Vibration issue!!!!

Firstly I need to add that my Xj has the 4.0 litre I6 with the Aw4 auto which is stock for here in Australia, it is regeared to 3.73:1 running an Old Man Emu 2" lift kit with rear airbags.

This issue reared its ugly head when preparing for our recent trip to Tassie (3000kms each way) whereby it would vibrate at 100kmh~ I had recently added an extra leaf to my OME pack as well as fitted a secondhand set of wheels and tyres, stock sized 225/75/15's on alloys which I had balanced.
I fitted a 2.5* castor wedge (I am running a SYE) to the rear but all the way to Tassie and back it vibrated at anything over 99kmh and it is a long way.
I noticed on the first day that 3 of the 4 wheels lost their weights so once I got back I had them rebalanced and was told by the tyre mob that stock Jeep alloys require special weights which they now had so redid it for free.

No improvement...

So I removed the extra leaf, no improvement...
I had an issue with the transfer case which meant the removal of the SYE, no change.
Lowered the rear by removing the inverted leaf from my OME pack, down to 1.5", no change.
Tried a second stock tail shaft, no change.
Tried a set of stock steelies with good cond 235/75/15's all freshly balanced, no change.
Refitted my modified output shaft with the SYE to my replacement transfer case, no change.
Fitted some 6* caster wedges with the SYE sitting at 2.5", diff @ 78*, tailshaft @ 80*, no change.
Tried another front drive shaft in the rear with the SYE, no change.
What I noticed today is if I lift off the pedal all the vibes disappear, as soon as I ease back on the pedal they come back, I also tried dropping it back to 3rd to see if it might be the harmonic balancer which changed the revs from ~2k to ~3.5k, no change.

Has anybody got any suggestions as this is driving me insane?

Cheers, JJ

Although you are dong a number of unconventional things, starting with the 2" lift with airbags,stock tires(!), and a SYE...

I would start with the rear driveline. By a "Castor Wedge" I assume you are talking about a shim under the spring to adjust pinion angle. Check your rear driveline angle. The SYE end, I "assume" is a CV, like the front. If so, the angle at the front is not as important as the operating angle is split between the u-joints. The relationship between the driveshaft and pinion should be about 1deg pinion nose down to make up for acceleration at what ever level you usually set your airbags at.

Checked the U-joints in the drive shafts and the pinion bearings?

WB9YZU said:

Although you are dong a number of unconventional things, starting with the 2" lift with airbags,stock tires(!), and a SYE...

I would start with the rear driveline. By a "Castor Wedge" I assume you are talking about a shim under the spring to adjust pinion angle. Check your rear driveline angle. The SYE end, I "assume" is a CV, like the front. If so, the angle at the front is not as important as the operating angle is split between the u-joints. The relationship between the driveshaft and pinion should be about 1deg pinion nose down to make up for acceleration at what ever level you usually set your airbags at.

Click to expand...

Firstly the 2" lift is very normal, in fact that is the maximum legal lift here, the airbags are to keep the ride height when towing etc, all pretty normal stuff, the SYE was fitted for reliability as I was getting noise from the slip yoke on a previous transfer case so I thought it would be a reliability improvement.
The 235/75/15's that I am running are the largest legal sized tyre that I can run here.
Even with the SYE with a little less than 2" lift using 2 different stock tailshafts it gave the same issue.
I have now refitted the SYE output shaft to a second transfer case and tried 2 different front DC tailshafts in the rear and the result is exactly the same.
That was with no caster shims, so I fitted a 6* shim to each side which now means the nose is actually sitting 2* up so it is too high, I have a pair of 2.5* shims which may be a better fit but I am yet to try them, these will result in the nose being 1* down, theoretically correct I believe?
Now upon some discussions with others I have managed to work out some more symptoms.
When cruising the noise/vibration cycles from annoying to audible, when under power it is constant, when accelerating it start to become just detectable at around 80-90kmh but it comes on quite suddenly at 105kmh and stays that way getting gradually worse up to 115kmh.

BUT!!!

As soon as I lift off the pedal all noise/vibration stops instantly regardless of speed, as soon as I ease back on the pedal the symptons return as above.

The diff was fully rebuilt earlier this year and regeared to 3.73:1 from the stock 3.55:1, I also fitted a detroit TruTrac and an aluminium cover to help dissipate some heat and run LS90 diff oil.
I am starting to think that it may be the pinion bearing as the DC tailshaft that I was running originally ever since I fitted the SYE has an extremely tight slip joint so maybe it was putting too much load into the yoke when being compressed, dunno if this would cause a problem, the replacement DC tailshaft is nowhere near as tight and slides smoothly.

If anybody has a method of checking/diagnosing pinion bearing issues I am open to suggestions, especially if it means not having to remove them....

Cheers, JJ

It's hardly important, but somewhat interesting as to what is legal down under, as the problem is unrelated to legal issues

SO, If you have now corrected your drive line angle to be 1 deg or less in direct alignment with the drive shaft, preferably a bit nose down, it's time to chase something else.

Your symptom of letting off the gas and having the vibration go away could have been two things. One was your 6deg shim causing the pinion to rotate up and increase the rear u-joint angle further...

The other are your pinion bearings. Block the tires, drop the rear DS, grab the pinion yoke. There should be NO in/out play. Side to side play will be about 1/8 turn (but more if there is a lock-right in there).

Good Luck!

A vibe at 50 mph and above normally is a sign of a driveshaft that is out of balance,bent, or has a bad u-joint.

It is very very common for ujoints to go bad right after doing any kind of lift. The ujoints get used to running on the exact same spot of the cross shaft. When you change that angle...even slightly you can cause the needles to wear on a different part of the cross shaft and they will fail very quick. You may have said you replaced them. I'm on my phone and didn't read it all. I would start there though.

WB9YZU said:

It's hardly important, but somewhat interesting as to what is legal down under, as the problem is unrelated to legal issues

SO, If you have now corrected your drive line angle to be 1 deg or less in direct alignment with the drive shaft, preferably a bit nose down, it's time to chase something else.

Your symptom of letting off the gas and having the vibration go away could have been two things. One was your 6deg shim causing the pinion to rotate up and increase the rear u-joint angle further...

The other are your pinion bearings. Block the tires, drop the rear DS, grab the pinion yoke. There should be NO in/out play. Side to side play will be about 1/8 turn (but more if there is a lock-right in there).

Good Luck!

Click to expand...

I checked the pinion movement today and there is zero in/out movement, zero side to side or up/down movement and rotationally is bugger all as everything was new about 9mths ago...

Well I swapped the axles even though they looked ok I thought I need to start eliminating some things as I gotta tow my trailer soon and if something is on the way out I need to find it.
So I took it for a run....

Still the same!!!!!!! Arghhhhhhhhhhhhhh!!!!!!!!

I am racking up heaps of k's testing this and it is driving me insane so on the way home I took it for a run near to my house as it is making some noise lower down that should be detectable at a lower speed.
Well even at 80~ I can hear it so I don't need to hit the motorway so that is a start...

So 2 transfer cases, 4 tailshafts (2 stock rears and 2 front DC shafts) and still the same, different pinion angles, different ride heights, different wheels & tyres all freshly balanced, different weight oil in the rear diff, different axles and all exactly the same (there was a slight shimmy when using the stock tailshafts that the DC shafts eliminated) so on the way home I thought I need to think differently and try some elimination (electrical fault finding is part of my job) so I thought why not remove the front tailshaft and at least eliminate the front end......

Front tailshaft out, noise goooooooooone!!!!!!!

I am going to try a different front DC tailshaft in the front as the one I removed has some play in the slip joint but I am guessing it is in the front diff, at least I can concentrate on the correct end of the car now!!!!!

I have had the original (tight slip joint) DC tailshaft soaking in penetron and it is still firm but better so it is back in as the is zero play in the slip joint.
When I was putting it back in I decided to remove the spare DC tailshaft that I had fitted to the rear and fit it to the front to see if it is the front DC shaft or the diff as the front DC shaft has a moderate amount of play in it, well while I was bolting it to the pinion yoke I found the problem.
The front pinion is loose, it has end float as well as sideways movement so I tightened the pinion nut some more but to no avail, so it is either a bearing collapsed or something worse.
I built the diff 2 yrs ago so if it is from bad set up it has taken 2 yrs to chew out the bearing.
Tomorrow I might have a looksy inside and see if there is any signs of damage to the gears etc (hope not) or if it is just the bearing.
I have offset balljoints in the front on both sides with the drives side just being used for caster and the passenger side used for caster and camber, I don't know if that has had any affect on things or not.....

Problem sorted, I stripped the front diff today expecting to find collapsed bearings and worn gears as it has been making this noise for over 8000k!!
Drained the oil, no excessive metal or contaminants, checked the crown wheel, looked just like a new one (well almost), pinion gear also looked excellent.
Found 2 problems, firstly I was able to lift out the centre by hand, without spreading the housing!!!
Secondly as expected found there was excessive play in the pinion, but to my amazement the bearings looked like brand new.
So I removed 1 x 0.010 shim which affects the bearing preload without affecting the pinion depth, then I torqued up the yoke, all sweet.
I then measure the backlash which was still the same as it was when I built it but I had too little carrier bearing preload hence lifting out the carrier without need to spread the housing so I added a 0.005 shim to one side which has too benefits, firstly it adds a little more preload as well as reduced the backlash by approx 0.002.
Re-assembled with the tail shaft and took it for a run, all sweet now up to 120kmh!!!!
This was the first diff that I have built so I am not surprised it wasn't 100%, I am much more confident now after helping to set up my rear d35 with guidance and instruction, lucky for me all the parts were ok....

Thanks for all the suggestions..