Vibration Under Load

My 99 has developed a pretty bad vibration under load above 2K rpm. Jeep is stock ride height and I replaced the rear drive shaft U-Joints in an effort to fix this problem with no resolve. 65-75mph it feels like its going to shake itself apart but goes away as soon as you let off the gas, atleast mostly. So the problem is mostly under load.

Anyone have any thoughts? I cant seem to find anything else that looks bad under the thing....

How many miles are on it? What trans and T-case? Where do you feel the shake most? In the seat or more in the dash? I would try checking the pinion bearings in the rear. Also check the trans and engine mounts. Under load the trans will want to torque up toward the floor raising the T-case output shaft and change the angle the driveshaft is running at. If the mount is bad it will move a large amount.

wheeling365 said:

How many miles are on it? What trans and T-case? Where do you feel the shake most? In the seat or more in the dash? I would try checking the pinion bearings in the rear. Also check the trans and engine mounts. Under load the trans will want to torque up toward the floor raising the T-case output shaft and change the angle the driveshaft is running at. If the mount is bad it will move a large amount.

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136K Miles, NP231 T-Case, It shakes the whole thing pretty bad but the dash definitly rattles. Pinion bearings at 136K sounds a little odd but the pinion seals definitly do leak. Trans and T-Case Mounts seem tight but I will check them again along with the engine mounts.

Any further thoughts while were on it, because im pretty sure all of those are going to check out good.

Usually a shake in the seat is from the back and a shake in the dash/wheel is from the front in most cases.

Working from that I agree that a rear pinion bearing going and being that bad would have a major leak by now and also shake in the back more.

Is this something that happened all at once or did it progress slowly?

I'm just going to throw some ideas out.

Do you wheel this at all?

Mud/random stuff stuck in the back of a rim?

Tossed a Drive Shaft balance wight?

How do the steer tires look? If your getting wear on the edges look for a toe problem that is causing the tires to drag across the road vs roll smoothly. Toe has to be really out of wack to cause this from what I've seen.

I'm outta ideas on what to suggest. I'll have to think about it on my wan home from work a bit more.

check the cardan joint on the front shaft, or better yet, pull the front shaft completely. When they go bad the driveshaft wobbles and creates a very noticeable and deep vibration.

I would try pulling the front shaft. If the shimmy remains you know it's likely in the rear driveline.

Answered your questions, thanks.

wheeling365 said:

Usually a shake in the seat is from the back and a shake in the dash/wheel is from the front in most cases.

Working from that I agree that a rear pinion bearing going and being that bad would have a major leak by now and also shake in the back more.

The rear seal doesnt seem to be leaking badly, the front does though.

Is this something that happened all at once or did it progress slowly?

No Idea, just bought the thing to drive during the winter, was like this when I bought it. Old owner claims to have never noticed it, i didnt think it was going to be a big pain in the ass, apparently I was wrong!

I'm just going to throw some ideas out.

Do you wheel this at all?

I never have.

Mud/random stuff stuck in the back of a rim?

No, and it only does it under engine load.

Tossed a Drive Shaft balance wight?

Drive shaft is rather rusty but does not show a missing weight.

How do the steer tires look? If your getting wear on the edges look for a toe problem that is causing the tires to drag across the road vs roll smoothly. Toe has to be really out of wack to cause this from what I've seen.

Tires are nearly brand new, mabey a few thousand miles but look to wear straight.

I'm outta ideas on what to suggest. I'll have to think about it on my wan home from work a bit more.

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87manche said:

check the cardan joint on the front shaft, or better yet, pull the front shaft completely. When they go bad the driveshaft wobbles and creates a very noticeable and deep vibration.

I would try pulling the front shaft. If the shimmy remains you know it's likely in the rear driveline.

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The front driveshaft rotates regardless of engine load though, Id assume if the front shaft was going bad it would vibrate all the time, not just under load. Let me know if im wrong though.

breecher_7 said:

The front driveshaft rotates regardless of engine load though, Id assume if the front shaft was going bad it would vibrate all the time, not just under load. Let me know if im wrong though.

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True the front shaft may be bad but unless its in 4wd its only driven by the front wheels. Its not transmitting engine power and would only be vehicle speed dependent. You could try running it in 4wd on the highway for a short straight run to see if it makes a difference with the front end under load.

How does it handle on the road overall? Does it wander all over the place or is the steering real sensitive to input? Either of those could be a caster issue. Caster thats super high would cause a definite shimmy at speed that gets worse as you go faster. Again its not engine load dependent though...

What transmission is in it? Do you feel any vibes from the engine/driveline at lower speeds when around 2k RPM? It could be a loose/cracked flex plate or converter. I would think there would be some kind of sounds associated with it beyond just a shake and it would do it anytime the engine is near that RPM.

wheeling365 said:

True the front shaft may be bad but unless its in 4wd its only driven by the front wheels. Its not transmitting engine power and would only be vehicle speed dependent. You could try running it in 4wd on the highway for a short straight run to see if it makes a difference with the front end under load.

How does it handle on the road overall? Does it wander all over the place or is the steering real sensitive to input? Either of those could be a caster issue. Caster thats super high would cause a definite shimmy at speed that gets worse as you go faster. Again its not engine load dependent though...

What transmission is in it? Do you feel any vibes from the engine/driveline at lower speeds when around 2k RPM? It could be a loose/cracked flex plate or converter. I would think there would be some kind of sounds associated with it beyond just a shake and it would do it anytime the engine is near that RPM.

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Overall it drives great, drives straight, does not wander at all. As far as what trans is in it, not exactly sure of the model but its a automatic if that helps.. Yes I feel the vibes at lower speeds around 2K and up but its not as severe at lower speeds. As far as sounds associated with it, just the hum of my dash and everything else rattling. Im going to put it up on the lift this weekend and check the flexplate for cracks as well as the converter bolts to the flexplate.

Im going crazy trying to figure this out.

I'll cloud your skies with two more ideas. Nearly new tires- My '96 had four brand new Dayton Timberline ATR tires on it when I bought it four years ago. They were OK at first, a rugged, SUV type tire on a rather light vehicle, with the rough ride. As they aged (wearing beautifully, bu the way), they seemed impossible to keep balanced. It seemed as though it was a rear-end problem, as the vibes were coming mostly from the back. Turns out three of the four were out of round. New Firestone Indys solved that. Seems that you can balance an egg, but it still ain't round...
Next up, what rear do you have? Two of our fleet have D35s with limited slip. Both (one worse than the other) exhibit similar problems to yours. Vibes on the gas, starting north of 60 MPH, and calm down on coast. The worst of the two will be getting an 8.25 transplant soon.
Crawl under there and tell us what ya got!

If its an auto trans its an AW4. They are pretty strong overall.

I'm really thinking that its more something in the driveline such as an axle or DS issue since its vehicle speed related. It might just be concidence that it occurs at 2k rpm. You said earlier in a post that the tires are fairly new. Is it possible that they are not balanced right or that one is out of round? Maybe try swapping them front to back?

heyhar said:

I'll cloud your skies with two more ideas. Nearly new tires- My '96 had four brand new Dayton Timberline ATR tires on it when I bought it four years ago. They were OK at first, a rugged, SUV type tire on a rather light vehicle, with the rough ride. As they aged (wearing beautifully, bu the way), they seemed impossible to keep balanced. It seemed as though it was a rear-end problem, as the vibes were coming mostly from the back. Turns out three of the four were out of round. New Firestone Indys solved that. Seems that you can balance an egg, but it still ain't round...
Next up, what rear do you have? Two of our fleet have D35s with limited slip. Both (one worse than the other) exhibit similar problems to yours. Vibes on the gas, starting north of 60 MPH, and calm down on coast. The worst of the two will be getting an 8.25 transplant soon.
Crawl under there and tell us what ya got!

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wheeling365 said:

If its an auto trans its an AW4. They are pretty strong overall.

I'm really thinking that its more something in the driveline such as an axle or DS issue since its vehicle speed related. It might just be concidence that it occurs at 2k rpm. You said earlier in a post that the tires are fairly new. Is it possible that they are not balanced right or that one is out of round? Maybe try swapping them front to back?

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Well I think we can kill the unbalance tire idea as its only under load that it has this vibration, pretty much goes away once your off the gas. As far as what axle is under it, I believe it is a 8.25 but I am not positive. As far as it being an axle or DS issue, i dunno. I removed the drive shaft, replaced the u-joints, greased up the slip yoke and reinstalled it. The shaft still appears to have all its balance weights.

Any idea how far the driveshaft slip yoke is supposed to engage the t-case output shaft? At factory ride height with vehicle weight on it, it has over an inch of exposed spline, is this normal?

Still at a loss here, I really dont want to start throwing un-needed parts at this thing.

Thinking about it more I'd rule out the tires also. I'm running out of ideas on what to suggest. There are only a few things that are inter related to both vehicle speed and engine load at the same time.

when you removed the driveshaft did you mark the yokes so you kept the U-joints in phase?


I still think you need to pull driveshafts to isolate the problem.

87manche said:

when you removed the driveshaft did you mark the yokes so you kept the U-joints in phase?


I still think you need to pull driveshafts to isolate the problem.

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Yeah I marked the driveshaft and slip yoke but I did not mark the yoke on the rear axle so it could be 180 degrees off, but the vibration is exacatly the same as it was before so I dont think thats the issue. But I will try rotating the shaft this weekend.

I may pull the front driveshaft as well, we will see.... Lots of snow coming this weekend though so that part of the troubleshooting may have to wait.

breecher_7 said:

Yeah I marked the driveshaft and slip yoke but I did not mark the yoke on the rear axle so it could be 180 degrees off, but the vibration is exacatly the same as it was before so I dont think thats the issue. But I will try rotating the shaft this weekend.

I may pull the front driveshaft as well, we will see.... Lots of snow coming this weekend though so that part of the troubleshooting may have to wait.

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well you can also pull the rear and drive it in 4hi on the front axle. I wouldn't drive around a lot, and it will handle a little funny, but we seam to be migrating that it's a rear riveline issue and the rear is a whole lot easier to take off than the front.

I'd start by pulling the rear driveshaft and see if the problem remains, at least then you'll know it's driveshaft back in the driveline, and not a motor/trans mount or something weird in the transfer case or something.

Fair enough, i may give that a shot but im affraid that the schetchy way it will drive with just the front wheels pulling may make it hard to tell if its still vibrating as its most noticeable at 60+ mph speeds and I reeally dont want to do that in 4hi I noticed that the exhaust is starting to rust out, not bad or anything but is there any way it could be causing that bad of a vibration? It doesnt seem to be rubbing on anything.

Only under load--describe the condition of the engine and transmission mounts. Missing any bolts from the engine mount to block brackets?

joe_peters said:

Only under load--describe the condition of the engine and transmission mounts. Missing any bolts from the engine mount to block brackets?

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Gonna get it up on the lift tomarrow and check it out, honestly I hope its as simple as a broke mount but I doubt it.

breecher_7 said:

Fair enough, i may give that a shot but im affraid that the schetchy way it will drive with just the front wheels pulling may make it hard to tell if its still vibrating as its most noticeable at 60+ mph speeds and I reeally dont want to do that in 4hi I noticed that the exhaust is starting to rust out, not bad or anything but is there any way it could be causing that bad of a vibration? It doesnt seem to be rubbing on anything.

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no, not likely that the exhaust is the issue. I wouldn't be scared to drive it at 60 MPH on the front driveline, I drove my junk home one time on the front driveline and a taco bell cup taped onto the t-case output to keep the fluid in it. The only time the funny handling will be noticeable is when you're not going in a straight line.

Re-reading all of this you say it happens at lower speed but is not as bad. Can you describe how the vibration changes as you speed up? Does it seem to vary with the same frequency as tire rotation, or is it exponential, and coupled with the driveshaft speed?

Stock sized tires are doing 12 and some change revolutions per SECOND @ 65 MPH, would you say that the frequency of your vibe is somewhere around that? Compare that with a driveshaft with 3.55 gears rotating at 45ish revs per second @65.

basically what that boils down to is that something that's vibrating and rotating at tire speed is going to be a very deep vibe tat you can feel, but is pretty much outside the range of human hearing.
Whereas a driveline vibe is a much higher frequency, more akin to the bottom note a subwoofer an reproduce.
Here's a test tone video on youtube at the frequency that a driveshaft/transmission output shaft speed vibration would produce in the 45-50 Hz range.
http://www.youtube.com/watch?v=BI1FzWG75oE