# Differential Vibration??



## leeklm (Mar 11, 2012)

Guys,

I was hoping someone may be able to help diagnose a drivetrain vibration that has me somewhat stumped...:confused

"seat of the pants" vibration around 60mph+ that is not impacted by engine RPM. Very noticeable on the 4spd shifter. Here are the troubleshooting steps i have taken so far...

--Roadforce Balance on all wheels, no vibration in the steering wheel.
--Balance driveshaft, new ujoints, etc. Stock angles on driveshaft, nothing funky going on with pinion angle, etc.
--Up on jack stands, run the car to 60+mph with no wheels or drums, most of vibration still exists, although it feels just a little different. 
--Replaced one axle that looked to have a tweaked flange and some wheel run-out- no noticeable change in vibration
--Removed the driveshaft and ran muncie trans up to 70mph. All nice & smooth, so culprit is behind the transmission.

Someone told me they experienced something similar on a newer Firebird (4th gen), and it turned out to be the ring & pinion.

I have a chevy 8.2" that was rebuilt by me (probably explains the problem right there ) with all new bearings and new Ring/Pinion. The rearend runs smooth & quiet. I have put about 1,500 miles since the rear rebuild, and everything seems to be fine, with no abnormal metal dust or chunks on the magnet after recent inspection.

After all of this, I am thinking the problem is somewhere in the rear axle. I have another unit I can bolt in place for trial, but was not real excited about going through the process just for the hell of it...

Anyone have a similar experience with a rear diff?

Thanks!


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## geeteeohguy (Feb 2, 2008)

Not with a ring and pinion. Have had bad pinion yokes that allowed the u-joint to shift off center. Get a ball joint dial indicator gauge and check the run-out on the driveshaft. Also, check the run-out on the rear axle flanges, and also the run-out on the wheels. Check for radial run-out on the tires (flat spots) too. Look at the rear brake drums and check for any signs of a thrown drum balance weight. Also, check for worn rear trans bushing (usually the seal will be leaking if the bushing is bad, though). Vibration issues can be one of the most frustrating things on an entire car. hang in there. You'll get it, hopefully sooner than later!


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## leeklm (Mar 11, 2012)

Thanks geetee. I still have the vibration with no wheels or drums, so rules that out. The pinion yoke holds the ujoint nice & tight, so should be ok. My trans does have some wear on it, including a whining 3rd gear, and does leak a little on the rear seal. Of course the shaft would likely not vibrate with no load on it, just spinning the trans freely. I will check for any play on the output shaft and go from there. I will be pulling everything out this winter, so may just live with it for the next month. 

Regarding the ring & pinion, if it was bad enough to cause a vibration, you would think the thing would be shredding apart!


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## BearGFR (Aug 25, 2008)

Are you sure about the driveline operating angles? That's what it sounds like to me. If you've changed anything on/near the trans crossmember or trans mount (taller urethane mount maybe?) that can cause it.

Bear

(edited to add a reference: http://www.hotrodhotline.com/md/html/drive_shaft_harmonics.php )


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## geeteeohguy (Feb 2, 2008)

Also, transmission to bellhousing and especially, bellhousing to engine trueness. There are offset dowel pins available. You need to pull the trans and use a dial indicator. As for the trans, some play or slop at the tailshaft is normal...the front slip yoke running on the bushing keeps it all true. I had a vibration and a rear seal leak on my '65's muncie, and when I replaced the seal, it still leaked and vibrated. When I tore it down the second time, I found and replaced the worn out rear bushing (an easy in car repair...same as a TH350 rear bushing) and that, along with a driveshaft overhaul, fixed the problem. My worn out front yoke caused the whole thing. Check the front yoke for wear, too!! If you had a ring and pinion out of true enough to cause vibration, it would be howling and grinding like crazy. Doubt it's in the gears.


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## leeklm (Mar 11, 2012)

I have never watched the shop balance a driveshaft, but would a worn yoke not show up during the balance spin?

I just checked the tail shaft of the trans, and there is some end play, but did not seem real excessive (maybe an 1/8 to 3/16" with some pressure applied via screw driver.) Trans needs to be gone through anyhow, so may just wait until I take it out.

Checking the shaft angles with a protractor would be a good idea. Just found an article on the topic, which allows about 3.5 deg of max angle difference. Crossmember, mounts, etc. appear to be stock, but based on the previous owner, anything is quite possible.


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## leeklm (Mar 11, 2012)

Making some progress now... Dug into the kids schools supplies and made me a low-tech angle indicator... Not quite as fancy as the digital units that mount on the ujoint caps, but should be in the ballpark. According to this article, the ideal scenario is to have the same angle at each end of drive shaft. In addition, you should have minimum of 1/2 degree of operating angle, and no more than 3 deg.

Free Driveline Assembly Troubleshooting Information - Drivetrain.com

I started with 4 deg at trans and 0 deg at differential. I moved my trans up and brought the difference to within 2 deg. After a test on the jack stands, and a short test drive, it does feel like an improvement!

I will be getting on the freeway on Sunday, and will see what kind of improvement i see, or feel.


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## jbs (Jul 23, 2012)

Had same thing in my 65 it was drive shaft yoke and tail shaft bearing


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