3278

I know what you're thinking: "Vibrations on a Cherokee? NO!" But these are a little worse than normal. I have a 1995 Sport with an auto transmission. It has an RE 3.5 inch lift kit, a 2000 transfer case with SYE, and no front driveshaft. [Don't ask me: bought it this way.]

It's in really good mechanical condition, and has been working great, but ever since yesterday, when I accelerate at speeds over 35, the whole Jeep shakes like crazy - not like death wobble, more like a really fast mechanical thumping - and you don't accelerate much because the force of the acceleration seems to be wasted on the vibration. Once you stop accelerating, it disappears completely and everything is, well, as smooth as it normally is. If you put it in neutral and rev it at those speeds, no vibration: it's not the engine making the vibes.

Any suggestions? I'm not even sure where to start looking. It feels like something's got some play in it, and when I accelerate, it slops around and shakes the car.

3278

And I've been hitting Search pretty hard, but the closest I've come is maybe the SYE yoke needs tightened down, which isn't impossible. I'll check it ASAP, but in the meantime, any other ideas?

dzeinert

Bad/worn engine mount?

cruiser54

That and/or trans mounts. First place I'd look.

Here's something else to look at:


From Jon Kelley aka 5-90 @ www.kelleyswip.com

This is a known issue.
1) Unless you're going to get silly with the skinny pedal, 2xSAE8 screws on one side won't be a huge problem. You'd be better off with three, but if you don't get silly you'll be okeh.
2) The Brown Dog engine mounts use two or three additional holes on each side, and spread the mount/clamping force over more of the block. This is invariably a good idea, and I'll probably be doing it as I refit my 88.
3) The primary reason that the screws snap is because, sometimes, the screw holes in the block aren't drilled & tapped deeply enough. This causes the screw to bottom out in the hole, so you end up torquing against the bottom of the hole instead of stretching the screw (as you're supposed to do. Not your fault - you aren't doing anything wrong.) The screw is not stretched properly, and the head is not butted up against the bracket, so there's some room to move there. Vibration then causes wear cycles on the screw, which generally leads to rupture.
The easy/cheap fix? When you replace the screws (3/8"-16x1.25", as I recall,) put two flat washers under the head before you screw it into the hole.
The check? Take a feeler gage (.003" to .005") and try to slide it under the head of the screw. You'll be able to get under the corners (look at the hex head, and you'll see that the surfaces curve toward each other slightly) if it's a standard hex head - if it's a flanged hex head, you should not be able to get under the head anywhere. If you can slip the gage under the head, you have a problem. If you can touch the shank of the screw, you have a big problem!
In no case should you reuse the screws after you take them out - they'll be stressed under the head, and you'll have a significant reduction in strength. Replace them outright, putting washers under the head as I mentioned before. The washers will make up for the slight lack of depth in the hole (two of them will be about 0.125" or so,) and allow the screw to be preloaded properly.

opie1988

I would check driveshaft...

cruiser54

That, too.

The underside of the OP's Jeep needs a good case of "noggin pokin'".

dzeinert

If it was the drive shaft, wouldn't you think it would do it at all speeds, not just during acceleration?

Bustedback

It's all about harmonics and frequency.

3278

Some dedicated noggin pokin' showed the u-joint where the draft shaft meets the rear differential is broken in an exciting way that's new to me, but not really surprising. Now, here's hoping that driving it this way for 50 miles didn't wear something else down.

Bustedback

If it's flat ground driving, just pull the rear drive shaft and cap the tail shaft housing on the transfer case. Then shift to 4hi and drive home. Losing a shaft while driving can really mess some stuff up. I hear a Gatorade cap works pretty good for plugging the transfer case.

cruiser54

Good job of noggin pokin'. Best of luck.

3278

Any recommendations on u-joint? I was just going to get the Crown from Morris, but if anyone has a better suggestion...

Jeepster242

Mine used to vibrate under acceleration, then i replaced the driveshaft u-joints and had the driveshaft re-balanced, vibration is gone! And for u-joints, i would recommend Spicer u-joints

cruiser54

Yep. Lots of different things can cause those kind of vibes.

That's why it's always a good idea to crawl under your Jeep and poke around. U-joints are usually caught before they fail during routine greasing.

3278

I would definitely like to get Spicers if I can. Does anyone know the part number for that application, for the u-joint where the drive shaft meets the rear differential?

What else will I need to do the repair? [edit: Actually, now that I think about it, does anyone know of a good write-up? It seems like a super-easy repair, but I'd hate to muck it up.]