First of all lots of good info here, first time poster, not a troll... I am a member of many other auto forums but first time here

SORRY FOR THE LONG POST!!

Little back story, helping aunt out. Has already spent 4k at a tranny shop, and having a hard time spending more... I still don;t know why they never call me first... HAHA

I have much knowledge in VW, Ford, GM.. first time working on a volvo.. but looking pretty much like every other vehicle...

02 xc70 70k miles AWD

Aunt takes vehicle to Tranny shop compaining about burning rubber smell, and harsh 1-2 shift. Shop diagnoses Trans fluid black, and harsh shift. Talks aunt into valve body replacement (does not change fluid, only valvve body, and what ever comes out with that)

test drives feels clunking. It is random, can be on first rollout from a light. most of the time it is when driving down the freeway, ... vehicle will randomly Thump, with varying degrees of BANG... they pull rear shaft. does not do it... so in goes viscous coupler. (rubber smell gone with this change) test drive states good to go...

60 miles later aunt feels same bump while driving... I inform her to not take veh back as they are just throwing the next part at it as they reccomended going to need bevel gear if still doing it....


I get vehicle, Agree coupler nec to change, has broken exhaust mount, and can not tell if valve body changed....(havent opened trans)...
Fluid is brown (to be expected it wasnt changed, only a couple qts to refil.... I am going to swap it out for her with the line removal method)

I pull the bevel gear fully expecting to see a stripped coupler. Banging noise is consistant sounding with a drivetrain issue, not a pot hole, or muffler banging... it is harsh, and loud like the wheels are trying to be stopped....HOWEVER coupler looks fine (unsure if it is common to have stripped on tranny side, but sounds as if this is not the case)

Splines look good ,move my attention to the bevel gear. Oil is full, pinion feels tight, bearings all look and feel good. Preloads are from the factory... Separate case to see if by chance something is wrong ... can see nothing... looks AMAZING inside... cleanup surfaces, , re-seal and it is sitting to dry now....

I am stumped... She can go a week with nothing, then it will happen a few times, then nothing again.. it is so random... and I thought for sure I was going to find some stripped splines or something....

so where to go next? pull the coupler anyway to see if by chance this splines are stripped on the output? thats a $200 part... hate to throw at it....

Rear shaft looks good, no vibrations.. have spun EVERYTHING everything feels tight nothing out of shape....

Im lost... going to refil the bevel gear tomorrow and reinstall.. and get the fluid change done... but I really would like to help her out... but I have never been stumped like this on a vehicle before....

Any of your help would be greatly appreciated!!!

 

OK. UPDATES,,,,,

I have been doing a little exploratory...

Pulled bevel Gear assembly out, spline looked good, no broken missing or stripped teeth.

Disassembled unit, nothing out of the ordinary. refilled with mobil 1 gl5 and reinstalled.

test drove. MAJOR banging out the back end... viscous coupler is brand new, so only item left is rear end... pulled rear end center section.. same as bevel gear. al bearings good, preloads good. reassembling now to install... park brakes look good nothing out of the ordinary, it really is stumping me.....

you can drive all over the place, it does not happen upon any certain circumstance... seems like more so after a sharp turn, or aroung 50mph....

is is so loud and sudden it scares me everytime... even though I am ready for it...

thoughts? other than this random bang, it runs/shifts/drives like a dream...

fyi it sounds in the rear, a few times like a hammer on the driveshaft... almost like the wheels are trying to stop, then broken free... or driveshaft unloading....

thoughts?

 

that sounds like a job for the hated chassis ears. not quite sure where you can get a hold of them though

 

So only thing I have noticed is rear tires are different size... 215/65r16 rear to 215/60r16 front...

I have ran the vehicle on stands, and can hear almost a grinding noise out of the VC area when off the throttle... Almost like if you were trying to push the tranny into a different gear, like a gear grinding sound...

is it possible the $1400 VC can be bad? or would this be a normal sound with chassis Unloaded on stands..
Maybe the coupling is "unloading" down the road, causing the aforementioned bang?This is my first venture into the VC arena... so I am lost as to how the internals work...

I am going out to install rearend, and driveshaft, and do the slow roll/park test to make sure the coupler isn't stripped on the tranny side... But I am really at a loss as to even which direction to head now...

 

I would not go any further until you have the same size tires all the way around. The noise you are hearing could be due to the drive train running different speeds due to the tires.

You are running a size difference that will cause issues.

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

215/65-16 5.5in 13.5in 27.0in 84.8in 747 0.0%
215/60-16 5.1in 13.1in 26.2in 82.2in 771 -3.1%

 

I am really heading to this is the issue... I wish I would have noticed the tires earlier... Never even dawned on me a tire shop would even install different size tires on this vehicle...

I don't even know how to explain this sound, it is harsh, it is loud, and it sounds like a huge hammer hit the driveshaft/rearend of the car...

 

Years ago (decades ) a shop I worked in had a full size Dodge Ram 4 by 4 have the same kind of issue. Drivetrain binding up and then releasing with a large loud jarring bang type noise. Turned out to have the same brand newer tires (installed at a different tire shop) all the way around but the front and rear were a size different. As I recall replacing two tires fixed things.

 

The manual points out how critical it is to match all 4 tires. The steering control require tires of equal size and condition (tread size) or the transmission will malfunction. It's not uncommon to end up replacing all 4 tires, after losing one to a road hazard. At the very least, the front and back pairs should be as identical as possible. Have the alignment re-set, too, as that can cause very premature wear on the tires. I ruined two new 80,000 mile tires in only 20,000 miles, because the back wheels were turned in too far and that wore off all the tread on the inside part of the tire. I just burned off a lot of tire on the road, without any clue there was something wrong. The wheel problem was spotted during an oil change.

 

I drove a 740 for years that would thump loudly when I cornered, at times. In variably, chaining the motor mounts fixed the problem. I guess the engine and transmission got in a bind at some point.

I was changing the motor mounts every 3 or 4 years, but it worked for me. Once, I think I broke them after running over a large piece of metal in the road, which did cause the car to bounce in the air a little.

I don't know how the suspension and anti-swereving devices work, but my guess is that they correct the car's motion with a brief application of the brakes on one or both back wheels. An inappropriate response could explain your thumping, so I wondered if there is something wrong with the sensors going to the car's computer.

If the transmission or drive train gears were involved, I think the noise would be constant and never let up. I also think they would have seized long ago.

Anyway, I keep an eye on my tires and I get the thing aligned at least once a year now, more often if one of the tires gets out of balance. Since we drive up to 80 mph in Texas now, it's more important than ever to keep good tires on the car and proper balancing and alignment. And, when my XC-70 is running right, it's a dream to drive.