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Unread 11-05-2013, 09:38 AM   #1
ridgeback_king
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1992 Jeep Wrangler YJ - Vibrations around 45mph

Hey guys, just looking for your opinions on the problem here.

I purchased a 1992 Jeep Wrangler about 3 months ago and I'm experiencing some vibrations from below (drive shaft area) when I accelerate between 42mph - 47mph. It does not vibrate unless I'm pressing on the gas.

I've replaced the U-Joints on the rear drive shaft and it calmed the vibration down a little bit, but it's still there.

I've taken the jeep to three separate mechanics and they said they weren't able to find anything wrong with the rear driveshaft or steering components. One guy recommended that I find someone to balance the drive shaft.

The tires are pretty worn, and need to be replaced soon... but I don't think they are the problem, because they'd vibrate regardless of whether or not I'm pressing the gas... right?

Everything is stock from what I can tell.
No lift kit.
30" tires.

I'm thinking about replacing the drive shaft all together. Would it be pointless to get a rear drive shaft with a SYE even though I don't have a lift? I want to add a 2.5" lift in about a year or so.

I just want to remove the vibration before I do anything.

Thoughts?

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Unread 11-05-2013, 10:37 PM   #2
jeepyj13
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Check you trans mount to see if it's worn out and motor mounts as well just to rule that out.
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Unread 11-06-2013, 04:43 AM   #3
SeanB95YJ
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^^^ what he said to start with

but given you're reporting vibes on accelleration indicates your pinion might be just a tad too high, that on acceleration, the pinion is "climbing" throwing the angles out of whack - and that wouldn't be evident to a mechanic just looking at the jeep sitting on the ground (or up on a lift)

also for your other question... no, it wouldn't be pointless to go SYE even with no lift... the slip yoke is a weak link - get rid of it
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Unread 11-06-2013, 08:39 AM   #4
ridgeback_king
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Thank you both for the suggestions. I know the motor mounts look great, but I'm not sure about the tran mount. I'll take a look.

As far as the pinion being too high, are there any tests or particular ways of checking to confirm this? I'll google and research, but I figured I'd ask.

Thank you guys for the help!!
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Unread 11-06-2013, 08:49 AM   #5
SeanB95YJ
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yes... a cheap ($5-10) magnetic base angle finder available at most hardware stores

for a stock driveshaft (single u-joint at each end with slip-yoke in back of T-case) you want the pinion angle to be parallel with the T-case output. if it is currently parallel, then you'd need to shim the pinion down a degree or two to account for the pinion climb during accelleration I mentioned above.
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Unread 11-06-2013, 10:48 AM   #6
ridgeback_king
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Gotcha!

Just a quick question. Does a SYE usually remove problems with pinion angles if they exist?

I obviously want to be sure that the angles are good, just a thought.

Thanks!
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Unread 11-07-2013, 05:08 PM   #7
ridgeback_king
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Here's the right and left of the mount... I'll be replacing this mount regardless this weekend, but just wondered if you guys think it's the culprit.
photo-2.jpg   photo-1.jpg  
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Unread 11-07-2013, 10:09 PM   #8
ridgeback_king
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Well I went ahead and installed a new transmission mount and no luck gettin rid of the vibration. The existing mounts looked very old, but probably still did their job.

Next step, get some readings on these driveline angles and we'll see from there.

Thanks for the help so far.
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Unread 11-10-2013, 04:02 PM   #9
ridgeback_king
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Had a pretty busy weekend and couldn't find time to take the rear drive shaft off for some angle measurements, but I took a quick photo and, in photoshop, placed some red lines where I estimate the angles to be. Take a look for yourself. It's hard to see this when you're under the Jeep, but with a photo it makes a lot more sense.

Does look to you guys like my pinion angle is too low?
photo-1.jpg  
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Unread 11-10-2013, 08:24 PM   #10
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Quote:
Originally Posted by ridgeback_king
Had a pretty busy weekend and couldn't find time to take the rear drive shaft off for some angle measurements, but I took a quick photo and, in photoshop, placed some red lines where I estimate the angles to be. Take a look for yourself. It's hard to see this when you're under the Jeep, but with a photo it makes a lot more sense.

Does look to you guys like my pinion angle is too low?
Looks to me your springs are starting to invert. Which changes the dline angles. And before you throw money at a new dline just to lift the jeep and need another one try getting your current one balanced. It's not to expensive.
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Unread 11-10-2013, 08:42 PM   #11
SeanB95YJ
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with a stock driveshaft, the output of the Tcase should be parallel to the pinion.... so given your pic (red lines) your pinion is too low and needs to come up. I made a quick add to your pic to show what the angle roughly should be... the yellow line is parallel to your Tcase output red line

dline.jpg
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