I recently completed swapping a 1969 Plymouth Belveder 8 3/4 rear axle into my 1984 Chrysler 5th Avenue. During the swap I replaced the axle bearings and races, all seals, the gear oil (with synthetic 75w-90), the driveshaft, and the u-joints (and I greased the u-joints). The drive shaft was replaced with what appears to be a driveshaft from a six cylinder car that was the right length for what I needed.

The car drove fine for two days. No noise, vibrations, problems, etc... Today, I took the car on a quick trip downtown, about five miles one way. On my way back home, suddenly with no warning, the car began to vibrate with a low noise that is best described as sounding and feeling like I was driving with snow tires. It is a very regular rythm that increases and decrease with RPM. I put the transmission in neutral and coasted and it is still there. When I slowed down to about ten MPH the vibration went away, but came back as soon as I sped up again. It got slightly louder when I accelerated. When i stepped on the throttle after coasting, there was a noticeable clunk from the rear axle.

Any ideas before I go blow my brains out? I just got this thing back together and now this. Argh. Driveshaft out of balance? U-joint failing? Axle bearing improperly adjusted? Drive gears need adjusting? Does it make a difference that the driveshaft might be a slant six driveshaft and my car has a 318?

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Casually looking for a Clifford hyperpak intake for cheap.

Get under it. Its prolly just a ujoint.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

I will check them out again, and shoot some more grease into them. I just replaced them about three days ago, so I would hope that they have not already failed. What are the chances it is a failing lock-up torque converter? Prior to the axle swap, I did note some lugging and a minor shimmy on the highway at cruising speed.

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Casually looking for a Clifford hyperpak intake for cheap.

The driveshaft doesn't know (and isn't smart enough to care) how many cylinders the engine has, so that's not a worry.

It's not a problem with the lockup function of the torque converter.

New U-joints make it less likely but not impossible that it's a U-joint problem. Did you use a good brand made in a country where U-joints get made out of something other than compressed yak poo?

Sounds like the driveshaft is out of balance — perhaps a balance weight broke off. Fixing this will require the services of a driveshaft shop. It could also be a problem with the U-joint angularity, which can arise suddenly as you describe if there's enough slop somewhere in the system (e.g. transmission mount, rear axle mounting) to allow things to shift and drift after installation.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The rear u-joint was made in the US, the front u-joint was made in Japan. Both were Napa heavy duty greaseable type. I guess I will be pulling the driveshaft and taking it to a driveshaft shop.

While under there I will check out the tranny mount and double check the rear axle mounting. It seemed good when I bolted the axle down last week, but who knows now that I have driven the car a bit.

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Casually looking for a Clifford hyperpak intake for cheap.

Pinion angle can be something of a moving target, especially on cars like your M-body with their "iso-clamp" rear axle mounting. It ought to be checked/measured per the FSM.

A bunch of years ago, before I had much of a real clue about pinion angle, I misunderstood the concept and so did something dumb that I thought was smart when I put an 8¾" in my '65 D'Valiant: I installed it with 4° wedges, thick end forward. The result was exactly the "THRUMMMMMMMMMM" kind of noise you are probably experiencing, complete with vibration visible in the rearview mirror. It persisted thru a driveshaft rebalance, a new transmission mount, and a new output shaft bearing. Higher speed = worse noise. Oddly, it went away when I drove the car from Eugene to Ventura very heavily loaded in the back. That is, it went away until there arose a "CLANK…CLANKaCLANK…CLANKCLUNK" and the noise/vibration came back at the same time as I saw a roughly 5" x 7" piece of sheetmetal go spinning into the ditch from under the car. It took me awhile, but I figured out what had happened: I had so loaded the car that the floor pan was resting hard on the pinion snubber, forcing the pinion angle downward ever so slightly, enough to take away the noise. The piece of metal I lost was the pinion snubber, and with it gone, the pinion nose jumped back upward and the noise came back.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I would suggest checking the wheel weights, it might have slung one or two off. It could also be a weight off of the drive line. I would also look at and re-torque the "U" bolts holding the axle. I know with a 4X4 it is recommended to re-torque them after a few miles of running to make sure they are nice and tight, after disturbing them when installing a lift kit.

Best of luck,

Mike

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An officialis Tyrde-Browne Racing membrum

Thanks for the suggestions. Whilst installing the 8 3/4, it was obvious that the "snout" of the axle stuck up more than I remembered the old one doing. I will check the pinion angle and maybe remove the snubber.

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Casually looking for a Clifford hyperpak intake for cheap.

Oops, no, don't remove the snubber. It needs to be there. If the pinion angle is incorrect, correct it with wedge shims between the rear axle spring perches and the springs.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yeah, this is getting hard to find a 'quality' u-joint now-a-days... The local Napa had handed me a pair of 'made in China' u-joints when I did the driveline in the Beater Valiant... Schuck's had a pair of Japanese made U-joints of which one immediately failed when I pressed the caps into the yoke... the metal was so soft and the needles were loose on the out of spec for clearance... I was lucky that my buddy's down at Davis Auto Parts (aka Baxter's) had a set of Dana u-joints with a 1992 stamp date on the box... fit good, installed good, and greased really nice....

One thing to ask Reed... which side/position did you put the grease zerk on?...the zerk should be placed under compression as the driveline turns...or else it'll fail prematurely...been there, did that, almost pogo'd a duster...lesson learned...won't do it ever again....


-d.Idiot

I agree with the pinion angle. Your shop manual should have a section in it on the proper angles (I believe you use a bubble level) and how to fix it. Also, you did get the bearings adjusted correctly, right? You got the lock in place on the adjuster?

I got the lock on the adjuster, and I am about 50% positive I adjusted the bearings correctly. I guess I need to pencil in a day to take the driveshaft off, check the u-joints, make sure they are installed correctly, find some shims, and get the rear axle positioned correctly. Going to be awhile though since I am swamped with work for the next three weeks.

DI- How do you tell what compression is? The zerk fitings were in the elbow of the X. They were offset fore and aft, but I don't recall which way I pointed them on install.

Thanks for the input guys, I'll get it straightened out eventually.

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Casually looking for a Clifford hyperpak intake for cheap.

Okay, I looked at my driveshaft as the car was sitting on the ground and the nose of the pinion is definitley pointing up. I have done some research and learned that this is almost universally considered a no-no.

As I under stand it, the angle of declination of the driveshaft coming off the tranny should match the angle of inclination of the driveshaft coming off the pinion. As my car is currently set up, I need to rotate the rear axle forward, past the 0 degree mark, and into the + degree angle vicinity.

Car Craft says 1-3 degree is optimal for a street car, but I have also read sources that say Mopar recommends 5-7 degrees pinion angle. Which is true?

Also, I have found shims for rear axles, but none of them have the large locator holes which corrspond to my M-body Iso-clamps. Is anyone out there aware of smis that have the M-body locator holes in them or will I have to purchase generic shims and drill them out?

Thanks for your patience in answering all my newbie questions about this stuff.

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Casually looking for a Clifford hyperpak intake for cheap.

So I finally jacked the car up and crawled underneath for a really good look at the rear axle. I discovered that the vibration is being cause by the fact that the end cap on one of the u-joint ends held in place by the straps on the rear axle yoke fell off! That is why I had the sudden vibration while driving. So now I am off to the parts store for a new u-joint cap. Hooray, no need for a drivelin rebuild or shimming the axle. Ahhhh......

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Casually looking for a Clifford hyperpak intake for cheap.

That was my first thought when this problem arose. Make sure you put the C-clips on all the U-joint caps otherwise you'll lose the next one. BTW, they are going to sell you the whole U-joint. Might buy a new strap kit too.

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Joshua