Drive train vibration. Scratching my head.

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JoePole1

JoePole1

A dude in a B body
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Hey Guys. Could use some help here. Drivetrain is LA 360/904/8.25. Combo has been in the car for over 10 years. About a year and a half ago I developed a vibration at about 55 and by 70 it is pretty heavy. It is only under power meaning if I take my foot off of the accelerator it stops. It is also not constant. It sounds like waaawaaawaaawaaa. This all seemed to begin with a gear and sure grip change from 2.76 to 3.55. I tried turning the driveshaft orientation even though I marked it. No change. Thinking it must be from the rebuild (new gears, bearings and crush sleeve eliminator) we went back in and checked the contact pattern again and it looked great. The only thing I noticed is that the pinion preload was at 14 inch pounds with a spec of around 30 but when we tried to adjust, we could not increase it and we were applying over 200 ft/lbs. After putting in fresh fluid with additive the vibration felt like it came in higher around 65mph. I can also feel the vibration seat of pants at 30+ but not pronounced.
I then started using 2 hose clamps and moving around the back end of the driveshaft in 90 degree increments to see if maybe it was out of balance. Vibration maybe changed a little but hard to tell.
At this point I am not convinced it is from the rear end but it was the only thing I changed.
I am not a drivetrain expert. I did the rebuild with a mechanic buddy of mine who has done quite a few similar rebuilds on GM's. He has not felt anything like this before.
Any help is appreciated.
 
Maybe the outer ring on the vibration damper spun on you....
 
JoePole1 said:
I never marked it. Any way to check in car? Timing off? Engine runs great.
Click to expand...


Use a piston dead stop tool and put the #1 cylinder at TDC, then check where the TDC mark on the outer ring of the damper is.....
 
It could be that by changing the engine RPM at a particular speed (diff gear ratio) you are noticing things that have been there all along but you didn't get to that engine rpm often enough to notice it? ( At 55 it is only about a 600 rpm diff????)

Dies it do it with trans in neutral coasting but keeping the engine at the same RPM that was needed to go 55?
 
Dana67Dart said:
It could be that by changing the engine RPM at a particular speed (diff gear ratio) you are noticing things that have been there all along but you didn't get to that engine rpm often enough to notice it? ( At 55 it is only about a 600 rpm diff????)

Dies it do it with trans in neutral coasting but keeping the engine at the same RPM that was needed to go 55?
Click to expand...
I don't think so but will try that.
 
My thinking...
  1. If the engine at the same RPM creates the same vibration sitting still in the driveway it is engine / trans.
  2. If it happens while coasting in neutral, engine ideling but still moving at 55 to 70 it is drive shaft, rear end related, or tires / wheels.
  3. Both could be trans to some extent.
What I posted earlier was meant to be the above.
 
Does it vibrate in park or neutral? If so, you just eliminated the drive train.
 
I think a couple of big clues are the vibration occurs at different speeds although I may not be feeling it so much at lower but it definitely gets pretty extreme from 60+ mph. Other thing is the tone. It's not steady. Kind of oscillates if that makes sense. WaaaWaaaWaaaWaaaWaaa...........
 
i had the same problem and it took a few days to figure but it turned out to be a crack in the race of the rear wheel bearing . i put new races and bearings in both sides and it now runs nice .
 
JoePole1 said:
Thanks KK. Gotta order one. SBM is 14mm....correct?
Click to expand...

I made one by gutting an old spark plug.
Ran a 3/8-16 tap through the body.
Rounded the end of a 3/8 bolt to contact the piston.

You might try driving the car in 2nd gear and bringing it up to the same rpm you see in 3rd at 60 mph. If the vibe happens I'd suspect the engine/trans. If no vibe then something in drivetrain after the trans.
 
If ur feeling it the seat then it’s in the rear. Drive shaft, pumpkin, axles or bearings or a bent housing. If u only got the torque on the pinion nut to 200 lbs after the fact then u already were asking for a problem. Kim.
 
oldkimmer said:
If ur feeling it the seat then it’s in the rear. Drive shaft, pumpkin, axles or bearings or a bent housing. If u only got the torque on the pinion nut to 200 lbs after the fact then u already were asking for a problem. Kim.
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The pinion nut was not tightened more when checking after rebuild/vibration. The rotating torque was at 15 inch pounds so still under spec. Odd. Not sure if crush sleeve eliminator played into that.
 
Holy crap! I literally came on here to post about dang near the exact problem. Above 65 mph is when it starts to get that low frequency vibration and "whoomp, whoomp, whoomp". 65+mph the vibration starts is higher (the oscillation gets faster) and the vibration gets a bit worse. Happened with my slant six /904/8.25, now it happens with my 360/904/8.25. Common denominator is the 8.25. Before we go in with a dial indicator, we pulled the inspection cover off and found the spider gears with wear and they move in an up and down fashion. My ring and pinion look fine. No shards of metal. Do your spider gears do the same? I have a video of it below.




Apologies I'm not trying to hijack your thread. I'm hoping I can provide anything you may not have looked at and vice versa.
 
I am going to dig a little deeper over the next 2 days using the recommendations I got from members here (thanks). I'll check spider gear play when I pull the cover as my sure grip unit is new. Can't comment if that is normal or not. Will report back here. No worries about the Hi-jack. Misery loves company.
 
JoePole1 said:
Hey Guys. Could use some help here. Drivetrain is LA 360/904/8.25. Combo has been in the car for over 10 years. About a year and a half ago I developed a vibration at about 55 and by 70 it is pretty heavy. It is only under power meaning if I take my foot off of the accelerator it stops. It is also not constant. It sounds like waaawaaawaaawaaa. This all seemed to begin with a gear and sure grip change from 2.76 to 3.55. I tried turning the driveshaft orientation even though I marked it. No change. Thinking it must be from the rebuild (new gears, bearings and crush sleeve eliminator) we went back in and checked the contact pattern again and it looked great. The only thing I noticed is that the pinion preload was at 14 inch pounds with a spec of around 30 but when we tried to adjust, we could not increase it and we were applying over 200 ft/lbs. After putting in fresh fluid with additive the vibration felt like it came in higher around 65mph. I can also feel the vibration seat of pants at 30+ but not pronounced.
I then started using 2 hose clamps and moving around the back end of the driveshaft in 90 degree increments to see if maybe it was out of balance. Vibration maybe changed a little but hard to tell.
At this point I am not convinced it is from the rear end but it was the only thing I changed.
I am not a drivetrain expert. I did the rebuild with a mechanic buddy of mine who has done quite a few similar rebuilds on GM's. He has not felt anything like this before.
Any help is appreciated.
Click to expand...
You did not say who's gear set you used.
After market gears are not like they used to be, some are noisier than others.
Point is good luck finding a current after market gear set that does not make some kind of weird noise. Even with a perfect pattern.
 
JoePole1 said:
It does not vibrate in park or neutral so yeah I do believe it's in the drivetrain. What about the new gear or new suregrip? Could they throw out of balance somehow?
Click to expand...

I wouldn't think so.
 
Have you checked the transmission mount to make sure it hasn't collapsed?
 
512Stroker said:
You did not say who's gear set you used.
After market gears are not like they used to be, some are noisier than others.
Point is good luck finding a current after market gear set that does not make some kind of weird noise. Even with a perfect pattern.
Click to expand...
I believe it's Motive gear set. No gear noise. It's the vibration that is causing the noise and only at high speed/RPM. Very faint at low speed (35).
 
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