Short Story - I have a drivetrain vibration at a very specific torque output(2000rpms and 10psi boost). What could it be?

Long Story:

I have a vibration(moderate) on my 86 diesel that occurs at a VERY narrow rpm band (2000-2200 rpms) that feels like, if I was forced to guess, it is coming from the carrier bearing for the rear driveshaft. It doesn't happen all the time, and isn't real bad. Well, after my connecting rod broke last spring, I just assumed it was the rod/engine and I was wrong as to the location of the vibration.

However, after the rebuild, it still happens. As part of the rebuild, I installed a boost gauge though and it tells the tale: While it doesn't happen all the time at 2000-2200 rpms, it ALWAYS happens at 2000-2200 AND 10 PSI boost. In other words, I can cruise the town at 35 mph in 4th gear (which is about 2000 rpms) without so much as a noise, but as soon as I mash on the pedal and raise boost to 10psi I get the vibration until 2200 rpm. It is most pronounced in 4th and 5th gear. It is barely noticeable at all in 1st through 3rd, but only because I think you pass any particular rpm band so quickly in those gears though.

In other words, this vibration seems to be very specific to a certain transmission torque output. Does anyone know/think they know what this is, or have a troubleshooting tip? I assume I should rule out driveshaft balance since torque seems to be the determining factor and not rpm.

During the rebuild I did try to address this in case it wasn't the engine. I have replaced trans and diff fluids and found nothing out of the ordinary, greased sliding shafts and universal joints, replaced fasteners at the universal joints, and replaced the transmission mount. I also inspected the carrier bearing and universal joints and they all seem fine. I can't for the life of me narrow this down to any one thing past the transmission since I am not sure what kinds of logic/troubleshooting techniques to use for this problem.

By the way, stuff in the front (loose change, etc) and the stick shaft does not vibrate any worse when this happens, just stuff in the back so I am pretty sure it is rear of the transmission.

NCDiesel

Noises are about the hardest thing to try and diagnois in the internet - but I will be daring and throw out my .02 cents worth. Ring Gear or Pinion Gear.

There is no noise to speak of, just a vibration. Just your average, garden variety vibration. Any way to confirm ring/pinion without removal? The diff fluid looked a bit old, but otherwise fine and there was nothing but a little very fine metal dust on the magnetic plug when I changed the fluid a last month.

Thanks for your guess, I do appreciate it. Any help is definitely welcome.

NCDiesel

hey spencer try putting about 200 lbs of something or other in the back end against the doors and then see if it does the vib, thing it sounds like it may be driveline angle and with the increase torque at that boost you could be twisting the pinion up to high , when you feel the vib start take the throttle back a bit and see if it stops , those things are notorious for the pinion angle vib, mine is driving me nuts , i am going to have a double cardan drive shaft made for it , the the angle wont matter much JERRY

I have two 100lb tractor wheel weights sitting beside the truck now. I will put them in and see when I go out tonight.



It absolutely stops vibrating. All I have to do is drop back off the pedal and get boost down to 8-9psi or lower and it will stop.

So a new, heavier driveshaft will take care of this ???

NCDiesel

Tried the weight tonight and it, for all intents and purposes, fixed the problem. Now if it could fix my back from lifting the 100lb wheel weights!

Does this prove pinion angle????

If vibration is caused from too sharp a driveshaft angle, why not use axle shims? I've seen them in 4 to 16 degree incements. they go between the axle and the spring, to tilt the axle to a less offensive angle, and prevent the vibration problem.
Of course I have no idea if they are still available, as this was 15 years ago. <img src="/forums/images/graemlins/zombie.gif" alt="" />

Check with Matt Brown at Independent4x. He sells the shims.

Matt's Web Site

Wow,
The angle that the drive shaft enters the diff housing caused the vibration...
I would never have even guessed at that one.
What is pinion angle, and please discuss how having the angle too steep caused this problem.
Also, NC, if you need custom shims let me know. I have a machine shop and would be glad to help out.
Eric
Burke, VA

Taking the following facts into account:

1 - Vibration caused by torque increasing pinion angle.
2 - Probably didn't do this when new (no one would buy them I suspect)
3 - Rear leaf springs are a bit loose and sloppy on this truck(and creaky and squeeky too!)

Could this actually be a suspension too weak to keep the axle from twisting upward? Would re-springing the leaves, new bushings, etc, fix this if the truck could stand having a little suspension work done on it anyways??? At that time I could add shims too.

Just some thoughts I would like a sanity check on. Especially #2. I just assume they didn't do this new....

PS: This web page has some great stuff about this on it.

PSS: Excuse my ignorance on double carden shafts Jerry. The above page straightened me out on that too.(-;

NCDiesel