Technical Low Frequency Roar in my 41 Plymouth

Finally got all of the needed materials and pulled the pinion and differential out today. Those bearings are definitely worn. I hope to finish assembly tomorrow and, hopefully, eliminate the roar.

Btw, the backlash was 9-10 thousanths.

 

I can’t remember what the book calls for,,,,,seems like .007,,,,but for an old differential,,,,9 to 10 ain’t bad .
And besides the new bearings might just tighten it some,,,try the bearings first .
Then worry about the lash,,,,you should be good after that .

Tommy

 

Not knowing the specs for that specific rear, but that is a tad loose. I have always gone with 5 to 9 thousandths back lash and had good results. Shooting for 7 thousandths as perfect.

 

6-8 is spec.

 

DANG! I finished rebuilding the rear axle and I STILL have that noise. It sounds and feels like I would imagine an out of phase driveshaft would sound but this one isn't out of phase. It isn't bent, the u-joints aren't loose and it's been balanced. I also experimented with pinion angles and that made no difference. I'm about ready to scream.

 

Hmmm....this is familiar. I recently finished working on a similar problem. I started at the back and worked forward because I initially thought it was a bad axle bearing. After replacing both bearings the car still had the noise. The problem ended up being the bushing in the tail housing. No leaky seal, but the bushing was worn nearly down to the outer steel shell. I don't recall any mention of this being a cause for noise, so I was surprised to find it had been the bushing. Apparently the driveline was oscillating enough to cause a harmonic that sure sounded like a worn axle bearing to me....lesson learned.
Something to check. Good luck!

 

Tires??? Swap front to rear see if any change. Ebrake if still on the trans releasing all the way? Have you messed with the rear brakes lately?

 

Last spring I had a strange noise that drove nuts. I had lost about 30 pounds and my wedding ring was loose, so I grabbed a ring that my father used to wear whish is smaller to keep my ring from falling off. I drive 90 % of the time with my window down and my hand on the edge of the opened vent window. Whenever I got above 40 I would get a high pitched whistle. It finally dawned on me it was the wind going under the stone setting in dad's ring causing the noise. Now, I keep the setting side toward my palm. No more whistle...

 

It is a rebuilt tranny with 5k miles on it. The slip yoke does not move laterally. I don't think that is it.

 

Big 'uns and Lil' uns. The tires have no cupping. They've only run on this rear end and there is virtually zero run out on the flange face. That's not it, either.

 

I'm riding with the windows up.

 

I'm about ready to slit my wrists.

This weekend, I tried different tires and wheels. No change.

I then decided to check my driveshaft length and discovered that I had apparently made my measurements with the lowering blocks in the middle position but then later decided to move it back an inch or so to center up the wheels. As a result, I had a new tube put in the driveshaft this morning....and guess what. No change.

This afternoon, I jacked the car up and put it jackstands under the rear axle, cranked it up, put it in 4th gear and got under it for a look see. (I probably should have done this soooner.) I observed eccentricity on the pinion end of the shaft. I measured it and it's about 24-25 thousandths.

Now it looks like, either the driveshaft is not concentric (doubtful), the yoke is not true or the pinion shaft has been tweaked. Is there any way to check the pinion and yoke without disassembly? If not, I'm looking at another teardown, it seems.

My plan right now is to go back to the driveshaft shop in the morning and ask them to check concentricity in the lathe. If it is true, then I have to figure out which of the remaining 2 parts is at fault. I am open to suggestions.

 

The rear yoke could have easily been damaged over yours of use. I had one where the u joint could still be shifted a little even with the strap bolts tight. A wedged screwdriver could walk the u joint back and forth. On that one, replacing the bolts and the straps solved that issue. It wouldn't take much wear off the holding tabs to add up to .024 offset you are getting. I'd look at that before sending the driveshaft back. I don't know of any way to check the out of round on a pinion or the yoke, if it was out of the housing. A .024-.025 bend or offset will be hard to see with a naked eye. Maybe you could set up an indicator on the housing and turn the pinion slowly to determine if the offset is in a direction the u joint can be shifted in the yoke. You may be able to loosen the u joint straps and shift the u joint in the yoke .010 to the bad side? Heck, there may be that much free play in the new u joints these days. Gene

 

I'm grabbing straws at this point. Thanks, Gene.

 

Possible you whipped the output shaft bushing by riding with a short driveshaft.
There’s a few really nice differential noises troubleshooting/diagnostic drill down charts. You could find one and study it then drive it or find someone who knows how to drive the car against the drill down.

 

Buy an automotive stethoscope,,,they are only a few dollars,,,,you can get one from HF that is suitable .
Have it on the jack stands,,,in gear,,,,place the probe against the pinion yoke area of the housing .
A roar will easily be heard,,,,,try different areas of the housing .
If no joy,,,,,move to the tail of the transmission,,,,if the rear bushing is bad,,,you will hear it .
And besides,,,I can’t imagine a bushing would roar,,,,vibrate yes,,,grind maybe,,,,not sure on the roaring part .
All of the moving parts will make a different sound on a car,,,surprising how you cant hear the little sounds .
Like the bearings in an alternator,,,,,front and rear,,,,,,they make a noise that can’t be heard running on the engine,,,,,except with a stethoscope .
But a bad one will roar like crazy with a scope .

Might very well be a bad pinion,,,,you did replace the bearings ,,,right ?
A friend had this happen,,,it was the small front pinion bearing after all.
Good luck .

Tommy

 

And the races,
And set the lash
And new crush collar
And cleaned all foreign materials out

 

Yes, I replaced races, bearings and seals. I installed them properly, as well.

I am very suspicious of the rear u-joint. It is one of those that adapts one driveshaft yoke to a different pinion yoke. I dropped it back off at my driveshaft shop for them to examine again, and possibly change the yoke to a symmetrical u-joint. I saw no wear in the pinion yoke and no shifting in the u-joint.

I also drained the transmission and found no foreign matter in the oil. It is not damaged and the vibration is definitely at the pinion, not the tailshaft

 

x2 on stethoscope. When you've got one you'll wonder how you ever managed without. $10 or so well spent. So sensitive. Totally unlike the wooden dowel, pry bar, socket extension etc. Great fun listening to everything.

Chris

 

As we say in the South, you ain't gonna believe this shit. I was sitting under the car looking at everything that could be metal touching metal and trying to figure out where this vibration was coming from. As you know, there is always vibration in the suspension and the driveline. Rubber bushings in the suspension isolate that vibration........when they are working.

As I looked, I spied that one of my rear leaf springs was cocked and, upon further inspection, I discovered that the rubber shackle bushings had given up the ghost. The shackle bolt was touching the leaf spring and the entire shackle was cocked so that the opposite upper corner was touching the chassis. I replaced those with new bushings when I built the car but the quality of the rubber looked like early Chinese to me.

I turned the bushings over until I can get new ones and guess what? Very little vibration and harshness now. I feel like a dunce. I spent a bunch of money and time trying to find out that 20 bucks worth of rubber bushings were my main problem. At least I have the peace of mind knowing that everything else is good to go.

Thanks for all your suggestions and attempts at helping me find this problem.

 

We all knew it was going to be one of those oddball little things that caused the problem, but there are so many little things that could have caused the problem.
I'm happy to hear you have found the cause of your trouble. Sure beats tearing the rear end apart again. Gene

 

Yea

 

A trick i've learned when trying to diagnose a driveline noise is to take a long rubber hose and run it from the vent tube on the rear axle or transmission, and stick the other end through the driver window. Drive down the road and you can hear clear as a bell. I had a problem vehicle at work and drove around with a hose on the transmission, transfer case, and rear axle- like a cheap multi-channel chassis ear!

 

Hey, it's gotta roar, with all of that Mopar Power.....love yer car !!

 

It will often be the cheapest thing that gets overlooked...

 

Thanks for sharing the final solution. So many times when the OP finds out it's something stupid they missed they never post the outcome. What you just went through may help another Hamber in the future.

 

Thanks for coming back with your solution !!

 

The reason my shackle bushings failed was that they had insufficient travel to compensate,for change in spring length as they compressed. Therefore, the weakest link, the small rubber bushings were basically sheared in half by the shackle bolts.

In order to prevent that from recurring, I made some "offset" shackles that added 3/4" of travel and put the shackles at near vertical when the suspension is at rest. This should solve my problem.

 

What a friggin journey, glad you found the culprit, I would replace that rear offset u joint or at least inspect it.

 

glad you found it, test drive it yet with new
Them itty bitty squishy bushings hold the whole damn car.