We've had a lot of people posting lately with drivetrain vibration problems. For those that mysteriously occur after a clutch or Transmission change, I have discovered something that I don't believe any one has mentioned.

Check the side to side location of the transmission crossmember. It must go back in precisely the same location as the factory installed it. Usually easy to tell if your car was undercoated....but you may want to scribe around the crossmember to mark it's loaction.

When you install the tranny...do not install the center mount bolt untill after you have aligned the Crossmember to the original location and tightened the 4 mounting bolt. Then install the center bolt. You will usually have to use a pry bar to align the Tranny mount to the center bolt hole in the crossmember. This is OK. It's usually not all that much....but it will ensure that your driveshaft is now phased correctly in a horizontal direction. We all think to check the vertical phasing...using magnetic levels to get the prop shaft in the correct position re: Tranny and diff.....but the tranny's got to be in the correct horizontal position as well.

Ran across this when we replaced a main seal on my car. A friend was helping me. He was bolting up the crossmember while I was doing something else.

Afterwards there was a horrible vibration on the freeway that was never there before. First thing I asked, was if he put the prop shaft back in the same position relative to the rear yoke. ( Prop shaft and yoke are marked with white paint for correct phasing ). He had. Only thing left was crossmember location. I've always made sure that the crossmember goes back in exactly the same position as the factory location.... but he was not sure if he had aligned the marks.

Sure enough. We got it up on a hoist and the crossmember was not in the same location. Now it was off only by about 3\8ths of an inch.....but that was enough to cause the problem. Undid the center bolt, loossened the crossmember bolts, realigned the xmember and tightened things back up. Tranny had to be pushed over 3\8ths of an inch to align center hole...but I knew that was normal.

Took it out for a test drive....ALL vibration gone.

Something to check guys. Happy diagnosing

 

Thanks for the heads up

 

interesting, very interesting, i thinks when the snow melts i'm gonna check this out, although i have no clue where my crossmember was originally maybe i'll try to move it a bit and see what happens

 

On my car if you let the tranny take it's own position the crossmember is positioned about 3\8ths of an inch too far to the drivers side. You might use that as a starting point if you can't see any pattern from the original install. Sometimes the original washer markings will be noticable on the crossmember. With my car it was dead easy because of all the undercoating.

Note that the engines in our cars are not installed dead center...they are offset a little to the....nuts I can't remember which way for sure...but I think it's to the passenger side. That would make sense with how the tailshaft of the tranny has to be moved to the passenger side slightly when lining things up.

Really made an amazing difference. You wouldn't think that
3\8ths of an inch would make all that much difference.... but it did.

 

The other thing to check is mounting of lower control arms.
I've had a vibrations at about 140 km/h.
First I thought it was a shaft. But after tightening (just a little bit) a bolt at lower control arm, vibrations gone.
By the way, the movement of LCA was rather hard to sense by hand.

 

thats an interesting thought too, but i don't think its my problem because this just went from no vib to bad vibe after some work on the car a while back, and the funny thing is i think it was related to the transmission......... by the way how fast it 140km/h in mph?

 

About 87 mph

 

WOW!!!!! Did a 6-speed swap...........horrible vibration above 70mph.........I am going to LS1 tranny mount instead of the poly mount. I will try to line this up this time around.

:lala: Best info in months!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Thank you