 1998 xc70

[JasonVolvoMan]

I just put in my brand new awd shaft from Colorado drive shaft. I have a 1998 xc 70. The install went great, I need a new crossmember mine is rusted out and 1 hanger I had to weld back up to it. Took it out for a test drive after, went on the highway and got it to 75mph. Their was not vibration of any sort or wired noises of any kind. So I got it back to the yard put in in reverse to get it in my stall and I felt and heard a little stump! So I took it back around to block, no problems again went to put it reverse again starting backing up again half way in to the stall I heard and felt the stump again. I just can figure out what it is. Does anyone have a opinion on this. Let me know. It only happens in reverse and its every time in reverse..

[jimmy57]

I have had viscous couplings make a first generation AWD bump after it has traveled backwards 20 to 30 feet. There is a one way clutch in the VC that will not let torque be transmitted in reverse. To give reverse power flow there is a ball clutch. The ball clutch is what I believe to cause the thump. It seems that is doesn't lock and when when you back a few feet it pops in and causes the thump.
I have not cut open a VC on a car with this issue. To test for it drive the car foward a distance, around the block would be good, and then pull into shop forward. Leave gear lever in neutral and jack up only one front wheel. Put a breaker bar and socket on the axle nut and turn it backwards and see if it turns. If it is the ball clutch in VC you will be able to turn the wheel which is turning the rear driveshaft. If the ball clutch is not engaging then nothing stops you from turning the rear driveshaft. I would not be surprized if you turn it a turn or two and it engages and stops you.

[JasonVolvoMan]

Is it something to worry about? Or can I drive it ? Don't want to wreak that new driveshaft!

[jimmy57]

Drive it. Back up with finesse.

[JasonVolvoMan]

Thank you!