I'm having tailshaft issues with my 740. I replaced one that had a mid-bearing support rubber badly distorted and partially gone. The symptom was a big rumble when going uphills etc. especially with passengers in the back. A bit embarrassing. The guy who sold me a replacement reckoned it had new bearings. Oh yeah? Salestalk. It was out of whack and vibrated around 50 mph. Anyway it gave me a chance to silicon up the rubber support in my original tailshaft. But when I put that back it vibrated at around 40 mph!

I don't know if that is because I pulled the spline coupling apart and may not have rejoined it in the same grooves. Does anyone know if tailshafts are balanced a piece at a time or with the two parts together? I did line up the threaded lubricating holes that only had screw plugs in them, and I replaced the plugs with nipples so the joints could be greased. I had to get the nipples from a ride-on-mower place, can you believe it? Anyway I oriented the nipples in the spiders so they're all in line, but maybe that's irrelevant. I notice the later models don't even have lubricating holes.

I mean th car goes OK but it would be nice to cut the vibrations. Any ideas on this?

 

AKA driveshafts, Yes, they are balanced together as a unit and when separating them for service the pieces should be marked so can be reinstalled the same way. Why did you change it for another? Usually just the bearing is changed...

To cut to the chase, your shaft is out of balance now and it needs to be rebalanced. There are shops that do that. I don't know how you could do that at home, may be someone knows a trick...

 

Thanks Lev - that probably explains the vibes. I haven't had a close look but I don't think the spline has factory orientation marks on it, and of course I didn't mark it. I had a thought of jacking up the rear wheels and using a crayon or something to pick up eccentricities on sections of the tailshaft and maybe rebalance it that way, trial and error. Or, as you say, take it somewhere but that usually means no car for a while.
I changed it for another because the rubber support for the bearing was pretty sloppy and didn't hold the centre of the shaft in place - you could see where it hit the chassis. The wrecker guy reckoned why not grab one for $50 - usually $120 - so what could I say? Trouble is that rubber surround was pretty well gone too.
I lifted the dropped rubber surround to centre it and filled up the bottom gaps and all around with silicon rubber. Could the added stiffness in the support cause any vibrations to be transmitted more to the chassis? The rubber flange is pretty thin and soft - maybe it's gotta be like that. Any thoughts on that?

 

the rubber donuts are just a few $$ from a volvo dealer. that and a new bearing is the correct fix for such problems.

re: out of balance shaft, I suppose you could mark it, and rotate it one spine tooth, reassemble it, and try it for a day or two, then repeat. keep notes as to which orientation is the smoothest, and when you've gone all the way around, pick the smoothest and put it back there. yes, I know thats a lot of work.

 

See if you can return that shaft to where you bought it. Even though it's used, still, a decent seller would understand. It's obvious that it is also "gone" rubber wise and that's the issue really.

Fixing the old one is better, you buy the rubber piece new, and it needs to be pressed by a machine shop (not a huge deal). Then you can try to fit it to see how it would match up balance wise. Then, it's a matter of getting the right orientation. Going to a balancing shop would be the easiest way, probably around a $100 job...

I know, it all sounds like a hassle but this is the learning process. My first car DIY fixing experience was after snapping the front U joint on the driveshaft of my '65 Ford Galaxy at 3am on the Oakland Freeway many years ago... Today with the internet, before embarking on any new-to-you car repair, do some research to avoid pitfalls.

 

Another issue can be your suspension. When the rear springs sag (as they tend to do on old cars) and with extra load ie. rear passengers or uphill, it will put more stress on the drive line, causing shudder on acceleration that you speak of.

When my springs were sagging on my 940, it had this exact same issue. Replacing the rear springs and restoring it to stock ride height fixed the problem. Its something to keep in mind. On the other hand, it sounds like there's more than just a sagging rear suspension at play here.

 

and of course, lowering a car excessively is just as bad.