Hey Dudes
1978 GMC C25 3/4 ton pkup
i am in the process of a restore and just rebuilt the 3 pc drive shaft, u-joints along with a new carrier bearing.
i also remove the helper leaf and 2 others to soften up the rear susp.
Everything looks great but I now have major vibration at about 40 MPH.
I pulled the shaft out to check the spline allignment and check the run out of the shaft at the carrier bearing, all looks good.

Any advice?

Glen

 

The driveshaft is probably unbalanced or slightly warped.

 

Agreed

 

tires and rims ok?

I'm not the best expert out there on your truck model or trucks in general, but here goes;

You can put a big hose clamp around the driveshaft, and see what that does. With quite a bit of trial and error, you may be able to confirm that the driveshaft is out of balance if by changing the orientation of the hose clamp the vibration changes too. Who knows, with luck you can find a spot where the vibration disappears Then you'd be tempted to leave the darn hose clamp in there, heh. Sounds shadetree, but it works as a diagnostic method.

if there is always an identical vibration no matter how you position the hose clamp, then the shaft probably is balanced ok.

You can also try to flip the u-joints 180 degrees, to see if it makes a difference.

Maybe put the rear on jackstands (jackstands under the axle, to maintain normal angle) and let the engine idle in gear, and look at the driveshaft to see if it turns smooth and doesn't wobble? You could try measuring the runout at several locations.

When you removed the spring, does that mean the ride height changed a lot? If you lower it a lot, the driveshaft may be at an angle that is out of spec, and it just doesn't like to run that way. Lift the thing up to normal ride height to see if that makes the vibration go away.

There are companies that make driveshafts that can run in abnormal angles, like www.driveshaftsuperstore.com (I have no connection other than being a customer). Granted, your situation doesn't sound like it would cause extreme angles. SOME angle is needed, I can't explain why, but the driveshaft really doesn't like to run absolutely straight. The U-joints need to run at an angle too to keep the bearing pins moving.

 

I would have to agree that changing the suspension back to see if the problem goes away would be a good start.

If all else fails, it wouldn't hurt to double check the balance of the u-joints again. Are they sealed, or do they need grease, and have they been greased? Also are the rag joints greaseable, and have they been lubed?

It's great to have a 3 piece system to help absorb the shock and load, but they can get out of balance easy, as you've just learned.

 

Did you replace the rubber bushing when you put on the middle carrier bearing?

 

Did you make shure all the u joints are in phase? They must line up the same, what I mean is they all must in line with each other.Lay the driveshaft in the cement, all the u joint caps should point the same way, best way is up

 

Thanks for all the suggestions... I ended up trying the hose clamp idea by straping hose clamps to the shaft to see if it showed a difference in vibration. I did see some change so I had the shaft rebalanced and the problem is gone. learn a bit about driveshafts along the way, use quality joint and always check the balance. There was a total run-out of .045 and the shaft dia. was out of round.
Thanks Again

 

No problem. George of Jungle just here to help.