[wildfire78]

I have a 78 Dodge D100 with a modified 318. I recently changed the rear end gears from 3.2? to 4.11's. Since that time when I get to around 4500 RPM I begin getting a severe vibration. I can't pinpoint where it is since it is felt through out the truck. I have checked pinion angle, drive shaft balance, tire/wheel balance, etc.. All of which is good. I'm now wondering if it is being caused by the rear end gear swap or if it was there already and I never pushed it to the 4500 RPM with the lower numerically gears. Am I overlooking something?? Any help would be appreciated. Thanks

[propaniac]

Is it more noticable at certain speeds or rpm? Does it vibrate at 4500 in all gears? Or is only in high gear at highway speed? Does it vibrate in neutral at that speed or do you have to throttle it up to get it to vibrate?

Let us know

John

[wildfire78]

Originally Posted by propaniac

Is it more noticable at certain speeds or rpm? Does it vibrate at 4500 in all gears? Or is only in high gear at highway speed? Does it vibrate in neutral at that speed or do you have to throttle it up to get it to vibrate?

Let us know

John

It starts to vibrate at about 4300 RPM on the highway and gets worse the higher the RPM/speed goes. I'm running the 727 automatic and there is no vibration in 1st or 2nd even when I am on it hard and RPM reaches 4000. I haven't tried shifting to neutral at that speed, the next time I am on the road will try this. I was wondering if maybe I needed a new harmonic balancer since this is the original from 78. I figured I would feel vibration even at lower speeds if this was it though. Thanks for the thought on trying it in neutal, that completely slipped my mind.

[johnsongrass1]

Drive shaft balance problem or u joint problem. Bad tire wear problem.

[propaniac]

I agree with Johnsongrass. The lower gears have sped up your driveline, ( thats why you didnt notice it before) See if you can find a good driveline shop and get them to balance it.(replace the ujoints when you have it out) I have a friend here that has a balance machine and it is interesting process to straighten and balance a shaft.

Also check the slip yoke and bushing in the trans.

Keep us informed. Nothing more frustrating than never hearing if we helped or not.

John

[poncho62]

Im thinking "U" joints also.......they can still be tight, but dried out which will cause vibration. Replace them both....they are cheap enough, and a simple fix.

[wildfire78]

I tried putting it in neutral today while on the highway and the vibration was still there. Both U-Joints have been replaced. I believe you'll may be on to something with the drive shaft since it still vibrates in neutral and as propaniac stated the speed of the driveline has increased compared with before. As soon as I get the chance I will pull the drive shaft and find someone to check for straightness and balance. I'll let you'll know what I find out. Thanks again for all the suggestions.

[poncho62]

Turning the "U" ,joint 180 degrees in the rear yoke may also make a difference.

[1931 steve]

Originally Posted by poncho62

Turning the "U" ,joint 180 degrees in the rear yoke may also make a difference.

I always mark them before I take the shaft out so to install it back in the same position. And yes I have had them vibrate by being 180 out.

Steve

[1982 SS]

Just a thought, nobody has said motor or transmission mounts. I replaced all of mine and it made a huge difference. I would imagine it could be the rubber mounts just given the age of the vehicle unless the have been replaced.

[IanRiordan]

Check the diff yoke, I had one side worn on a 9 inch and it chopped out 2 rear unis in 2 weeks. Also consider driveshaft diameter, check the shaft from a factory 4.11 geared truck - they may differ. Is your pinion bearing preload ok? It all points to a problem between the g/box planetary set and the pinion as this part of the powertrain is the one that has seen the increase in rpm.