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Discussion Starter · #1 ·
And, no, these are not good vibrations! cwm1

Vibration is felt more in the seat than steering wheel.

It can be felt from low-to-mid 50 mph, and diminishes above 70 mph.

It definitely does NOT follow engine speed.

The only change is in intensity.

I started with wheels and tires, and found no reason for vibration with them.

One shop claims it's possibly a "body flexing." Meaning the body-to-frame "doughnuts" are "bad" or gone.

I have a small but growing leak at the rear of the transmission... Plus, last January, a pumpkin leak was fixed, which required the shaft to be disconnected from the differential. So, I'm thinking either bearings at the rear of the trans are bad, or the DS was re-attached to the diff 180 degrees out...

I think it's drive shaft vibration. I'm almost certain most of you will agree.

What say you???
 

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The only time I've maked a DS was when I shortened one. I've never marked one in relation to the diff and never had a problem because of it. I dont know why so many how-tos say you have to do it.

Sounds like u joints or a tossed wheel weight to me.
 

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Discussion Starter · #4 ·
The only time I've maked a DS was when I shortened one. I've never marked one in relation to the diff and never had a problem because of it. I dont know why so many how-tos say you have to do it.

Sounds like u joints or a tossed wheel weight to me.

Thanks for your replies Big and Zephyr.

Since the wheels and tires are no longer suspect, looks like U joints are the next on the check list.
 

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pieces of metal. this happened to me with my 94 caprice. it would eat up the abs sensor inside the diff. and at speeds of 70 it would shake the car.
 

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Discussion Starter · #8 ·
Is there any other test I can do before taking the diff apart?

Thanks again for your help, BiG!

Just replaced the DS yoke bushing, tail housing seal, output seal and although this stopped the tell-tale leak at the back of the trans, it didn't help with the vibration much...

U-joints are good...

It still feels like a wheel/tire bouncing about an inch, except I cannot tell which corner... Could worn shocks be the culprit?

I had my son drive along both sides, and he cannot discern any unusual wheel/tire bouncing...

This is driving me crazy! Like fingernails screaching on a blackboard bothers others...
 

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I'm in for the final result you find. I was gonna say a shock too based on your diagnostics. Maybe still do a visual for fluid or the fender bounce test. Depending on mileage just replace them anyway to remove a variable.
 

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There may be a speed range where the spring has a "harmonic" and without good shocks, it can become a positive feedback loop, increasing the displacement with each cycle. The shocks may be the answer.
 

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If the tranny has been leaking for a while, have a look at the rear transmission mount. I had one go bad from the fluid breaking down the rubber.
 

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id check the u joints just replaced it on my wagon..was getting vibration until BOOM!! driveshaft blew out because of the ujoint..check it
 

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Discussion Starter · #13 ·
Just replaced the drive shaft yoke bushing, tail housing seal, and output seal... And the transmission mount... U-joints are good...

Rear shocks are the next cheapest thing to replace... They are about a year+ Gabriel "coil-overs," and they pass the fender bounce test. However, the wheels and tires are quite light, and it still feels like a bouncing tire, but only slightly...

I've wanted Bilsteins in the rear to match the fronts anyway... Can't hurt...

If that's not it, I'll open the pumpkin and check for metal fragments, and check the ABS senser... then have the drive shaft balanced... ARRRRGH!!
 

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can you find another set of wheels/tires to try? Maybe even another driveshaft? Any other B-body folks around who's tires/wheels you can try out?

If this has been something that has come on gradually it might be the tires themselves. I've had tires balance out just fine that were amazingly lumpy and/or "square". Had the vehicle at a shop and we spun the tires and I was amazed at how out-of-round they were. Even though the tires were very new we replaced them and it was amazing how much better it drove.

I also have had another tire get such a big bump in the tread that it would fall over when rolling it through the garage. These were all tires that had much more sidewall than yours, but figured it might be worth mentioning. They were also national brand tires, not inexpensive ones.
 

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Good idea on OOR tires. Just jack the back and run in slow gear should give you an idea if that's even close to the problem though.

You're running out of sources though. I thought of bad front A-arm bushings setting up a resonance thing, but that's a stretch.

This item, [quote: "Vibration is felt more in the seat than steering wheel."] made me remember I had a dull harmonic thumping from the front end at certain speeds on a Riviera once. Kept getting worse over a few months and turned out the windshield was attached by only a couple feet along the bottom and had setting up a pulse in the cabin.
 

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There may be a speed range where the spring has a "harmonic" and without good shocks, it can become a positive feedback loop, increasing the displacement with each cycle. The shocks may be the answer.
I agree. Had a 87 Crown Vic wagon that I went through the same song and dance with. Turns out that if you normally drive the car with the same load, or if you have the auto level system, the shocks spend most of the time only cycling through an inch of travel. This area wears and the shock may seem to be working but in that particular area of travel is not, thus your harmonic vibration.

Try putting some wieght in the back end (or pull the auto level fuse) and see if the vibration goes away.

Bill
 

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Discussion Starter · #17 · (Edited)
I doubt its the tires. I had it a while before replacing the tires, and thought it was the old tires fault. This vibration has been around for both sets (several months), and the new set has twice been "road-force" tested by Big Brand. Neither the tires or wheels are to blame. Though the tires and wheels are considerably lighter than the OE set...

I've always thought it felt like a small wheel bounce... I ordered Bilsteins for the rear yesterday... Installing them today... fingers crossed!

One mechanic tried to convince me it was body flex from worn out frame "doughnuts." The trans bushing was only slightly squished (half inch or so). Can't imagine the body-to-frame bushings would wear out differently though...

The shop where I take my car tried the free-wheeling, checking for any drive train wobble, out of balance, busted or broken whatevers... Drive shaft, u-joints, wheels and tires all looked "right."

I carry about 100 lbs of music gear in the "way-back" every week... Doesn't seem to matter... Putting trans in neutral at 65-ish doesn't matter either... it feels like one or both of the rear wheels is bouncing about an inch between 55 and 70...

Auto-level was removed years ago...

Could a broken rear spring be the cause?

Shocks will be on by tonight... "I'll be back."
 

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I would mark the driveshaft, then remove it, install new u joints, and then reinstall flipped 180 degrees. See if that helps. Also balance the tires. If that makes no change, it may be time for an aftermarket shaft, its possible you threw a balance weight.
 

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Discussion Starter · #19 ·
...just goes to show... even well-meaning friends can miss something!

We put the wagon on a lift, ran it up to 80 mph and one of the wheels was slightly wobbly (so much for the "road-force" balancing!!). Removed both wheels, and the drive shaft vibrates at a higher speed than before the drive shaft yoke bushing, tail housing seal, and output seal were replaced! They missed it!!!

So, no shocks yet... Driving to Denver tomorrow (with vibrating DS)... Will have it balanced when I return.

I think an underhanded shop is to blame for me getting the shaft! I discovered they were breaking "stuff' when it was in their shop for maintenance... I told the "owner," whom I liked, then took my business elsewhere (to support a friend's shop, though much further away). You just can't trust some shops...

So, once the shaft has been balanced, or replaced, I'll report back... Maybe that's it and maybe not! I still intend to replace the rear shocks with Bils...

Thanks to y'all for the help so far!
 

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If you have a wheel wobble, don't rule out a bent axle flange.
 
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