High speed vibration and grinding noise

[whtgmc68]

I need some help with my 68 GMC LWB 4x4. I put in a Smedding Peformance 383, Dodge 27 spline with GM input shaft NV4500 and matching fixed yoke NP205. It had a mild vibration above 70 mph that progressively got worse until it become awful at anything above 45 mph and I found a failed u-joint at the rear transfer case output.

I replaced the failed joint and the truck was a pleasure to drive up to 70 mph so I took it out on the interstate to see what it was like above 70 and that is where everything seems to have gone south. The truck drove great until the speedo hit 80 and the vibration came back along with a serious whine (almost a grind) that seems like it is in the transfer case. Once I dropped below 80, the whine and grind went away and there is no vibration. Truck is smooth as glass as long as I stay under 80. The truck is lifted 6" and the engine was set back 2" during the swap so i dont think driveline angle is the issue. I have checked to make sure the hubs are not locked and the transfer case is in 2H.

Anyone have any ideas? The entire driveline only has about 2,000 miles on it.

[Zoomad75]

This may come off a little blunt, but no offense is intended. You don't think it's a driveline angle problem, but you haven't checked it? New u-joints don't just fail in 2000 miles. Well, Chinese ones maybe but normally they don't fail that quickly without help.

Keep this in mind. What didn't seem like much to you initially when you felt it at 70 was beating up on the joint. Without checking the angles you truely can't rule it out as a source of your problem. You've got a 6" lift and moved the drivetrain back 2" from stock. Plus running the nv4500 and the 205. You don't have one item left stock except the axles in the drivetrain. If you didn't have to adjust the driveline angles or shaft length I'd be surprised.

I'm saying this since I made the same assumption. Shackle flipped the rear springs on my K5, added a 14b full floater and by uncalibrated eyeball it looked ok too. That was until it spit out the rear u-joint on the rear shaft. Lucky for me I caught it before it pogo'd the truck at speed. I might have had 50 miles on the new joints in the rear shaft when it let go.

That little event forced me to re-evaluate the rear driveshaft angles. Typical working angles need to be within 2 degrees of each other. Mine was 7 to 9 degrees off. After much measuring and checking I ended up rotating the pinion up 8 degrees and replacing the rear shaft all together with one built with a cv joint on the front side. In that case I needed the pinion angle to match the rear shaft down angle. The 8 degree shim lined it up right and the new rear shaft allowed it to run smooth at any speed.

Put the truck on a level surface. Measure the angles with a magnetic angle finder or download an app for your phone. I used one called "clineometer". You want the shaft oriented so that the rear u-joint cap on the axle yoke is pointing down. That will give you a flat surface to measure against. Again, if you are not running a cv joint in the rear shaft you should have no more than 2 degree difference in working angles from the front joint to the rear joint. The working angle is found by measuring the joint angle on the front and the shaft. Subtract the smaller from the larger number, which ever that one is. That result is your front working angle. Do the same at the rear joint and reuse the same shaft angle number. Again subtract the smaller number from the larger and that is the rear working angle. If they are both at 2 degrees or less your angles are ok. Time to move on to another issue. If they are higher than 2 degrees it's time to start shimming. Might need to drop the rear of the t-case down a little to limit how much the rear pinion might need to come up.

Whether it looks ok by eye won't cut it. Driveline vibrations are rough on parts. I've had them take out pinion bearings on RV chassis when the angles were not set right from a stretch job. The shop couldn't figure out why new bearings started making noise 500 miles after install. When they called me for help they finally measured the angles and found they were way out of line.

[whtgmc68]

No offense taken and I need to clarify a mistake in my original post and further explain. The engine was moved forward 2" for the distributor to clear the firewall, not backward. Also, the truck has a 14BFF rear and a Dana 60 front axle along with custom length driveshafts.

I had these axles and lift in the truck before I added the 383 and NV4500 and I never had any driveline vibrations. The Dodge 27 spline tranny and transfer case setup is about 1" longer than the previous SM465 and NP205. Based on this, I could see the front driveline angles maybe being a problem, but it would seem to me that the rear angle is now better than it was before the changes.

Also, I believe the u-joint was bad right from the start as the transmission had a roar under load in 4th gear that I believed was a harmonic roar that many say they experience with the NV4500 and since replacing the joint, that roar is completely gone.

[obijuan]

what type of rear driveshaft do you have?

[whtgmc68]

It is a 2 joint drive shaft and the 205 has a fixed yoke.

[obijuan]

what drivetrain was there before? what were the driveshaft rpms prior at the same speed?
are you following that diagram on your ujoint angles?

just because you didnt have it before doesnt mean shaft angles arent the cause. you changed it, even just a little.

[whtgmc68]

The only thing that was changed was the tranny and transmission which changed the length of the drivetrain by about 1" longer and then it was moved forward 2" so the driveshaft is about 1" longer than it was with the SM465 and 205 that were in it before. Both 205's have fixed yokes.

[Zoomad75]

Originally Posted by whtgmc68

No offense taken and I need to clarify a mistake in my original post and further explain. The engine was moved forward 2" for the distributor to clear the firewall, not backward. Also, the truck has a 14BFF rear and a Dana 60 front axle along with custom length driveshafts.

I had these axles and lift in the truck before I added the 383 and NV4500 and I never had any driveline vibrations. The Dodge 27 spline tranny and transfer case setup is about 1" longer than the previous SM465 and NP205. Based on this, I could see the front driveline angles maybe being a problem, but it would seem to me that the rear angle is now better than it was before the changes.

Also, I believe the u-joint was bad right from the start as the transmission had a roar under load in 4th gear that I believed was a harmonic roar that many say they experience with the NV4500 and since replacing the joint, that roar is completely gone.

Thanks for the clarification. You moved the drivetrain forward, but the nv4500 and 205 is a inch longer than what you removed. You still need to check the angles. The front really isn't going to give you any issue since I assume you are running around in 2hi on pavement.

I know an inch doesn't seem like much, but it makes a difference. When you are measuring down to a half of a degree it could be a major factor. Until you check it you don't know. I can't tell the difference between 2 or 4 degrees by just looking at it. Nobody can. Take the time to check it. If for any reason, just to rule it out.

['63GENIII]

When I swapped in my 4500, It took a while to get the rear end shimmed correctly. Then a year later, while swapping the rear D60 for a 14BFF, I was at it again only this time I was able to set the pinion angle and weld the perches in. At that point, even though the 14 bolt pinion was shorter than the 60 by 1 or 2 inches, I still had to put a 1/8" flat shim under the rear of the transfer case mount to get it back in place. It took a bit of time to get it right and the adjustment was very minor but made a huge difference.

I suspect that you'll have to get the DL angles right before attacking the transmission sound. I could see the vibration wiping out a bearing or bushing in the trans pretty easily.