speed related Vibration (approx 55-75) seeking opinions

[Keith Seymore]

Worst part is I have no idea how to troubleshoot it without simply replacing parts.

If you can follow this thought process you can diagnose and fix anything.

It's all about "divide and conquer". If you can rule out large chunks of the vehicle then that allows you to focus on the remaining parts (regardless of any random suggestions those trying to help might throw at you).

K

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[Keith Seymore]

My vibration feels like a buzzing. I get up to 45 and I get vibration, it stops through 60 or so, then 65 and I get a buzzzzz..... buzzzzz.....buzzzzz. Its not continuous, it pulses like that. Looonnnggg annddd sloooowwww. The faster I go the worse it gets. 75+ is... out of the question.

"Looonnnggg annddd sloooowwww" is called a "boom/beat" in the biz. That means you have two disturbances, very close in frequency, that are playing off each other. The sine waves are out of synch just enough that the amplitudes add, then gradually subtract, then gradually add, causing the boom/beat effect.

In our trucks it is typically 2nd order of driveline (2 kicks per rotation of the shaft, due to u joints) and 4th order engine (firing frequency of a V8 engine).

The good news is that if you can make one of the disturbances go away then you've fixed the boom/beat problem.

K

 

[Oldsnut]

Thanks for this information. It will take me a while to fully investigate as I have too many projects going also. I am so thankful that I thought about running it up to speed with the rear shaft out as I had convinced myself that it had to be coming from the rear end. Absolutely rules out so many possibilities! A friend told me that his father had an early 90's model Caddie with the same engine trans combo that had a similar vibration and they found that there was a bearing in the tail of the trans that was bad. He claimed that they replaced the bearing with out removing the trans. Not sure this is the issue since you and I have different transmissions, but I guess anything is possible.