Transmission or pinion angle vibration. Unknown

Leo Opalewski · October 20, 2020

My pinion angle i believe is fine. Truck is lowered..... Trans -6 shaft -3 diff +5 it started happening about a couple months ago. U joints were replaced. I think its 4th gear when I step on the gas pedal if feels as though I running over a rumble strip between 58-63 mph. The more rpms I am at at this mph range the vibration is stronger. Stops after the speed. No vibrations from 65 and above. Did a few WOT runs, no traffic on highway. No abnormal vibrations at 0-55 65-97. Only happens upon acceleration. 3.08 gear ratio. 2010 gmc sierra CC.

sefiroxx · October 20, 2020

Did you get a new driveshaft to lower it. It might be out of balance.

Check your mounts.

Leo Opalewski · October 20, 2020

Same driveshaft. I replaced the u joints. But that was way before this started happening. Maybe I used the wrong u joint clip size and they shifted a little. Just hoping its not my trans. Looking for some answers.

Leo Opalewski · October 20, 2020

Also motor mounts were replaced last year and transmission mount is fine. No cracking etc. Have to really push on it to move it.

Doug_Scott · October 20, 2020

Is this the 4spd or 6spd? Put shifter in 3 and see if it is the same. U-joint vibration is usually lower in vehicle speed, and does not matter what gear it is in. You may want to remove the shaft and move the u-joint through its range of motion, feeling for smoothness through out the entire range. You can take off the two caps with the shaft off of the differential end of the shaft and look for lots of lubrication. You should not see any shadow lines that look like the bearings have been stationary.

Leo Opalewski · October 21, 2020

Yeah when I installed them they were moving fine. Even used a runout gauge to text axial play. Transmission is 6l80

Doug_Scott · October 21, 2020

7 hours ago, Leo Opalewski said:

Yeah when I installed them they were moving fine. Even used a runout gauge to text axial play. Transmission is 6l80

The vibration only just started didn't it? U-joints cannot be checked with the driveshaft installed. If one trunnion is starting to seize it will not be detectable with shaft in place, it must be removed to check for any tightness. You should also check to see if the lockup convertor is working correctly. Easiest way to check the convertor is to get the convertor to either unlock, or select a gear with the shifter that does not have the lockup as part of the gear. I don't know if the 6 speed unlocks the convertor when you brake, but you can try to just touch the brake pedal with your left foot enough to turn the brake lights on. Leave your right foot on the gas while doing this and see if the RPMs increase. If it does, try to get the vibration to occur. You will need to keep the brake lights on to keep the convertor from locking up. If that does not work for unlocking the convertor, perhaps there is a fuse for it.