Help me find a vibration!

I've been chasing a vibration in my 1989 Chevy Caprice Classic wagon for probably 3 years now to no avail, so I guess it's finally time to ask you guys. The vibration is intermittent, and is the worst right around 60 mph. It comes and goes above that speed, and feels more or less like it is wheel speed frequency. It has persisted through three sets of rims and four sets of tires, along with three different engines, two transmissions, and two driveshafts. Both front wheel bearings have been replaced, and has had two sets of spindles (stock and dropped). There have also been three different sets of shock absorbers. The rear end is a GM 8.5" 10 bolt that has never been apart, but I have run the car on jackstands at 60+ mph with the suspension at ride height and while there is a little vibration, it is not severe. Unless being under load or with weight on the tires can change things, I don't think it's coming from the rear. When the vibration happens, it's severe enough that I can't tell if it's coming from the front or rear. The only other correlation I can come up with is that, when I'm towing, the vibration might be less severe or less frequent. Hard to say, though.

Any ideas? I'm pretty well stumped, and the vibration makes an otherwise fun car feel like a piece of junk when it starts shaking its way down the highway. I also have no particular desire to start throwing parts at the rear axle on the off chance it will fix something.

What parts did you reuse when doing the engine, trans, and driveshaft swap?

Do the angles for the rear end and trans match up? You may need to shim the rear at the springs to correct any misalignment.

Next, have you replaced the u-joints? If not, I would give that a try.

If you have already replaced those, I would try turning the driveshaft and flange and/or propeller shaft 180 degrees ( one at a time so you don't add too many variables.

Does temp affect it? Do you have a clutch fan on it? Check the clutch fan, it may be the source of vibration, especially since its intermittent, which may be related to the engagement temp for the fan clutch.

Has it ever been in an accident?

Bad transmission mount?

My dad had a chevy van that no one could figure out a vibration of .....it turned out to be a really bad u joint that in park locked it self to one side of the wallowed out caps but at speed would float out of the wallowed section and vibrate

jstand:

I think by now, basically everything is new. The engines were all different blocks, and one transmission was a 200r4 automatic and the other an NV3500 manual transmission, and the truck it came out of drove just fine. The shaft of the driveshaft is the same, but the yokes are different. u joints are new. I had not thought of rotating the yokes, but seems a bit too violent and low frequency to be the driveline? I'm not well acquainted with what an unbalanced driveshaft feels like.

I have not checked driveline alignment...I would probably need adjustable upper arms since it's a coil sprung 4 link and there isn't much to shim. It could use new bushings back there though.

Temperature doesn't affect it, and it's an electric fan now.

Antihero:

Never been in an accident, aside from backing it lightly into a basketball pole a few years ago. Trans and engine mounts are new and are polyurethane, since I destroyed the stockers the first time I shifted hard from first to second with the new engine...

I guess it could be the u joints, but they've only got maybe 20k miles on them.

If it can be reproduced easily, and is distinct enough that is stand out, then you could try using something like a Sirometer to determine the frequency.

Have a friend ride along with you and hold the Sirometer on a solid surface and adjust the wire length until the frequency is identified.

Once you have an idea of the frequency you can start identifying the source based on the frequency:

i.e. Does it match up with rpm, wheel speed, or some harmonic of another rotating assembly?

Carrier bearing in the rear end?

If the rear end is the only thing you haven't changed.......

I'm not saying just race out and swap out the rear end, but I'd be paying particularly close attention to it. Disk or drum? An unbalanced drum could give some interesting feedback.

You mentioned drop spindles, I'm assuming the rear is lowered. Did the problem exist prior to lowering? Pinion angle or a decreased distance between diff and trans could have a negative effect.

jstand: I had to look the sirometer up - that's a pretty cool idea, and would definitely help with the frequency rather than just guessing as I am right now that it's wheel speed.

dean1484: I suppose it could be. I'm newer to the whole solid rear axle thing and have no idea what that would feel like, though the rear end isn't making any noticeable noise. Would be easy, if messy, to check by pulling the cover off.

daeman: 11x2 rear drums, so there's a lot of mass back there. I did check that they at least still have their balancing weights, and they do. Minimal axial runout on the axle, as in probably 1/16". No radial runout, at least from just pushing on it.

The rear is lowered by a few inches, and the problem was there before that. It's been there since I got the car, which has shockingly been something like 6 years ago.

So it sounds like I need to check pinion angle, pop the diff cover off and see how things look in there, and possibly invest in a sirometer.

In other possibly related news, my current transmission yoke is leaking (TH400 wet style that occasionally blows that rear seal out) due to having had a cap welded on the end rather than a press-in seal that I hadn't noticed before. I have a new yoke and front u-joint in the mail, so we'll see if that changes things.

U-joints or driveshaft balance/alignment issues.

Many Fords from the sixties and early seventies had a vibration damping driveshaft that was put together like a harmonic balancer, when the rubber got old they became a vibration inducing driveshaft, but looked fine. Does GM have a driveshaft damping device that could similarly fail?

Sirometer sounds like a great tool for this job! Seriously, I'm still puzzled that the sirometer thread wasn't a wash in praise and general excitement, but I'm an engineer and that stuff really revs my engine.

Check pinion angle and also pinion play...could be a worn pinion bearing. Those can get quite bad before they start to cause vibration.

HappyAndy: nope, no rubber bits in this one. It's a straight 3.5" steel tube. I need to measure it so I can figure out the critical speed - it's pretty long.

Is there a good way to check all these rear end bearings for wear? It seems like there is so much weight or preload on everything that the usual shake-and-listen test might not actually show a failed or failing bearing. I'll check the pinion bearing for play when I have the driveshaft out to replace the yoke.

Well, at your behest, I pulled the driveshaft last night. Pinion yoke doesn't seem to have any play in it, and the u joints didn't thunk when it was installed, but I did find this:

So what does this mean? Seems to me like the operating angle on the joints might be too low, so the rollers are never moving? All of the bearing surfaces look like this, but this is the worst one of them. The trans yoke is worse than the diff joint. They were properly greased. I'm going to throw some new joints in for now, but would prefer to not have to replace them every 15k miles.

I also did some math to make sure I wasn't hitting driveline resonance - 1/2 critical speed is between 86 and 88 mph, and maximum safe vehicle speed is 117 mph, so that shouldn't be coming into play on the street. Uncomfortably close to my expected entry speed going into turn 1 braking zone at Putnam though...going to have to keep an eye on that.

Do you have a good truck driveline shop in your area? after you replace the U-joints drive it over there then should have wedge shims for the rear and flat shims for the trans to give matching angles with an offset so the needles roll over the lube.

There is one halfway decent shop in town; I'll have to ask about that. How would wedges help in the rear, and where would I put them? I can see how it would apply with leaf springs, but am not quite making the connection with coils even though literally dozens of people have recommended it to me.

Might be worth finding a friendly guy with a wheel balancer, and have him spin your drums.

Well, the new u joints are in, which seems to have helped with a high frequency vibration, but the vibration that prompted this thread is still there as much as ever. Also, the pinion bearing in the rear end felt good with no play. I need to get the lemons car done so I can use the rest of my jack stands to check driveshaft angles, but given the vibe frequency that changed with u joints, I don't think that's going to be it.

I did discover that both of the rear axle seals are leaking very slightly, but that's likely not related.

I'll ask around about spinning the drums on a balancer. I have a motorcycle wheel balancing stand as well, but I haven't come up with a way to keep the drum centered on the shaft to try to use that.

Of no help but empathy - My dad had a 73 Chevy van that we grew up in. Super cool... He put a warmed up 400 in it and it was awesome.

It developed a vibration at around 55 that he, a very smart and skilled man, could not find. He tested and checked everything , to the point of putting a new crate 350 in to replace the 400, a summit tranny, new torque converter, driveshaft balanced, etc etc. never found it.