Significant vibration problem

[dogstar]

Hello all!

I have a bad vibration problem in my 73 series III at certain speeds and conditions. I have checked all the U joints, one was bad in the rear shaft at the transfer case end and I have replaced that. I also found that three of the bolts holding the rear drive shaft to the differential were loose and one was missing. I bought four new identical bolts and secured the shaft. Despite fixing those obvious problems the vibration is still there and actually may even be a little worse. The vibration occurs between 25-30 MPH. It is significantly worse under hard acceleration. If I very gently accelerate through this speed range I can greatly reduce the vibration. If I am in this speed range and disengage the clutch or shift to neutral the vibration goes away. At speeds greater that 30MPH I can cause a less severe form of the vibration by accelerating hard.

This really has me stumped. I am wondering if a bad motor mount could cause such problem. I have one mount that looks rather shabby, but I don't seem to have what I would consider abnormal engine movement when I rev the motor.

Any suggestions would be greatly appreciated.

Thank you,

Dogstar

[fruitpunch]

Who did the drive shaft check ? were they balanced ? if not possible cause could be drive shaft not balanced, worn splines and so on good drive shaft place can rebuilt them. How about your tires are they unevenly worn ? Try the whole proceedure and while accelerating engage 4wd.... my guess it will be less shaking. If you have free wheeling hubs lock them.

[bamarover]

Check the transfer case mounts for missing or loose nuts. Look just like the motor mounts, but on the transfer case.
Greg

[jp-]

Try the whole proceedure and while accelerating engage 4wd.... my guess it will be less shaking. If you have free wheeling hubs lock them.

Are you sure this is a good idea? I know that you can press the yellow knob down while moving, but it has been my general understanding that it shouldn't be done above 20mph and not while accelerating. I pressed it once at approx. 30 while accelerating, and it didn't make a good noise.

If the driveshaft has been running on one bolt (I've seen this before) the ujoints are quite possibly shot. You can check this by having the truck parked and out of gear (with the wheels chocked). Get under and turn the shaft back and forth. Look for wobble in the joints, it should be easy to see.

[dogstar]

Who did the drive shaft check ? were they balanced ? if not possible cause could be drive shaft not balanced, I checked the shafts. I saw no obvious problems other than the one U joint I replaced on the rear shaft. Did not have them spin balanced, but I would think if it were off balance I would feel increasing vibration with increasing speed. Is that a correct assumption?



worn splines and so on good drive shaft place can rebuilt them.
The splines on the rear shaft were tight and looked good. The splines on the front shaft seemed to have a LOT of play until I greased the shaft. I did not remove the front shaft so I have not visually inspected the splines, just going by the wiggle on the slip yoke. The play was not torsional, but lateral (wiggle up and down). As I mentioned previously it tightened up once I greased the slip yoke.



How about your tires are they unevenly worn ?
My tires are evenly worn and in good shape.



Try the whole procedure and while accelerating engage 4wd.... my guess it will be less shaking. If you have free wheeling hubs lock them.
To my total amazement you are 100% correct! With 4WD engaged the shaking went away almost completely. Just wee bit of vibration left. What does that mean? I'm totally puzzled now.

--dogstar

[dogstar]

Anyone have any suggestions?