High Speed Low Frequency Vibration

[JimKueneman]

I now have well over 500 miles on my Coronet 500 restoration and it is working very well until I take it on the freeway. At about 70 I can start to feel a very low frequency vibration that quickly gains intensity and by 75 it has the console door vibrating. It is a very low frequency vibration, in the order of a few seconds. I just had all the tires balanced and everything is new. The drive train is 100% factory original but I have rebuild everything including new mounts and driveshaft u-joints. I am not sure where to start. It does not seem related to the wheel rotation speed and if I let off on the gas it quickly dies down. Thoughts?

318, 904, factory limited slip '741 rear axle with 2.94 gears.

Jim

 

[dieseldazzle]

What's it do when you throw it in neutral and let the engine idle while the cars still rolling 70-75?

 

[JimKueneman]

What's it do when you throw it in neutral and let the engine idle while the cars still rolling 70-75?

smooths out, seems like it is in the drive train.

 

[JimKueneman]

It it does have an Energy Suspension polyurethane transmission mount

 

[khryslerkid]

Check tail shaft bushing for play and proper amount of front U joint yoke into tail shaft. (Not too much hanging out)

 

[JimKueneman]

Check tail shaft bushing for play and proper amount of front U joint yoke into tail shaft. (Not too much hanging out)

Ok, I would have expected the bushing to be replaced with the transmission overhaul but I will look at it, why would the yoke be at an incorrect depth if everything is original?

 

[khryslerkid]

Ok, I would have expected the bushing to be replaced with the transmission overhaul but I will look at it, why would the yoke be at an incorrect depth if everything is original?

It should be good if original. Just have to throw that one in there when we're talking about these 50/60 year old cars. Some have been through quite a few Mechanics.

 

[Lionized]

Driveshaft balanced?

 

[JimKueneman]

Driveshaft balanced?

I was wondering about that, is there any reason a driveshaft should go "out of balance"? The car only had 88k family driven miles before it was put in storage in 1984. It is about a factory original as it gets.

 

[BeeKool]

Hard telling, the family could have had an unruly teenager. It could have dropped the drives haft once. The driveshaft would be next on my list.

Did you do any repairs to the rearend during restoration?

 

[dieseldazzle]

If it goes away i neutral and you are still doing 70-75 the driveshaft is still spinning so rule that out.

 

[JimKueneman]

Hard telling, the family could have had an unruly teenager. It could have dropped the drives haft once. The driveshaft would be next on my list.

Did you do any repairs to the rearend during restoration?

The front u-joint was the factory Chrysler part (still had the little clips that held the caps in place in the factory so they did not fall of on the line), the rear had been replaced.

The rear end I pulled the chunk, cleaned everything, new seals, packed the bearings (they looked fine), set the axle end play, new gaskets, fresh gear oil (correct for the sure-grip additive)

 

[Lionized]

If it goes away i neutral and you are still doing 70-75 the driveshaft is still spinning so rule that out.

It's still spinning but you're decelerating.

You could have a ding in your driveshaft or one of the balance weights could've flung off. Often they're just epoxied on. I had mine welded on.
Worth having it checked out, IMHO.

 

[dieseldazzle]

At a dead stop hold the engine in neutral at the Rpm you get the vibration. I've had weights on converters wrong and I've had fan clutches give me harmonics. Even had a new napa water pump vibrate till it ate itself

 

[Skey]

If it goes away i neutral and you are still doing 70-75 the driveshaft is still spinning so rule that out.

I do not totally agree, there is no load on the driveshaft in neutral which can effect a driveline vibration of this type. If it was more violent I would say not, but the way it is being described I think it is possible. Not saying I would bet on it but something I may check. I will also say finding a tire shop these days that can balance a tire properly is like winning the lotto, so I would start with the basics and have that checked also. My shop which we work on newer imports find tires being misbalanced the #1 complaint for shimmys shakes and vibrations at speed. The common reply from customers is we just had them balanced. This usually occurs at least once a week minimum. Of course lots of things could cause this just throwing a couple darts at the board! Good luck hope you come up with the answer quick!

 

[JimKueneman]

I do not totally agree, there is no load on the driveshaft in neutral which can effect a driveline vibration of this type. If it was more violent I would say not, but the way it is being described I think it is possible. Not saying I would bet on it but something I may check. I will also say finding a tire shop these days that can balance a tire properly is like winning the lotto, so I would start with the basics and have that checked also. My shop which we work on newer imports find tires being misbalanced the #1 complaint for shimmys shakes and vibrations at speed. The common reply from customers is we just had them balanced. This usually occurs at least once a week minimum. Of course lots of things could cause this just throwing a couple darts at the board! Good luck hope you come up with the answer quick!

The shop that balanced them did it with me standing there while they did the alignment and balance. Jeff is very conscience and I am sure he did it correctly (if his machine was in calibration). It just does not feel like the right frequency for wheel balance, it is a _very_ low frequency drone that seems to emanate from the power train.

 

[Lionized]

The shop that balanced them did it with me standing there while they did the alignment and balance. Jeff is very conscience and I am sure he did it correctly (if his machine was in calibration). It just does not feel like the right frequency for wheel balance, it is a _very_ low frequency drone that seems to emanate from the power train.

Before I had a new driveshaft built my entire car shook and felt like it was going to enter another dimension. My vibration happened around 90mph and scared the living $&it out of me.

 

[Skey]

The shop that balanced them did it with me standing there while they did the alignment and balance. Jeff is very conscience and I am sure he did it correctly (if his machine was in calibration). It just does not feel like the right frequency for wheel balance, it is a _very_ low frequency drone that seems to emanate from the power train.

I understand, it's hard to diagnose something like this if your feeling it let alone it being described to you via internet, just wanted to throw it out there. To me it sounds like a driveshaft issue based on your description but always like covering the basics first. Do you have any friends with the same car you can swap driveshafts with? Or a good driveshaft shop that can check the balance?

 

[miller]

The front u-joint was the factory Chrysler part (still had the little clips that held the caps in place in the factory so they did not fall of on the line), the rear had been replaced.

If the front u-joint is the original, suggest you get a hand on it, and check for any play. If you get any movement in the joint itself, there's your vibration. Would be a bad day, if it came apart on you, going down the road!

 

[JimKueneman]

If the front u-joint is the original, suggest you get a hand on it, and check for any play. If you get any movement in the joint itself, there's your vibration. Would be a bad day, if it came apart on you, going down the road!

It was original, everything has been replaced.