Help me figure this one out

Oliver's

Oliver's

Drive Line & Gear
Joined
Jul 16, 2005
Location
707 E. 12 1/2 St. Winston Salem NC 27101
We have a super clean 92 YJ 4.0 231J, 5sp, 34,*** OG Miles in the shop. Came in with what I would have called a drive-line angle issue from just having a 4.5" lift installed. Only thing that struck me kind of funny was this vibe really only came alive at high rpm 3rd gear thru 4th gear, after that you can't hear anything due to the wind noise.

Guy wanted to go from 3.55's to 4.56 gears, SYE and new cv rear shaft. Run of the mill, do it once a week.

Once all the work was done, now this vib comes in in 3rd gear like a light switch getting flipped on. Gear ratio difference, I get that, not the problem. First I assume it is the rear shaft and check the balance, it's good. Next I assume maybe the sye is bad, AA brand. I take it all back apart install another SYE and Build another shaft, exact same problem.

At the point I'm like WTF? I start ruling parts out. Ran it both on the road and on jack stands in 2wd and 4wd with both shafts installed, then No front drive shaft, then just the front shaft, no rear shaft installed, same exact thing.

My gut tells me it's in the t-case, but it's clean inside and all the bearings look good. The trans (ax15) shifts fine, the clutch seems to be fine. Tires are good and have been checked for balance. Checked the run out on the rear axle shafts, all good.

What else can I check?

Thoughts?

Thanks,

Dave
 
3rd and 4th gear share the same shaft and bearings in trans? Overdrive probably goes strait through the mainshaft. Especially if it stops. Drain the trans oil, maybe the plug has a magnet. Sounds like everything else is rulled out.
 
Jody Treadway said:
So it does it on stands without any driveshafts installed?
Click to expand...

Negative, quiet on stands with no shafts installed, but I would expect that with no load on the t-case.
 
Wow, I'd never think to check that @Jody Treadway .
Dave if you want to try a whole different wheel and tread for a reason I got some here ready to bolt up. Used 33s on aluminum wheels.
 
Man people must love bringing these mystery vibe problems to you. Reminds me of that vibe in Richies 69 c10 you guys fixed, checked absolutely everything that made sense and ended up with rear wheel bearings.

If it moved rpms with the gear change than it is obviously attached to output shaft speed, gotta be either bearing in Trans or t-case. Have you tried running it through 3rd/4th in low range? That may atleast help determine if it is before or after the planetary in the case.
 
NC-V said:
Man people must love bringing these mystery vibe problems to you. Reminds me of that vibe in Richies 69 c10 you guys fixed, checked absolutely everything that made sense and ended up with rear wheel bearings.

If it moved rpms with the gear change than it is obviously attached to output shaft speed, gotta be either bearing in Trans or t-case. Have you tried running it through 3rd/4th in low range? That may atleast help determine if it is before or after the planetary in the case.
Click to expand...

I haven't tried that, but I will.

BTW, Riches truck still baffles me, never in a million years would I have guessed it was the wheel bearing(s), no roar, just a crazy vibration.

Thanks
 
I know, he kept calling me with what was tried and not tried and I never would have thought rear wheel bearings, crazy.

Good luck with this!
 
GotWood said:
Have you checked the rear wheel bearings?
Click to expand...

I did, they looked fine.

Wheel bearings tend to roar and normally can be unloaded in a turn, the vib we have seems to be coming from the center of the jeep.
 
If it makes you feel any better....that od green thing in my carport has a flipped 300 and a 5 speed out of a TJ. NV something or other.

Its doing nearly the same damn thing. 4 inch spring. No t drop since its all been sorta engineered. All on the list to redo.

It comes directly from the center on harder pulls 3rd and 4th. Feel it in the floor board and through the shifter. I really think it's in the trans. Well worn 3,4 gear area. Im only guessing on mine as I haven't done nearly the work to check it over.

Original owner had new shafts front rear and every thing else is bolt on except the trans mount.
 
Trans/t-case mount worn out and causing a harmonic at the rotation speed and torque load of 3rd&4th gear?
 
Worn motor mount(s) causing driveline vibes? To go with @jeepinmatt's post.

If the transmission internals were worn to the point of causing the vibration,I'd have to suspect there would be noise associated with it. Plus, that vibration within the transmission housing would have to damage the internals of it as well as the transfer case. AX15s are pretty darn tough, let alone with such low mileage.

Long shot. How does the harmonic balancer look?
 
jeepinmatt said:
Trans/t-case mount worn out and causing a harmonic at the rotation speed and torque load of 3rd&4th gear?
Click to expand...

Trans mount appears to be OK. We loaded it up and really doesn't move.
 
Jody Treadway said:
Worn motor mount(s) causing driveline vibes? To go with @jeepinmatt's post.

If the transmission internals were worn to the point of causing the vibration,I'd have to suspect there would be noise associated with it. Plus, that vibration within the transmission housing would have to damage the internals of it as well as the transfer case. AX15s are pretty darn tough, let alone with such low mileage.

Long shot. How does the harmonic balancer look?
Click to expand...

Motor mounts are in good shape as well. We tested them at the same time as the t-case/trans mount.

Balancer I would assume would do it with the shafts out as well, but with no shafts installed there are no vibs.
 
NC-V said:
If it moved rpms with the gear change than it is obviously attached to output shaft speed, gotta be either bearing in Trans or t-case. Have you tried running it through 3rd/4th in low range? That may atleast help determine if it is before or after the planetary in the case.
Click to expand...

In low range with just the rear shaft installed, the vib comes in in 5th gear.
 
Oliver's said:
Trans mount appears to be OK. We loaded it up and really doesn't move.
Click to expand...
Rubber or poly mount? If poly, does it look pretty new? When I went from a rubber to a poly mount, I got a ton of vibes that didn't used to happen. Was very happy when I went back to a rubber mount.
 
jeepinmatt said:
Rubber or poly mount? If poly, does it look pretty new? When I went from a rubber to a poly mount, I got a ton of vibes that didn't used to happen. Was very happy when I went back to a rubber mount.
Click to expand...

It has a stock rubber mount.
 
Oliver's said:
In low range with just the rear shaft installed, the vib comes in in 5th gear.
Click to expand...

If that is the case then the problem should be after the planetary in the t-case. Should have nothing to do with the trans. If it was trans related the vibe should have stayed in the same gear range. Since it took more trans output speed to get the vibe it must be after the reduction in the t-case.
 
At Dave's expense this a fairly interesting. Sorta fun being detectives....sorry Dave. I wish I could help more. I am teaching night classes this summer if you need a hand.
 
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