Troubleshooting a driveline vibration

[WissaMan]

So our 68 300 has a high-frequency vibration that comes and goes when driving down the road. I just recently had new springs installed and KYB shocks at all 4 corners, so it's sitting at stock height now but that didn't do anything to make the vibration better or worse. The garage we took it to said they saw some dents in the driveshaft and the exit shaft at the transmission had some play in it, but they said the U joints seemed good. The transmission itself seems to perform great with crisp shifts and the speedometer is way off (reads 80mph when you're going 60) which makes me wonder if it was swapped in from a different car with a different rear end ratio.

I notice that sometimes it seems to be dependent on driveline load (i.e. accelerating or decelerating) so if you modulate the throttle to remove the load, or drop it into neutral, the vibration will go away almost completely -- though not always. Sometimes it does not go away, which confounds me that's it's inconsistent like that.

It is not dependent on engine RPM and it does not vibrate when sitting still in neutral and revving the engine. The frequency of the vibration feels too quick to be a tire.

Anyone care to take a stab at guessing where the vibration could be coming from? There's a shop in town that specializes in transmission and driveline work, but I'd like to have at least some ideas of what it could be before I take it there. Could it be from the play in the exit shaft? U joints? Rear end? Driveshaft?

 

[65sporty]

If the driveshaft has some dents I would start there. Pull the shaft, send it to a driveshaft shop have them spin it and check it to see if it's bent or out of balance. If it's bent have them build a new one. Put a tailshaft bushing in the trans also.

 

[Big_John]

they saw some dents in the driveshaft

Not easy to dent a driveshaft... In fact, I'd have to say that if the driveshaft got dented, it's bent. It doesn't take much bend to be out of balance. Even worse, and it can drive you nuts, is if the shaft has a twist in it and the U-joints are out of phase with each other. AMHIK.

 

[rags]

dents = game over.

 

[1978 NYB]

You can take the drive shaft to a drive line shop and they can spin balance it for you.

 

[WissaMan]

I'd just as soon replace it with a new one rather than have it balanced. Is a driveshaft something that a competent shop can fabricate or do you buy them pre-made for the specific car? And as for the tailshaft bushing, does that get replaced with the trans in the car or does the trans need to be removed and disassembled?

How can a driveshaft get dents in the first place? Bottom out the car somehow like driving off a high curb or over one of those concrete parking stops?

 

[Ross Wooldridge]

New driveshaft required without a doubt. Next, the pinion angle is likely out of whack, as you describe a "come and go" vibration, which is one of the hallmarks of pinion angle. The pinion angle can be adjusted with shims under the axle perches.

As well, if the vibration has been ongoing for a long time, then the tail shaft bushing is likely torn up and possibly the slip yoke is scored. If it's not too bad it can be polished up.

The speedo reading being off indicates that either the trans has been changed or the rear end swapped out (more likely) with one of a higher numeric ratio, resulting in the higher than reality speedo reading. This can easily be fixed with a new speedo gear.

New drive shaft, tailshaft bushing, pinion angle adjustment, speedo gear.

 

[livininharrow]

any drive shaft with dents in it is junk. it needs a new tube for Christ sakes.

 

[cbarge]

Our C's had the two halves of the shaft molded with rubber at the rear axle end of the shaft.
50+ years and the rubber can be rotten.
Dents in the shaft and replace it no question.
Check the pinion bearing and also as mentioned the tailstock bushing for any play.
Worn ring and pinion with improper lash can cause vibration.

Process of elimination,hope this helps.

 

[rags]

extension housing is removed from the transmission in car. bushing and seal replaced on the bench. in truck repair 've seen worn bushings, loose pinions, bad carrier bearings, teeth broken off gears, etc, never cause vibrations. but a bad driveshaft can cause some of these other problems.

 

[Triple Pickle]

Drive train vibrations are hard to isolate. Start with the driveshaft itself to eliminate that. Check for complementary angles on the trans and rear yokes.

 

[CBODY67]

I'll concur on the driveshaft issue, BUT that doesn't take the "load factor" into consideration. An unbalanced driveshaft will probably get worse as the shaft rpm increases, linearly.

I'm more inclined to suspect the tailshaft bushing and possibly the front u-joint. Both of which a load situation might make an existing vibration worse, to me.

Our '66 Newport always seemed to have a light vibration at 70mph. I figured it was a tire issue. With my right arm on the front seat top, while driving, my arm would shake a bit. Higher frequency, but nothing serious.

Later, it developed a vibration and squeak when taking off from a red light. Sounded like the wheel covers squeaking as the wheel turned. Took the front wheel covers off, no change. Took it to Fenner Tubbs C-P to get checked out. The service advisor came back after a quick trip around the block. Front Y-joint was the diagnosis. I thought that odd, as everything seemed tight last time it was on the rack. But one front y-joint later, all was quiet and smooth again.

I doubt a driveshaft with a "ding" is not going to change the balance that much, if any. A kink or bend would.

Find a good driveshaft shop and let them check it out. Make sure they can check the balance, as some might not do that, just build a new shaft from known accurate tubing and ends. So ask that question. If it needs new u-joints, let them do that too. You can also check the slip yoke's snout for indications of wear, too.

Just some thoughts,
CBODY67

 

[WissaMan]

This is all good stuff, thanks everyone for the responses. I'll be sure to post when we get it sorted out. (if...)

 

[rags]

note for safety. the springs were replaced. if you haven't already, retorque u-bolts on axle housing now that the car's been driven.

 

[dart4forte]

Don’t rule out something as simple as a worn out tranny mount

 

[MEV]

Or a broken/separated engine mount

 

[david hill]

Here is my approach that is tried and proven to have good results. First rotate tires front to rear and re-road test, note any change. If the primary vibration still remains, tire balance is not the problem. Second raise your car up and support w/ jack stands w/ tires aboveground by several inches. Chock front tires, start car w/ eng warm and place in drive w/ the eng idling. Note any driveshaft for wobble or run out. Any beyond 1/8 inch is too much and a candidate for a rebuild. Check the rear diff. W/ a stethoscope for unusual noises. Us the same method as done/ in as used in checking the driveshaft, starting and putting in drive. No noises coming from the carrier, pinion or axle bearing areas, different probably not your problem. Now remove drive shaft for inspection. Any stiff or frozen joints bent weld yokes are grounds for a replacement or rebuild. If you have any questions, PM me.

 

[WissaMan]

There's a shop in town that advertises transmission work and I've sometimes seen old (classic) cars sitting for repairs. I went there and talked to the owner and he sounded pretty knowledgeable and mentioned many of the things talked about here on his own. He also said they'd keep it inside at night which I thought was very nice of him to offer. But mainly, he sounded upbeat about working on it and thanked me for coming in, as compared to some garages where if you took a car like this they'd be hedging and giving the general impression they were not too thrilled about working on it.

It'll be in his hands on Monday so hopefully he'll be able to get it sorted out.

 

[Nadir Point]

Don’t rule out something as simple as a worn out tranny mount

Or a bad U-Joint.

 

[John Kirby]

The speedo being way off is due to when the transmission was replaced they didn't swap the speedometer gear. Had the same issue with my 66 New Yorker after doing a rebuild at a transmission shop. I figure they must of swapped the transmission with a different one. You need to pull it and count the teeth on it. Put a pan under it so fluid doesn't drain all over the floor. Mine had 26 teeth (i think, been a while)and I changed it to a 34 tooth. Speedo now agrees with my GPS. You can find them on ebay by looking for mopar speedometer gear. There are two different shaft lengths. You need the long shaft version. If you know your rear end gear ratio there is a chart on the net somewhere that lists the correct speedo gear. It should be in the service manual too. They are color coded by # of teeth. The mount has tooth numbers printed on it for correct orientation.