Help with driveline vibration

[TJBob]

Need some input on a driveline vibration I’ve developed. I have a couple ideas of where it may stem from. Before I start throwing parts at it, any ideas?

A little info on the Jeep... it’s a 99 TJ. 150,000 miles. 4.0L 5speed with a 4” skyjacker lift w/TC drop. Fixed lower control arms (I assume they came with the skyjacker lift. PO installed this lift) Stock upper control arms. 231 t-case. On 35” mastercraft courses MTX’s. About a month ago, I had some frame rust repair done on the cheap. And about a week ago I replaced both motor mounts and the transmission mount with rubber OEM replacements from crown automotive.

The vibration doesn’t seem to be any better or worse at any particular speed. It’s most prominent when taking off. Almost like a worn out transmission mount.

I’ve tried dropping the front driveshaft, not much change. I’ve tried dropping the rear driveshaft, again not much change.

So my theory is, I’ve gotten u-joints in both shafts that are on their way out (what are the odds of this ). My other theory is, when the rust repair was done, it dropped the skid a bit lower, causing the vibration...?

Should I start by replacing u-joints, or should I go grab a couple handfuls of washers and re-shim the skid?

Photos of rust repair:

Photo of driveline angle:

I know the proper repair for this is a SYE w/DC driveshaft. That is coming in the future, but isn’t in the works for a bit. Any suggestions?

edit: the TC drop is square tube style.

 

[Chris]

You need a SYE and DC driveshaft, but you already know that.

The only other way to fix this is to lower your transfer case even further with washers and longer bolts, and add a MML.

Really though, at 4" of lift, you really, really ought to have a SYE and DC driveshaft.

 

[TJBob]

You need a SYE and DC driveshaft, but you already know that.

The only other way to fix this is to lower your transfer case even further with washers and longer bolts, and add a MML.

Really though, at 4" of lift, you really, really ought to have a SYE and DC driveshaft.

thanks for the reply Chris! SYE and driveshaft are definitely in the build plans. I just don’t have the funds to cover at the moment.

Is there any reason why it didn’t have this vibration before the work I did to it? The same TC drop was reinstalled after the rust repair.

Dropping the skid any further isn’t an option. Because the guy that I had do the frame work, installed the bolts into the frame, instead of the nuts... So changing bolt lengths are out of the question at this point. This poor Jeep is a mess brother. Feel like ke a dog chasing my tail.

Only reason I’m even trying to bandaid this, is because it’s my DD. Don’t want to be dropping a driveshaft in the rain one day. Cuz we all know, it’s never nice out when shit decides to grenade haha!

 

[Chris]

Well, dropping the skid plate lower wouldn't cause the vibrations to get worse, that would likely just make them get better of anything.

However, there's always the chance the vibration isn't coming from your driveshaft at all. In fact, to test this out, remove the rear driveshaft, then put it in 4WD (which will then just be powering the front wheels). Is the vibration still there? If so, then you know it's not driveshaft related most likely. I'd actually rule that out first, since this is a very easy test to do.

 

[broken scope]

Looks to me like your rear pinion is set up for a double cardan and not a single but im just a newbie.

 

[TJBob]

Well, dropping the skid plate lower wouldn't cause the vibrations to get worse, that would likely just make them get better of anything.

However, there's always the chance the vibration isn't coming from your driveshaft at all. In fact, to test this out, remove the rear driveshaft, then put it in 4WD (which will then just be powering the front wheels). Is the vibration still there? If so, then you know it's not driveshaft related most likely. I'd actually rule that out first, since this is a very easy test to do.

I have done that test already. The vibration is still there. If there is any change at all, it isn’t noticeable. I only get a vibration as I’m trying to take off. It’s about the same as a transmission mount being worn out. I know that isn’t the issue, as I have installed a new one. Along with new motor mounts. All rubber bushings.

 

[TJBob]

What are the odds of it being clutch chatter?? Clutch feels solid, and no slipping in any gear... I’m really at a loss here.

 

[TJBob]

Looks to me like your rear pinion is set up for a double cardan and not a single but im just a newbie.

I thought that as well, but the arms are fixed arms. No adjustment to rotate the axle.

 

[Jerry Bransford]

Looks to me like your rear pinion is set up for a double cardan and not a single....

You're exactly right. That rear pinion angle was raised by the previous owner who didn't know what he was doing when he raised it which is at least one cause of your vibrations. LIkely the only cause but...

The rear pinion angle needs to be dropped some to get the rear axle's pinion angle back parallel with the tcase output shaft. Like shown here...

 

[TJBob]

How would one do that with all fixed arms?

 

[Jerry Bransford]

How would one do that with all fixed arms?

You don't. Those arms were apparently set up to create the correct pinion angle with a SYE and CV driveshaft as should be run with a 4" suspension lift. You could install a set of lower adjustable length arms and lengthen them to lower the pinion angle down.

 

[TJBob]

You don't. Those arms were apparently set up to create the correct pinion angle with a SYE and CV driveshaft as should be run with a 4" suspension lift.

So the only way to get the vibrations on takeoff to go away is with a SYE.

Even though I had no vibes before the rust repair and new mounts? Not contesting here,as I know the SYE and drive shaft are the CORRECT fix. Just don’t make since why I didn’t have this issue before all the work. Bad bushing covering it possibly?

 

[Chris]

Jerry nailed it! I totally missed that. Your pinion angle is completely wrong!

I can't explain why you didn't have vibrations before and now you do, but one thing is for sure, that rear pinion angle is completely incorrect for setup.

 

[TJBob]

I thought it looked off as well, just wouldn’t have guessed that to be the issue, since it wasn’t doing it before. Bummer... but it looks like a SYE just moved up the mods list! Any suggestions on a SYE kit? I know Tom woods shafts get a lot of love from this forum. So that’s what I’ll be going with as far as the shaft. Also, with the SYE, I can remove the TC drop as well correct?

 

[Jerry Bransford]

Jerry nailed it! I totally missed that. Your pinion angle is completely wrong!

It wasn’t me, Broken Scope is who first noticed the pinion angle was too high.

 

[pagrey]

The pinon angle hasn't changed. It didn't used to have vibes. Are you guys smoking something? Forget about the pinion.

I'd check and see if the frame repairs put something in contact with the engine or transmission. That'll cause vibrations. I'd look close at the exhaust and the transfer case.

 

[broken scope]

When you changed motor mounts were they identical as in height? Thanks Jerry but you and others actually taught me most of what l do know about a TJ.

 

[TJBob]

When you changed motor mounts were they identical as in height? Thanks Jerry but you and others actually taught me most of what l do know about a TJ.

Yes they were the same in height/width. Originals are what came off, crown OEM replacement is what I put back on.

Long story shorter, I purchased this Jeep about 4 years ago. Was already lifted. I just drove it. Has been relatively maintince free. Rust finally got to the frame. Came out to go to work one morning, and seen this:

So I took it to a local shop (on a trailer) to have the rot cut out and patched. I plan on getting a new frame in the future that’s why I didn’t do safety caps. When I got it back I noticed it had a vibration for just a couple seconds, right when you first let the clutch out and take off. About what you would expect to hear when a u-joint is failing, or a bad trans mount. From reading several other threads on here, I came to the conclusion that I should start with the trans mount, since the skid did fall out and it had a lot of stress on it. Sure enough, it was toast. So I ordered the crown OEM replacement kit. All rubber bushings. Came with both motor mounts and the trans mount. Installed, the vibration is slightly better, but still very present. Triple checked all mounts, everything is tight.

Maybe it’s a possibility, that with the bushings in the motor mounts and trans mount being original, and being 20 years old, they were masking the vibration caused by the incorrect driveline angles

I checked all the u-joints when I had the driveshafts out of it. No play to speak of. Everything rotated smooth.

 

[broken scope]

The picture Jerry provided is correct. So regardless l would want to set it up correctly anyhow.

 

[TJBob]

That’s been the plan from the get go... just wondering how to minimize it for the present time.