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Old 08-06-2019, 08:23 PM
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Strange noise at 30 mph

This past weekend my son commented that there was a strange noise coming from the Jeep starting at 30 mph. In case you've been following my other thread about a dead battery that resulted from a wire shorting out the interior lights and leaving them on constantly, you will understand that I've been occupied taking apart the dash of my Jeep to find the little bugger. Spoiler alert, it was the wire running up the roll bars to the sounds bars / lights that got pinched behind the shoulder belt bracket.

In any, case now that the short issue has been dealt with, I took the TJ out for a spin to recharge the battery and heard what my son mentioned. I cannot hear it at all until almost exactly 30 mph. It sounds like a whirring sound from under the hood (I think). Some other facts about it, if I let off the gas above 30 mph, the sound stops even thought I'm still moving at 30 mph +. It doesn't seem to be connected to the brakes or rotation of the wheels, and the sound will resume as soon as I touch the gas pedal again.

In my mind, I've kind of ruled out a wheel bearing or the transmission since it only happens when my foot is on the gas. Let me know if you think those are bad assumptions.



Thoughts?

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Old 08-07-2019, 07:23 AM   #2
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Just reread your note and editing. Since letting off the gas mitigates the sound, I'd look to u-joints.

But does rpm play any role? 30 mph in 3rd vs 30 mph in 2nd, etc

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Old 08-07-2019, 10:09 AM
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Quote:
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Just reread your note and editing. Since letting off the gas mitigates the sound, I'd look to u-joints.

But does rpm play any role? 30 mph in 3rd vs 30 mph in 2nd, etc

I sat in the Jeep and in park ran the RPM up past 3k and no sound, seem that movement and acceleration are needed for the sound.
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Old 08-07-2019, 11:25 PM
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Just reread your note and editing. Since letting off the gas mitigates the sound, I'd look to u-joints.

But does rpm play any role? 30 mph in 3rd vs 30 mph in 2nd, etc
So I plan on taking it on a test drive tomorrow to try and narrow down the possibilities. Assuming for a moment that its the u-joints, how can I test that other than switching out four universal joints?

Bob
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Old 08-07-2019, 11:48 PM
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I found this old posting on the JeepForum and it refers to multiple members that have the same issue, even a few with sound clips that are spot on with my issue.

https://www.jeepforum.com/forum/f310...noise-1343093/

Can I disconnect my front drive shaft and drive the Jeep to see if this is it? Will either the diff or the transfer case leak when I pull the drive shaft out and run it?

I changed the pinion seal a few months back as it was leaking and isn't anymore. I didn't see a problem with the u-joints, but who knows.
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Old 08-08-2019, 01:47 PM   #6
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Disconnecting the front shaft is a common method of isolating problems, yes. Just disconnect the front from the pinion yoke and tie it up. Don't shift into a 4wd mode, of course, so it won't turn from the TC.

Once tested, if you still hear the noise, shaft ruled out. Do some lower speed sharp turning. Generally axle u-joints make more noise at sharp angles. Definitive u-joint testing is physical. Try to wiggle them on all axes. Jack up a tire to get some play for the axle ones if necessary.

Beyond u-joints, which are so commonly in need of replacement, you'd have to start looking to bearings, TC, pinion, hub, etc. So many things after this point. The fact you changed a pinion seal might play here. Did you do it yourself? Preload on the pinion bearing is critical.
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Old 08-08-2019, 04:04 PM   #7
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plenty of lube in the diffs?
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Old 08-08-2019, 04:16 PM
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Diff's are both full, although I will check again tomorrow.


Front pinion seal was replaced a few weeks back, but the Jeep hasn't been driven much while waiting for some of my re-build projects to be completed.


I will inspect the u-joints tomorrow, looking for play. I may just detach the front axle (and tie it up as suggested) and take it for a test run up and down the street.


This TJ has 208,500 miles on it and I have no idea if or when the u-joints were replaced, but from my memory when replacing the leaking front pinion, they seemed solid.
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Old 08-08-2019, 04:40 PM   #9
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I would remove the front drive shaft then take for a spin.
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Old 08-09-2019, 02:49 AM
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Quote:
Originally Posted by Shark_13 View Post
I would remove the front drive shaft then take for a spin.
Just so I’m clear, leaving the yoke in the front diff, and the drive shaft secured but disconnected?

Bob
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Old 08-09-2019, 07:55 AM   #11
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Just so Iím clear, leaving the yoke in the front diff, and the drive shaft secured but disconnected?

Bob
Correct. You'll be unscrewing 4 8mm hex heads holding straps. The straps go over the u-joint cups. Tape the u-joint cups to keep them from falling off. Very securely tie the shaft up. This isn't to be taken lightly, as the shaft hitting the road at speed can destroy the vehicle.

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Old 08-09-2019, 10:21 AM   #12
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Or reach back over the skid and disconnect it from both ends and just remove it, gives you a chance to inspect all u joints and centering ball too
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Old 08-09-2019, 10:45 AM   #13
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It is possible that when the front seal was replace the pinion shaft nut was either over torqued or under torqued ( aka Pre-load is out) and you are hearing pinion shaft whine, which will still be heard with the front driveshaft disconnected.
Engage 4-Hi and drive to 30mph and try to determine if "whirring sound,' is still present.
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