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post #1 of (permalink) Old 08-11-2015, 03:17 PM Thread Starter
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Cj-8 Vibration problem -Help!

OK, so I'm trying to track down this annoying weird vibration problem. here's the deal:

-The original T5 transmission suddenly seized up on the freeway one day. I figured this must have been the problem. Replaced it with an AX-15 conversion. But the vibration still happens.
-It comes and goes. It usually happens when I drive over a bump, then goes away. It doesn't seem to matter if I'm in gear or not.
-It's a fast vibration like driving over a washboard. It rumbles and I can feel it in my seat. I do NOT feel it in the steering wheel/pedals/front end.
-Replaced the yoke/output shaft from the Dana300 because a U-joint bearing cap was spinning a bit.
-Had rear driveline rebuilt and balanced, new U-joints.
-Got brand new wheels and tires, still vibrates.

-Still vibrates occasionally for short periods of time, then goes away, usually when I change speed.

-I noticed about .015" of play between the Ujoint retention tabs in the rear diff yoke. Could this be the problem, or part of it? (I did replace the Ujoint mount straps with new ones).
-I noticed the front output shaft from the Dana300 has some play, and the front driveline is not straight (I can see it wobbling when in 4wd with the hubs disengaged). Could this be turning a bit when I'm driving even in 2wd causing a vibration? I used plastic twist ties to hold it still to see if the vibe goes still happens when I know the front driveline is not turning.
-I noticed my front wheel bearings have a little play.
- One of my engine mounts is not aligned well, and the rubber block is tweaked pretty good to make up for the misalignment, but seems to hold well.

Any thoughts?
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post #2 of (permalink) Old 08-11-2015, 04:10 PM
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A very clear description of the problem. Good job!

Play in the U-joint tabs can certainly cause vibration, and hitting bumps could make it come and go. The cross can slide freely in the caps, so a bump while it's turned a certain way could slide it off center causing vibration, and a bump when it's turned the other way could slide it back into its proper position. In any case, you should fix the problem.

A dial indicator riding on the driveshaft tube will show if the shaft is running true. Thump the shaft with a dead blow hammer to move it to its extremes in the yoke. Any run out will cause vibration. If you find runout you may be able to shim it out, but eventually you should replace the yoke.

If it comes and goes in a steady cycle it suggests that there are two components vibrating. When they are in sync you feel it, when they drift out of sync, because they're spinning at slightly different speeds they cancel each other and you no longer feel it. For example, if the front driveshaft were spinning with the tires, it and the back shaft could turn at slightly different speeds due to different tire sizes or different gearsets in the differentials. Since you've stopped the front shaft with ties it can't be that, but bent axle shafts could cause the problem.

You don't mention if your CJ has stock suspension, but a lift can change the driveshaft angles in a way that causes vibration. Normally one would expect it to be constant at constant speed, but old Jeeps have a way of doing things not logically possible. And the same thing is possible if the power plant is sitting slightly sideways in the frame, although I would think the angle would have to be extreme for it to show up.

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Last edited by Jim_Lou; 08-11-2015 at 04:12 PM.
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post #3 of (permalink) Old 08-12-2015, 08:00 PM
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OK, you have provided a very detailed description to your problem. And Jim has gone into many aspects of what the problem cause might be.

Let's just take this gut first!

A Scrambler will not react the same as CJ7 with a rear drive train problem. If you have a rear drive shaft problem on your CJ7... It's gonna feel like the world is coming to an end!

So to that end, I would recheck the rear drive shaft, u-joints and everything related. There shouldn't be much room to move the drive shaft around.

Here's a question for you... Have you replaced any motor or transmission mount with anything other than rubber?

Geeze I hated that, had to go back to rubber.


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post #4 of (permalink) Old 08-12-2015, 09:21 PM
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If you have the same driveshaft as when the transmission locked up, take the driveshaft out and set it on a flat surface. Make sure the u-joints are still lined up on each end and the driveshaft didn't twist. For example, the following picture of my front driveshaft shows a distinct twist.
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File Type: jpg 1334607-untitled1.jpg (48.2 KB, 1 views)

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post #5 of (permalink) Old 08-21-2015, 02:01 PM
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I'd check the pinion nut to make sure it's tight and then I'd drop the salad bowl er...diff cover off the AMC 20 and check to see if the spider gears survived the trans locking up. I have a differential case in my garage right now that cracked where the pin goes through.

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post #6 of (permalink) Old 08-31-2015, 10:32 PM
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And so Momo ?

Let us know whatever....


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