Rear driveshafts relationship to front driveshafts...

[SDWA]

I recently replaced the center bearing assembly on my two-piece rear driveshaft, because the rubber isolator was disintegrated. Before I removed anything, I marked the position of the rear u-joint bearings in relation to the pinion yoke on the rear axle. I didn't mark the position of the front slip yoke/shaft in relation to the transfer case, because I didn't think it mattered, and that shaft isn't keyed. The center slip yoke stub is keyed to the rear driveshaft, so I didn't mark that either (but wished I had, because it would have been easier to align the keying later!)

Apparently something didn't get back together right. I now have a vibration/booming between 36-45 MPH that wasn't there before.

I checked all my bolts and they are tight. The welded u-joint yokes on the front and rear shafts of the rear two-piece driveshaft are 180° to each other. I'm assuming this is correct. The service manual references phasing of the rear driveshaft u-joints to each other (on the same shaft) but doesn't mention anything about the relation of the rear driveshafts to each other - I'm assuming the keyed stub shaft takes care of that alignment.

So - does it really matter where the front stub driveshaft is re-installed onto the transfer case output shaft? If it does, and I screwed up by not marking it before removal, how do I determine where it needs to be re-installed to? And if it does matter, why isn't that shaft keyed also???

Thanks!

Scott

[SDWA]

Ok, here's what I've found upon further investigation:

There is a keyway on the forward yoke (into transfer case) but no matching bridged tooth on the transfer case shaft.

Up on the stand, the vibration is worst at around 37 mph, disappears around 49 mph.

Took both rear wheels off, vibration still there.

Rotated rear u-joint in pinion yoke 180° (opposite of markings before removal), vibration still there.

Rotated forward yoke on transfer case 90°, still vibrates.

Ran without rear driveshaft installed, no vibration.

So, it would appear the rear driveshaft is out of balance. I'm seriously doubting that the bearing replacement process did that, as all I did was remove the rear driveshaft and carefully lay it on the ground. The original balance weights are still in place, nothing appears to have been knocked off.

But here's a theory I came up with - maybe the rear driveshaft has been out of balance from the factory (I missed it during the first few months...) and that's what ORIGINALLY disintegrated the rubber fins of the center bearing isolator (the bearing was fine). So the bearing free-floating inside the rubber isolator had essentially "isolated" the imbalance, to some degree. Now that I've replaced the bearing/isolator, that imbalance is again being fully transmitted to the frame/body.

My next step is to have the rear driveshaft rebalanced. Or maybe both driveshafts - on the rear shaft there is a sticker that says "Do not replace this shaft without rebalancing mating shaft" - I'm not replacing it, just rebalancing it. So do I need to take the entire assembly in (rear shaft, front shaft & center bearing) or could I just balance the rear shaft?

Or does this theory seem way out there?

Scott

[NutNbutGMC]

Ok, here's what I've found upon further investigation:

There is a keyway on the forward yoke (into transfer case) but no matching bridged tooth on the transfer case shaft.

Up on the stand, the vibration is worst at around 37 mph, disappears around 49 mph.

Took both rear wheels off, vibration still there.

Rotated rear u-joint in pinion yoke 180° (opposite of markings before removal), vibration still there.

Rotated forward yoke on transfer case 90°, still vibrates.

Ran without rear driveshaft installed, no vibration.

So, it would appear the rear driveshaft is out of balance. I'm seriously doubting that the bearing replacement process did that, as all I did was remove the rear driveshaft and carefully lay it on the ground. The original balance weights are still in place, nothing appears to have been knocked off.

But here's a theory I came up with - maybe the rear driveshaft has been out of balance from the factory (I missed it during the first few months...) and that's what ORIGINALLY disintegrated the rubber fins of the center bearing isolator (the bearing was fine). So the bearing free-floating inside the rubber isolator had essentially "isolated" the imbalance, to some degree. Now that I've replaced the bearing/isolator, that imbalance is again being fully transmitted to the frame/body.

My next step is to have the rear driveshaft rebalanced. Or maybe both driveshafts - on the rear shaft there is a sticker that says "Do not replace this shaft without rebalancing mating shaft" - I'm not replacing it, just rebalancing it. So do I need to take the entire assembly in (rear shaft, front shaft & center bearing) or could I just balance the rear shaft?

Or does this theory seem way out there?

ScottInteresting theory which sounds perfectly reasonable for consideration.

You are a diligent individual.... :D

I say have them both balanced while at it. Then there lies no doubt upon reassembly.

[SDWA]

After a $66 same-day balance of both shafts by Aukeen Drivelines in Kent, WA (highly recommended!) the truck is running smoother than ever! It didn't even feel like the rear driveshaft was connected!

The center was "way off spec" according to the shop. There's now a new weight at front stub shaft end, and on each end of the rear driveshaft.

I now have no doubt that the factory balance was off, and that is what originally destroyed the center bearing rubber isolator, and as a result what hid the imbalance.


Lesson learned: If you've got any type of unexplainable speed-related vibration that can't be blamed on the wheels/tires, you might want to check the driveline balance. Even if the truck is brand new!

Scott

[NutNbutGMC]

After a $66 same-day balance of both shafts by Aukeen Drivelines in Kent, WA (highly recommended!) the truck is running smoother than ever! It didn't even feel like the rear driveshaft was connected!

The center was "way off spec" according to the shop. There's now a new weight at front stub shaft end, and on each end of the rear driveshaft.

I now have no doubt that the factory balance was off, and that is what originally destroyed the center bearing rubber isolator, and as a result what hid the imbalance.


Lesson learned: If you've got any type of unexplainable speed-related vibration that can't be blamed on the wheels/tires, you might want to check the driveline balance. Even if the truck is brand new!

ScottMost excellent. Glad you got-her-did. :D