Vibration from 30-45 MPH

[Robyn]

My 84 Blazer has a strange vibration that I think is drive line related but not sure. The 700 was replaced with a built K case unit recently and the drive lines and u joints are all good. The tranny locks up very early but thats not a real problem as I dont tow with it. The vibration and noise sounds like what you have when you lug a stick shift, if you feather the throttle is goes away. I have checked drive lines and motor mounts to no real avail. I have heard of cracked main webs causing vibrations but have never been around a 6.2 with broken webs and seen or observed what they do. The engine does not leak a lot of oil and runs smooth till the converter locks and the RPM drops. Oil pressure is good too.
I have done a lot with the 6.2 over the years but never seen anything quite like this. The rig was wrecked some years before I got it and it could have a drive line angle problem which would certainly raise hob with a smooth operation. I would like some input from anyone who has had a 6.2 lose the main webs and vibrate a lot. What type of vibration does this anomaly usually show??
Thanks troops
Robyn

[More Power]

Somewhat tall gearing (i.e. 3.08 or 3.42), early TC lockup and low rpms will transmit the engine's torque impulses through the trans and into the driveline. This usually causes a "ringing", that can sometimes be felt as a slight vibration.

If it's a rear u-joint angle problem, remove the rear driveshaft, then take it for a test drive in 4WD.

MP

[Robyn]

MP
Thanks
I was thinking this may be the case as this rig has 3.42 butt gears.
Unfortunately taking the rear shaft out is tough as it has a slip yoke the goes right into the back of the 208 and without it there is no support for the output shaft and the oil pours out.
I do believe what you are saying is most likely the cause as originally these rigs were set to lock at around 50 mph. This one will lock at 35 in 4th and some times its tough to get it to drop out above 40. When the trans was built the decision was made to convert the TCC to a full hydraulic lockup control and this I believe now was a bad choice. This power train package is going to be installed in My Humvee replica and I think I will switch it back to electric lockup control with a switch to keep it off at low road speeds. The worst thing about building a real nice 700R4 is that when you mix and match things get wierd as components are not matched to the original design and dont always do exactly what the original parts did. My 94 6.5 sub does not engage lockup untill 51-52 and falls out at 50. there is absolutely no lugging of the engine. The blazer on the other hand will rumble along and you can definately feel the engine as though it was a stick in too low of a gear.
Been hearing of 6.2's with broken webs in the bottom end and the vibrations they do and was begining to get a tad goosey. My Blazer is smooth up through 25-30 and then will start to vibrate some and then when the TCC locks its quite noticeable untill above 45 then from 50 up its smooth. Rigs been rolled before I got it ans who know what all else it was subjected to. very well could have been tweeked to even cause a driveline side angle. At this point I dont care about the Blazer as all I want is the engine tranny and Tcase for my project Hummer replica. Was worried a bit though about the engine and dont want to spend time and $$$$ on a dud. I bekieve I will remove the pan when I take the engine out of the Blazer and have a look at things in the bottom and if its ok (NO Cracks visible) reseal the thing and call it good. My Hummer replica project (http://www.bigborecanyon.com/badlands05.jpg)

[More Power]

Buy a little extra ATF, then let it run out of the transfer case when you pull the driveshaft. Once you determine whether it's the rear driveshaft u-joint angles or not, re-install everything and top-off the txfer case.

If you're concerned about dust getting onto the exposed output shaft of the txfer case, just make a cardboard tube that you can slip over the output housing then tape on a plastic bag.

MP

[Brianlar]

The symptoms you describe exactly match what I was getting til I replaced my U joints. I know you said they were good, but that's what I thought too.

[Robyn]

I replaced not only the joints but the shaft too trying to get rid of the vibration. I have not looked at the angles of the joints to see if both ends have the same working angle though.I guess at this point I will see what the setup does when its installed in the Hummer project as it drove smooth while it still had the Suburban body on it and none of the mounts or locations of gear boxes or axles has been changed. If the vibration turns up in the new rig I will have to be suspect of the drive train that came from the Blazer. Tranny is all new inside and the transfer case was fine. New torque converter (that run true).
Thanks guys.