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Unread 10-10-2005, 11:45 PM   #1
dporta
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Low Speed Vibration

I'm stumped. I have an 85 CJ7, with AMC 20 rear end, 4.0 MPI conversion. Jeep runs great, but lately I have noticed some rattling on the drive line. Not sure if it is the drive shaft or transfer case. I was driving home the other day and started feeling a vibration in the rear end at low speeds when first accelerating. The vibration lessened as I increased speed. I also sensed a slight burning smell, not sure. My gut tells me some gears are not sitting together properly. I did the 4.0 MPi conversion myself and it went well, but I know very little about drive trains.

I have also noticed a little fluid on the floor of my garage directly under the rear differential. I've seen some similar postings and my initial impression is that I have a loose drive shaft, again not sure. When I clutch the vibration goes away, but when going slow and shifting uphills, I hear a slight "clanging" noise coming from the drive train. I'm totally stumped. It almost seems like when I am on an incline that the drive shaft is loose and sliding around.

This is not an alignment or tire issue.

Anybody have any advice?

Thanks All!

Doug

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Unread 10-10-2005, 11:55 PM   #2
80cj
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Sounds like a bad U joint in your rear drive shaft. Get under there and shake the shaft around at the U joints. There shouldn't be any play.
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Unread 10-11-2005, 06:00 AM   #3
SgtSnake
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Could also be low fluid in your differential due to a bad oil seal. I notice the same thing as far as fluid under my rear diff, and it only seems to show when I'm parked with the nose downhill. Look for an oil trail down the case of the diff to where it dripps from. That should tell you if that's what it is. I'll have the seal replaced in mine in time, but til then, I get my diff level checked everytime I change the oil, and try to park my nose uphill. Jeeps like climbing better anyway. They were bred for it.
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Unread 10-13-2005, 09:00 PM   #4
dporta
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Thanks guys. I finally made time to get under there. The rear drive shaft is pretty tight, I can wiggle it a little but it seems pretty tight. I did notice a ton of fluid all over the under body of the jeep above the rear drive shaft. Also looked like there was a trail of fluid running down the drive shaft. I do think that maybe I have some bad seals and the quick fix for this is to just add fluid. I'll work on it this weekend and keep you posted.
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Unread 10-14-2005, 07:06 AM   #5
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When you replace the seals, make sure the driveshafts are inline also. A u-joint at a weird angle can make the seal problem come back. The U-joint will spin okay, but at higher speeds the vibration will start. I found that out when I first put the new driveshafts on after my engine upgrade.
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Unread 11-04-2005, 08:23 PM   #6
dporta
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Finally took the drive shaft out. The rear u-joint was done. It was craked on both sides...the internal studs of the u-joint were craked in half on both sides. Replaced the u-joints last night, re-attached the shaft and she is running like normal.

Thanks guys for the advice.

DP
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Unread 11-05-2005, 06:02 AM   #7
cjbilt4me
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It's nice when it is something that simple!
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Unread 11-05-2005, 11:16 PM   #8
dporta
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For me every time it breaks it's a new experience....but it was a pretty easy fix. The more it breaks the more I learn. Thanks Again!
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Unread 12-01-2005, 10:35 PM   #9
calcallender
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Quote:
Originally Posted by SgtSnake
When you replace the seals, make sure the driveshafts are inline also. A u-joint at a weird angle can make the seal problem come back. The U-joint will spin okay, but at higher speeds the vibration will start. I found that out when I first put the new driveshafts on after my engine upgrade.

What do you mean to make sure the driveshafts are inline? I just replaced my rear ujoints and had the driveshaft balanced and now there is a new vibration at 35 mph.
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Unread 12-05-2005, 08:20 AM   #10
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Unless the driveshaft is straightgoing from the rear end of the transfer caseto the rear diff, and also forward to the front diff when you have the 4-wheel in, you will get vibration while the u-joint turns. It's like if you've used a u-joint with a socket wrench. You can turn the nut in weird angles, but you feel the difference in the way it turns. Straightening out the u-joint makes it so it doesn't have to move side to side everytime the driveshaft turns. After my engine replacement. The driveline would vibrate badly, and my problem was the parts store gave me the wrong motor mounts. The engine and tranny were too high up, so the length of driveshaft from the transfer case back to the rear diff was at an angle. When I put the right mounts on, the engine was a bit lower, and the vibration was gone because the line was straighter. Some guys loosen the u-bolts on th differential tubes, and rotate the differential to get the u-joints in line when they do a driveline lift to get more ground clearance, but I'm thinking yours may be there was a pre-existing angle somewhere, but since your driveshaft wasn't balanced, it didn't show up until it did get balanced. I'd find out what they did to balance it first, and go from there.
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Unread 12-05-2005, 12:05 PM   #11
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I think what Sarge is saying is: your T-case and axle should be parrallel to each other if you are running a stock (two universals) driveshaft. If you are running CV style (three universals) then the rear differential should be pointing at the T-case. Both scenarios are on flat ground at normal load rate. SGTSNAKE...any chance you are in Jacksonville?
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Unread 12-09-2005, 06:17 AM   #12
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You got it right there John. The transfer case universal needs to point directly (linearly) at the universal that the other end of the driveshaft connects to the diff universal at. Any offset left/right or up/down from straight at it will cause vibration.

Nope. I'm not at Jacksonville, John. I'm near Greensboro (Note the change to my location under the chevron). I was stationed at Cherry Point for awhile while I was in the Corps though, so I know the area. Met my wife in Newport, NC not far from there.
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Unread 01-01-2006, 08:38 PM   #13
WR4NGLER
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Quote:
Originally Posted by dporta
Finally took the drive shaft out. The rear u-joint was done. It was craked on both sides...the internal studs of the u-joint were craked in half on both sides. Replaced the u-joints last night, re-attached the shaft and she is running like normal.

Thanks guys for the advice.

DP
Was your vibration decently loud, at like 0-5 mph, starting at a low interval and getting faster as the vehicle went faster, only lasting a few seconds? That's what I got. Just out on RE 3" coils but think PORC sent me 4.5" rear coils.
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Unread 01-02-2006, 07:08 AM   #14
SgtSnake
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Quote:
Originally Posted by WR4NGLER
Just out on RE 3" coils but think PORC sent me 4.5" rear coils.
Was this part of a lift? A lot of guys get that after a lift, because the angle at the universals is different. Also, the driveshaft is further out of the tube, and has less contact with the splines (the grooves in the tube that make the two pieces of the driveshaft turn like one piece). If so, you can get rid of some of the vibration by rotating the rear differential slightly upward on the driveshaft side. You'd have the loosen your spring connections though I believe. I know that with plate springs, the procedure is to loosen the u bolts that hold the diff. in place, rotate the front of the diff. until the angle is more directly pointed toward the output side of the xfer case, then tighten it back down. What it comes down to is anything other than a straight shot from the diff. to the xfer case is like shooting around a corner. It doesn't work to good.
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Unread 01-02-2006, 07:37 AM   #15
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This is correct, but the one detail missing is that steel degree shims are used to point the differentials.... just tossin' that in.

Also, if you use degree shims on spring lifts, the centering pin that holds the leaves together will need to be replaced with a longer one. All you do is use a C-clamp or big pipe wrench to hold the springs together when you take that pin out or the leaves will fall apart... even if they do, it's not a hassle to put them back together.



Quote:
Originally Posted by SgtSnake
Was this part of a lift? A lot of guys get that after a lift, because the angle at the universals is different. Also, the driveshaft is further out of the tube, and has less contact with the splines (the grooves in the tube that make the two pieces of the driveshaft turn like one piece). If so, you can get rid of some of the vibration by rotating the rear differential slightly upward on the driveshaft side. You'd have the loosen your spring connections though I believe. I know that with plate springs, the procedure is to loosen the u bolts that hold the diff. in place, rotate the front of the diff. until the angle is more directly pointed toward the output side of the xfer case, then tighten it back down. What it comes down to is anything other than a straight shot from the diff. to the xfer case is like shooting around a corner. It doesn't work to good.
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