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740 Rumble around 40-50mph

petiww

petiww

Member
Joined
Aug 7, 2011
Location
Hungary
Hi folks!

I recently serviced a very good condition '91 740 to pass MOT. I changed the diff seal, so pulled the driveshaft, and as the center support looked very tired, i decided to change the bushing and the bearing too.

For the diff seal i used the trusty paint marker to line everything up before disassembly, and torqued the nut exactly where it was.
Driveshaft was marked aswell, pulled it apart, pressed new bearing on with bushing, rotates freely.
U-joints are moving freely by hand.

So here comes the problem, it has a weird rumble when cruising around 40-50mph. As i recall it is more present when just keeping speed or decelerating. No real changes when clutch is pressed in. Its a rather fast and deep rumble, so i'm thinking its driveshaft related, maybe an u-joint?
I have never experienced a bad u-joint before, so im not sure at all. Can they act like this?
Maybe one thing to mention, the previous center support was holding the bearing more freely, the current one is a tougher design.

Owner mentioned it had a slight rumble before but now it changed a bit and got stronger.

Any tips or advices?
Thanks!
 
It's possible that it was unbalanced before but since the old center support was tired, it basically soaked up all the vibrations that the stiffer, new center support is unable to do without making a fuss
 
The true story would be R rated:language, tension, some violence.
Click to expand...

Customer service at it's finest degree.

But seriously it sounds like its off balance if it happened prior to the center support but got more aggressive after a new one.
Off-balanced driveshafts rumble reliably at certain speeds while rumbles caused by pinion angles happen on and off throttle usually. This is my understanding at least.
 
Thanks for the inputs! Well, after a 100-150km this has turned into a deeper rumble at higher speeds, and definitely speed related. Vibration and rumble around the previous speeds decreased somehow. Sounds like a bearing is going out.

As i touched the diff around pinion I'm a bit worried that i may messed up pinion bearing preload if its a crush sleeve type. I put back everything to its previous place and position, but did not check that i exceeded the maximum torque during tightening. :omg:
Pinion shaft however does not have remarkably high turning resistance...

Other concerning parts are the U-Joints. They moved freely by hand, but can they bind once they are spinning with road speeds?

InTheWhite said:
The problem is the new centerbushing. Nearly all that changes it get the problem.
I had the same **** when I changed mine. Tried several different and finely found
one from "KL Racing" that works proper.

https://shop.klracing.se/sv/artiklar/stodlagerhallare-740_-760-_-940_-960-for-508mm-kardanaxel-.html
Click to expand...

So this is basically version that lets the bearing move around more?
 
agreed. If you are able to move a u-joint freely by hand it is good enough to not bind while in use. (unless it's mounted wrongly)

The driveshaft is made of 2 parts, have you seperated those at any point? Have you put them together in the same position relative to each other?
What about the preload on that centre bearing? (after mounting it with the bolts still lose you can shift the assembly forward/backward a bit) Is it in the same position as before?
Have you tried spacing out the center bearing assembly (with some washers) so it sits lower. This would straighten it a bit which might be enough to get rid of the vibration.

In order to really get to the bottom of this you could put the rear suspention on jackstands so the engine can drive the wheels without the car moving. Maybe you can see or hear or feel the vibration while the driveline is turning. (be careful when you do this!)
 
Janspeed said:
agreed. If you are able to move a u-joint freely by hand it is good enough to not bind while in use. (unless it's mounted wrongly)

The driveshaft is made of 2 parts, have you seperated those at any point? Have you put them together in the same position relative to each other?
What about the preload on that centre bearing? (after mounting it with the bolts still lose you can shift the assembly forward/backward a bit) Is it in the same position as before?
Have you tried spacing out the center bearing assembly (with some washers) so it sits lower. This would straighten it a bit which might be enough to get rid of the vibration.

In order to really get to the bottom of this you could put the rear suspention on jackstands so the engine can drive the wheels without the car moving. Maybe you can see or hear or feel the vibration while the driveline is turning. (be careful when you do this!)
Click to expand...

Yes, driveshaft was apart, went back in the same spot. Preload, well, i tightened the support bushing carefully with no pressure to the bearing and bushing at all.
Car is a stock height one by the way, so spacing the assebly higher or lower is not needed imo.

By the way i finally put the rear end on jackstands and tried it. Well, the rumble definitely comes from the driveshaft center support, as i could listen with a screwdriver from bottom. At 90kmh it is very intense in the cabin, like a heavy subwoofer...
So InTheWhite was right, new center support bushing does not let the bearing move around enough to absorb the vibrations. It almost acts like a solid support its so tough.
 
Just because you put the drive shaft back the way you found it doesn't mean it was aligned correctly before you took it apart. Make sure the arrows stamped into both halves are aligned.
 
hiperfauto said:
Just because you put the drive shaft back the way you found it doesn't mean it was aligned correctly before you took it apart. Make sure the arrows stamped into both halves are aligned.
Click to expand...

Cleaned it, i can see them now. They are only a few degrees away, but they also seem to be hand stamped, so its ok. Rotating one spline away would throw it off way more i'm sure.

Checked a few threads too, some people experienced the same with the aftermarket more stiffer center support. Few of them had success with lifting the center bushing. Well, tried 10mm first, it did change the vibrations, but only shifted them into lower speeds. This is still only a jackstand test, but I'm fairly sure this would be still crap on the road.

Will order a Meyle one and check its design. If its a tough one too i'll may try just cutting some of the current bushing rubber to match the original style. If that does not work out I'll just swap in the original 25 year old volvo center bushing from a laying around driveshaft...
 
petiww said:
Will order a Meyle one and check its design. If its a tough one too i'll may try just cutting some of the current bushing rubber to match the original style. If that does not work out I'll just swap in the original 25 year old volvo center bushing from a laying around driveshaft...
Click to expand...

New meyle has the same stupid design, so i left that at the shop.

Removed the 10mm spacers, cut some material from the current new aftermarket center support, and finally it feels way better. By hand it is still stiffer than the original support.
Tested on jackstands, it only makes a slight little rumble around 40mph, and no vibes or rumbles around 50-60mph, super silent. On the road it feels nice, little vibes come thru during deceleration, but it is not intense at all.
 
"Cleaned it, i can see them now. They are only a few degrees away, but they also seem to be hand stamped, so its ok. Rotating one spline away would throw it off way more i'm sure"

Hey,
The marks (arrows and/or paint dots) do appear to be scribed/made by hand. If these are not lined up I would wager that is the cause of your vibration.
 
moustacio said:
Hey,
The marks (arrows and/or paint dots) do appear to be scribed/made by hand. If these are not lined up I would wager that is the cause of your vibration.
Click to expand...

They do line up, I just wrote that they are off a few degrees. Assembling one spline away clockwise or counterclockwise would throw it off way more than that few degrees.
 
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