Driveline Vibration Diagnosis Help Please

[ft bronc]

Before I go back to the driveline shop that balanced my rear driveshaft, I could use some opinions. I've got a completely stock '73, 302, C4, factory posi front & rear. Tranny died two years ago & I finally got it back on the road. Here's the work I did while it was down:

Front:
Had the tranny rebuilt.
Replaced the T/C mounts w/ new urethane mounts.
New u-joints on the front axles & new ball joints.
New brakes w/ new drums.

Rear:
Rebuilt the rear drive shaft. All new spicer joints & new CV collar. Did not change slip joint as it seems OK.
Had the diff rebuilt - all stock.
New axle bearings.

Got it back on the road, and it rides and drives awesome, except for a high speed harmonic vibration that seems to come from the rear driveshaft. Pulled the shaft and had it balanced. No improvement, maybe a bit worse. Got the tires balanced today & rotated them. No change at all in the vibration - still feels like it's coming from the rear shaft.

The driveshaft had a lot of weights on it stock, and the shop added more weights to it on both ends. On one end, 180 degrees opposite the old weights, and on the other about 150 degrees opposite. They also added weight to the slip joint, which I had never seen done before.

The only other possibility I can think of beside the driveshaft is that one or more of the tires flat-spotted from sitting so long, but I did move it a few times while it was down. Tires are Michelin LTX's, with maybe 7,000 miles on them.

Any ideas? The vibration was not there when the tranny died two years ago.:-[

 

[bronconut73]

If it is flat spots they will go away pretty soon. Some highway miles at max psi will help it along.

 

[kc15842]

check the angle of the rear pinion. How much lift do you have?

 

[SteveL]

I had a output bearing on the t-case do it years ago. Unbolt d-shaft and see how loose output is.

 

[ft bronc]

check the angle of the rear pinion. How much lift do you have?

No lift. Everything is stock.

 

[ft bronc]

I had a output bearing on the t-case do it years ago. Unbolt d-shaft and see how loose output is.

How much play should you feel? None?

 

[DirtDonk]

Have you pulled the rear driveshaft off and driven it around in four-wheel drive? Effectively making it "front" wheel drive. This can confirm or eliminate the rear shaft as a culprit.

Can you sight down the driveshaft to verify that the joints are "in phase" still? While not common, I've seen even experienced driveline shops put the two halves back together one spline off on the slip yoke.

If I'm not mistaken though, this usually results in a fairly serious vibration. Not just a minimal one like you're describing. However, it's still worth a look-see.
Just make sure that your joint yokes are in the same plane.

Paul

 

[SteveL]

Not sure of the exact oamount of play the output should have. When mine went it was obviously too loose.

 

[ft bronc]

Have you pulled the rear driveshaft off and driven it around in four-wheel drive? Effectively making it "front" wheel drive. This can confirm or eliminate the rear shaft as a culprit.

The vibration doesn't start until 55-60 mph. I guess I can drive it that fast with only the front shaft connected?

Can you sight down the driveshaft to verify that the joints are "in phase" still? While not common, I've seen even experienced driveline shops put the two halves back together one spline off on the slip yoke.

Never thought of that. I didn't take the slip yoke apart from the shaft, but I guess its possible it was that way to begin with. I'll take a quick look. There was always so much noise/vibration coming from the rear axle previously the vibration could have been there & I wouldn't have noticed it.

Thanks for the help!

 

[DirtDonk]

The vibration doesn't start until 55-60 mph. I guess I can drive it that fast with only the front shaft connected?

Sure can. It's not going to feel like normal of course, and how it does feel will depend on the condition of the front end too. But it's not really an issue.
If there's any question though, just don't stay at the speed any longer than you need to, to determine if the vibration is still there.

Never thought of that. I didn't take the slip yoke apart from the shaft, but I guess its possible it was that way to begin with. I'll take a quick look. There was always so much noise/vibration coming from the rear axle previously the vibration could have been there & I wouldn't have noticed it.

Hmm, well then it wouldn't be as likely to have been a fault of the driveline shop after the fact. I said that when I thought the vibration started after the balancing. But obviously you were having it balanced because to the vibration.
It's still possible that they missed it when they did the work I suppose. But the chance of that is getting slimmer by the minute.

Paul

 

[1sicbronconut]

60 mph is where you will feel a tire/wheel vibration most of the time, have another set of wheels/tires you can try? I would aslo try driving it without the rear drive shaft first.

 

[ft bronc]

Sure can. It's not going to feel like normal of course, and how it does feel will depend on the condition of the front end too. But it's not really an issue.
If there's any question though, just don't stay at the speed any longer than you need to, to determine if the vibration is still there.

Thanks so much for the suggestion. Took out the rear shaft & drove it down the freeway.

Good news: I did a good job on the front axle - nice & smooth in FWD mode.
Bad news: the vibration is coming from the rear axle.

I've never felt a harmonic vibration like this from a rear axle before. If I stepped into this cold, I'd swear it was a tire issue, but I had the tires balanced and rotated front to rear, and the vibration didn't change at all - not even a little bit. I guess to be absolutely sure I could try a different set of wheels & tires on the rear, but definitely don't have any handy.

What in the rear axle could be causing a vibration like this? As I mentioned, the diff was completely rebuilt & new wheel bearings installed.

 

[DirtDonk]

Well at least it's getting narrowed down.
Could it be the tire tread type? What tires are they, and what size? Did this start suddenly, or was it doing it all along?

Reason I ask is that I can't think of a differential issue that would feel like what you describe. Maybe an axle shaft out of true or the flange end actually bent could do it, but not sure about the gears and/or diff.
What ratio by the way? I've heard of some specific number ratios causing harmonics on some cars. Like a GM Corporate 10-bolt and a 3.42 ratio for instance. But I don't remember hearing of anything like that with a Ford 9" setup.

Maybe someone else will know about the diff aspect, but an axle shaft or tire is sounding more like it.
What's the condition of the springs, bushings, and u-bolts?

Paul

 

[DirtDonk]

Wait. You're saying it went away without the rear driveshaft?
Then it's VERY likely the rear driveshaft. Or the rear output of the t-case maybe.

That sound logical?

Paul

 

[1sicbronconut]

You can balance a square tire find another set to try or find somepne that can force balance them just to make sure. http://www.youtube.com/watch?v=XsaJyq7FsFg I'm not there to drive it but I've been a Ford tech for 25+ years and I'd try another set of tires or have them force balanced at this point. HTH Steve

 

[ft bronc]

Wait. You're saying it went away without the rear driveshaft?
Then it's VERY likely the rear driveshaft. Or the rear output of the t-case maybe.

That sound logical?

Paul

Sorry, the vibration is still there with the rear driveshaft out. You feel it less in the vehicle since the shaft isn't there to transmit it up to the t-case, but it's definitely still there.

I'm with you that it doesn't seem logical it could be coming from the diff. The tires are Michelin LTX's, so real mild tread pattern. I can't remember the exact size, but not far from stock. I don't believe the vibration was there before I did all the work, but it's possible it was & I couldn't notice it over all the other noises and vibrations from the completely worn out diff & wheel bearings.

HTH Steve - the tire place has a RFV machine, so I was thinking the same thing. Michelin has a very good reputation for uniformity compared to other brands, but you never know. I've gotta ask around to see if I can find someone with a couple wheels & tires I can throw on for 10 minutes to see if it goes away.

Man this has been frustrating. Thanks for all the advice!

 

[RG75Bronco]

I don't know if you resolved this vibration issue, but I dealt with the same issue for over a year. At 50 mph the truck vibrated so bad it was dangerous. I went through all of the trouble shooting you have. I had the pony up on a lift about to pull the T-Case out when a friend dropped the drive shaft from the rear end and turned the yoke. He said stop what you are doing and turn this yoke. So I did and it was smooth then it would tighten up. We pulled the chunk out and found a spot on the ring gear that was pitted. Borrowed another chunk and the vibration disappeared. I hope this helps.

 

[Viperwolf1]

It's also possible for the top of the D20 rear output assembly to contact the support under the floor. Usually happens with old sagging body mounts and new tcase mounts.