(Fixed) Need help diagnosing oscillating / low pitch vibration

[Troutbum]

Happy New Year everyone!

I have a problem, that just recently started but is quickly getting worse. I have a 97 TJ 4.0 manual with plenty of lift on 35's. About 2 weeks ago, I started to feel a vibration. Not a high pitch vibe. More of a low pitch shake/oscillation akin to an out of balance driveshaft. I actually was hoping that that was the problem, but I wasn't that lucky.

The abnormal condition does not seem to be tied to engine RPM. More to vehicle speed regardless of RPM. It is worse in 2wd and almost completely disappears in 4wd. I have tried it in 4 high and 4 low. It is still there but greatly reduced. If I press the clutch in while it is shaking, the shaking is also greatly reduced. It shakes worse if I step on the gas hard and make the drivetrain work.

It was pretty faint when it started. My daughter borrowed it last week while her car was in the shop. I estimate that she put no more than 100 miles on it. The shake has gotten considerably worse in that time, so something has changed/is changing quickly.

Today I tried to do some in-depth diagnosing. Here is what I have done and the result:

Removed the front shaft and went for a ride. Still shakes. Put it back in.
Removed the rear shaft and drove in 4wd. Shake is almost completely gone.
Noticed that a u-joint was dry on the rear shaft, so I replaced it before putting the shaft back. Still shakes.

At this time, I am thinking something is wrong in the TC, but I am not sure what it could be that would cause this issue. It shifts in and out of 4wd without any issues or noise. There is no discernible noise from anything during the shaking and the Jeep tracks straight. This is not a DW kind of shake. I seriously was sold that it was an out of balance shaft until now.

Any thoughts on where to look next would be greatly appreciated.

 

[someguysjeep]

how tall is it lifted?
and do you have adjustable control arms, on the rear?

 

[Troutbum]

Approx 5 inches of suspension lift. 2 in Body lift. Motor mount lift. Adjustable rear uppers. The pinion to shaft angle is pretty spot on.

Just some more info; if I shift into 4wd while it is shaking, the shake is 90% unnoticeable. I do have a Detroit Locker in the rear end too, should that have a contribution.

I'm seriously scratching my head on this one. No shake when revving the engine in neutral. Shake doesn't seem to be tied to the manual trans and the shake starts in the last little bit of 2nd gear, carries through a big chunk of 3rd gear and tapers off from there. It is worse if more torque is applied to the drivetrain.

I hate having to replace things in hopes of making a problem go away.

 

[someguysjeep]

sye and double cardan drive shaft?
you didn't have it b4, and it goes away with the rear shaft out. it sounds like it's in the shaft, sure those joints are in good shape?

 

[Troutbum]

Yes on the SYE and DC shaft. I replaced the axle side joint as it was dry. The other two joints in the DC feel good. No slop or roughness. I definitely can replace them though, just to eliminate the possibility.

Yes, the TJ has 140k miles on it. The shake just started last week out of the blue. Nothing that I can think of as a trigger. No recent work done to it.

 

[someguysjeep]

it could be transmitting from the TC or rear diff.
TC would be the hardest to pinpoint. next time the shaft is out, make sure there's no slop in the diff pinion or TC yoke. but since your searching, the U joints are the cheapest place to start, and may be it.

or have you changed the weight in the rear end? did you add weight for winter? did you remove stuff you've been carrying? if so, it could need a tweek on that pinion angle.

i'm sure some of the others will have some advice as well.

 

[Jerry Bransford]

If you're feeling the vibration up through your seat it could be from an improper rear pinion angle.

 

[Troutbum]

What about axel wrap? How much misalignment can I have before I would feel the oscillation?

I am familiar with what is inside an NP231 and I can't think of anything in there that would cause what I am feeling. It definitely feels like some sort fo driveshaft problem. I used to have a 73 GMC that had a pinion nut that would work itself loose. That is the type of shake that I am feeling now in this TJ.

I did check the output shaft out of the rear of the TC and it is solid, not loose. I will replace the other two u-joints and see how that affects the problem. I am also going to stick my gopro under the jeep and see if I can capture the problem. I will post what I find out if anything.

I have also fired off an email to Novak as they are just about a mile away from where I live. I hope that hey can lend some support my way. Any info will be appreciated.

Keep those ideas coming! I hope that between us all, we can figure out what is going on.

Thanks everyone!

 

[Goatman]

Have you checked out your fluid levels in the drivetrain? Could it be as simple as losing a wheel weight or two? Check all your control arms are tight and give them a pump or two if possible. Inner belt coming loose in a tire?
I always try all the cheap and easy stuff before thinking something is about to implode my credit card.

 

[someguysjeep]

i've read that once (forward) torque is applied the axle/pinion can raise just a bit. i keep my pinion about 1* below my drive shaft angle, this has worked for me. but i have no visual proof that it's true, just what i've read. and it gives me no issues so i went with it.

i would like to others opinion of this.

 

[Troutbum]

Another update. I took it for a little ride this morning before pulling into the garage to let the snow and ice melt off. If I accelerate very slowly, there is no shake. When rolling in neutral, there is no shake. The harder I accelerate, the harder the shake.

I'll pull the rear driveshaft and replace the other two u-joints. I'll also check all my fluids just to be on the safe side. I'll get a small jack to press on the pinion and see how much play I have in my control arm bushings.

It was 15° F here yesterday. It's supposed to be warming up to 35° today. My garage isn't heated, but it is sheltered. It will be nice not freezing my butt off today. I need to build me a man cave with a jeep lift in it.

 

[Troutbum]

Last update everyone. It was the center u-joint in the rear shaft. It felt solid upon the original inspection on the bench, but when I removed it from the cardan joint, one of the cups had a crack in it. SomeGuysJeep wins the prize today.

Got her all put back together and there is no more shake. Lesson learned.

Thanks for all your ideas and help!