Slight "shudder"/vibration when accelerating

[terry1167]

Good Morning Y'all! Need some help with identifying a small problem. I have a 72 F250 Camper Special w/390 2bbl and C6 auto transmission, 2WD and 76K original miles. I've noticed that when accelerating between 20-30mph (approximate), there seems to be a very slight "shudder" or vibration. It's almost unnoticeable but I can feel it but a passenger probably wouldn't notice it. It does not feel like a suspension issue. There is no "noise" associated with it. Once the truck exceeds 30mph, the shudder stops and the truck operates smooth and flawlessly. I had the transmission flushed and a new regulator put on and everything checked out 100%. There is a slight "clunking" sound and "jerking" motion when I place the transmission from park to drive or reverse but otherwise the transmission operates perfectly while driving. I also had the rear diff. fluid changed, wheel bearings serviced, and new brakes.

Does this sound like a U joint matter or other driveline matter? How can I check this out with a process of elimination so I am not throwing money into something that is not "broken".

As always, your assistance is greatly appreciated and go Denver Broncos!

[forrestbump]

Terry, could be some wear in your drive shaft. Chock all four wheels, set parking brake, put trans in neutral and crawl under your truck to slightly turn your drive shaft CW and CCW by hand. You won't be able to turn the drive shaft completely around due to the parking brake being set...but that's OK as your looking for any slop.

Look at all three U joints, the carrier bearing and slip joint carefully. What you're looking for is any excessive movement or gaps that shouldn't be there. All of these connections should move immediately when you turn the drive shaft clockwise and then counter clockwise. Also check that all fasteners for attaching these components are securely tightened.

[70_F100]

To add to Forrestbump's comments, the only way to "truly" check the U-joints is to pull them apart. I've seen lots of them that were bad and had so much rust built up in them that you couldn't identify the wear by turning the driveshaft by hand. One other way is to have someone in the truck and have them shift from forward to reverse while closely watching them (but with driveline slack, you still may not be able to detect the wear this way).

The carrier bearing and slip yoke, as mentioned, are also sources of vibration due to wear.

Two other things come to mind, bad output shaft bearing on the transmission (would probably cause the rear seal to leak, though) or driveline angles. Bad/sagging motor or transmission supports or sagging rear springs could cause the driveline angles to be off.

There are probably a dozen other potential causes that I haven't mentioned, but with the things I identified, along with Forrestbump's excellent advice, you will hopefully be able to identify the problem pretty quickly and easily.

[terry1167]

Forrest Bump and 70 F100, thank you for the quick and excellent responses. I'll give that a shot today. Would these issues by typical of causing the vibration just at that low speed vs. all speeds? Like I said, it seems to flatten out nicely with no vibrations once the truck exceeds 30mph. Also, I should have mentioned that on the truck is a large cabover camper that weighs around 2800#. I can't recall if the shudder occurs, or not, when the camper is off of the truck as it's been a little while since I've had the camper off of the truck. If the shudder does NOT occur when the camper is off of the truck and only when ON the truck, I wonder if the weight is causing the drive shaft angles to be slightly off as you have suggested??? The camper is a BIG one and it does cause the truck to sag in the rear slightly even though it is a camper special but after 40 years, the leaf springs are probably a tad tired. Thoughts?

Thanks!

[forrestbump]

Terry, harmonics can be a funny thing where worn parts show no vibration at certain speeds, yet rear their ugly head at other speeds. I don't think loading your truck with a camper would change drive angles as long as all other things like motor mounts and such are in good shape.

The heavy loads WILL strain a faulty part to the point of failure, at the worst possible moment of course.

[70_F100]

Larry, I have to respectfully disagree with your post.

Adding the extra weight to older springs may cause them to sag more significantly than when they were new.

This sag of the springs MAY be enough to change the driveline angles enough to cause a vibration. Remember, the carrier bearing holds the front driveshaft at a consistent angle, while the weight on the springs will lower the rear of the truck, in turn changing the operating angles on the two rear u-joints.

Is this enough to cause a shudder? As Bugs Bunny would say, "MMMMMMMMMMMM... Could be!!"

[forrestbump]

Great Point! Thank YOU for fleshing this out!

[BobbyFord]

Check engine/trans mounts, also.

[70_F100]

One other thing comes to mind.

A slight shudder like you're describing could be a cylinder mis-firing under a load.

[terry1167]

Thanks again Larry, 70 F100, and Bobby Ford for your advice and input. I chocked the wheels and checked it out. The U joints are very rusted and probably haven't been greased in years. Am I supposed to be able to turn the driveshaft a little, while in neutral or is that considered "play"? Im thinking that between the vibration and the "clunk" when I put it into gear, the U joints need to be replaced. How do I tell if the Carrier bearing also needs to be replaced? If I am going to replace the U joints, should I just go ahead and replace the carrier bearing while Im at it or is that a "big" job? I've never replaced U joints before. I've read a few threads on it and watched a generic DIY video on it.

Thanks!

[fordman]

the carrier bearing is pressed on. the place you buy it from usually will replace it for free if you buy the parts from them. you can tell the bearign is worn out if the rubber is broken or cracked pretty good. when the carrier bearign goes completely bad it will halt the truck while driving at low speeds. you can back up and unfreeze it. but not for long.