I have a bad vibration that occurs in 4th gear (OD) only...it comes on around 62-65 mph at 2000-2500rms... If I shift into 3rd it goes away. If I get on the gas it goes away. I've ruled out the drive-shaft and u-joints, they are all good. Rear end, pinion angle, and wheel bearings look/sound fine. Trans and TC are brand new.

The possible culpits I'm considering are:

-trans and/or motor mounts
-CV joints in the front

....is there anything else I'm missing?

Stroker2112

 

Is it lock up related? Can you get a scanner and watch the TCC for when the vibration occurs?

Chris

 

...yes, it starts right after the TC locks up.... I'm finding alot of other threads and Silverado owners with a similiar same problem. Wondering if it can be resovled by the changing the TC settings or something else in the tune?

 

Just another idea:

When you're scanning & driving it, check for mis-fires. Quite a few times I thought I'd be looking for a drivetrain vibration, to find out it was engine mis-fires under light load.

 

....thx that sounds possible. If it was happening why would it only misfire in 4th and not when I shift trans into 3rd?

 

Often, an ignition misfire is load-related.

 

Wondering about a couple more areas...

- Alum block and underdrive...lighter engine maybe prone to more torque/vibration on motor mounts?
- Old MAP sensor
- incompatible TCI flexplate / Yank TC combo?

what's curious is how it always occurs in OD 4th gear at 60-65mph in low rpms and nowhere else

 

Did you get your vibration figured out? I think the vibration on my truck is the TC also. But mine is only when I let off the gas.

 

I heard from a friend of my Dad's who worked as a transmission tech for a GM dealership for a long time that there is a TSB out for the driveshaft on the extended cabs. He said the TSB tells you to take the short and long driveshaft apart and grind down the key slot so you can rotate them a couple teeth over and reinstall. Its supposed to make a vibration at highway speeds. Might be something to look into.

Also your vibration may only show up when in 4th because there is no buffer when in lock-up. In third gear you have only a fluid coupling (torque converter) but when the converter locks up in 4th and there is a solid 1to1 connection you have nothing nothing to buffer things like misfires when they happen.