[vegadan]

I posted this at the A-body site and thought I would post it here as well. I have a vibration issue with my 73 GA. below about 60 everything is fine. But above 60, the car experiences a vibration. It lasts for about 3-4 seconds, subsides then happens again. It is consistent and is not felt thru the steering wheel but is like the whole car shudders. Its not significant but it is enough to be annoying and somewhat concerning. Since it doesn't happen consistently, I don't think it's wheels, tires, brakes, front end components but then what do I know. Thoughts, suggestions? Thanks, Dan

[camerjeff]

How new are the U-Joints? When they seize up in one direction only they can give you odd vibrations at only certain speeds or RPM's. I would drop the driveshaft and check the feel & movement of both U-joints 1st.

[77 Canamman]

Bet it's U-Joint, trans bushing, or driveshaft balancing issue.

[vegadan]

Thanks guys. I had a new driveshaft w/new u-joints made when I switched from the auto to a 4 speed. I just reused it when I switched back to an auto. I don't recall if I had that issue when I switched to the manual, so even tho the joints/balance should be good, I will revisit them. I don't recall the diameter of the new or the old shafts. Do you happen to know what is the diameter of an original? Do you think if the new one is smaller that it might be vibrating at certain rpm? Also, how do you change out the trans bushing? Sorry for all the questions. Thanks again for the help.

[mlewisariz]

Two thoughts. One easy thing you could try is to disconnect at the rear and rotate shaft 180 degrees. This has worked sometimes. Another idea comes to mind. Check with whomever made your shaft when converting to 4spd and find out if auto application calls for a longer shaft. If too short there may be excessive play at front which could cause your vibration. A visual inspection would show if there is an exposed shinier section on the yoke that was was riding further into the 4spd.

[gtospieg]

Trans bushing is a simple fix...unbolt it from the crossmember and the trans and then lift the back of the trans and pull it out....reverse procedure to replace. Good luck

[78w72]

Originally Posted by gtospieg

Trans bushing is a simple fix...unbolt it from the crossmember and the trans and then lift the back of the trans and pull it out....reverse procedure to replace. Good luck

thats for replacing the trans "mount".... the bushing(s) is inside the trans.

[Keith Seymore]

A "pulsing" vibration is what is known in the biz as a "boom/beat".

It is caused by not one but two vibrations that are close enough in frequency so that they sympathize. The "fade in-fade out" aspect is a result of the amplitudes adding and subtracting as they pass by each other. We used to experience this regularly when I was in band class in high school and actually used this characteristic to help tune our instruments (slower boom/beat frequency means the two vibrations are getting closer together; faster means farther apart).

If you can find and fix one or the other of the two offending components then you fix the boom/beat aspect.

Engine firing frequency (4th order of engine rpm) and driveline u joints (2nd order of driveline) are a couple usual suspects, because they are often pretty close together with a 1:1 trans final drive ratio. Hopefully this gives you some ideas.

K

[vegadan]

Thanks guys. I had the driveshaft replaced and want to check to make sure it is the proper length. Does anybody know the dimension between the u joint centerlines?

[vegadan]

Wondering how to check for a bad motor mount. And how to replace them. Thanks.

[gtospieg]

You can usually jack the engine up one side at a time and see if there is any space between the motor mount halves. Sometimes you can replace them by jacking the engine up while it's in the car...if there's enough room to get at the bolts on the block...and sometimes not.

[stobin]

Check the driveline angle. When you switched back to the 4-speed it may have changed.

[stobin]

Check the driveline angle. When you switched back to the 4-speed it may have changed.