Source of driveline vibrations
 

I've been chasing a vibration in my truck for quite a while now.

The local transmission shop recently replaced the rear transfer case output bushing and seal along with the rear u-joint.

Sadly although improved, I still have a vibration at a frequency that matches the prop shaft frequency.

My question is this: Besides the prop shaft itself, what other parts could be out of balance and cause a vibration at the prop shaft frequency?

I've read that misalignment of the components can cause a vibration. My truck is at stock height although the T-bars are cranked fully and I have longer shackles installed due to the t-bars and leaf springs having sagged.

Thanks!

Follow up: I removed the front drive shaft and the vibrations were gone. According to my accelerometer, they were 10 times smaller. The front u-joint was stiff so I think that might have been the problem.

does that shaft turn when in two wheel drive?

On the 236 transfer case that Christine has in her truck, it will be quite stiff if you were to try to turn it by hand and will spin when in 2wd due preload in the viscous coupling inside the transfer case. On the 231/233 transfer cases, the front driveshaft should freely turn when in 2wd, but still may turn while driving due to parasitic drag in the driveline.

This is what the proprietor of the local transmission shop told me about my 236, i.e., "4-button", transfer case.

Thank you Swartkik for the always thorough explanations!

This I hope will be the last update on this.

I installed a replacement front drive shaft today. It was a Dorman unit I got from RockAuto. It was a little harder to get in than the old one was to get out since the new one was constructed from a hollow tube whereas the original was a narrow solid piece of steel. This made for a bit less clearance at the crossmember when inserting in from the front.

I then repeated my measurements of the vibrations as I had done before.

To compare the amplitudes of the vibrations, I took the standard deviation of the amplitude of the vibrations. The results were this:

• With original front drive shaft: std. dev. = 0.1953 g with pronounced well-defined oscillations with peaks at +/- 0.4 g. This was extremely noticeable.
• Without any front driveshaft: std. dev. = 0.0501 g with only very small oscillations at the wheel rotation frequency. This made for a very smooth ride.
• With the new drive shaft: std. dev. = 0.1088 g with no pattern to the oscillations in the frequency measurements, i.e., no dominate peaks at the wheel or driveshaft frequenies. The vibrations were back but very much reduced as felt through the steering wheel.

The only source for this remaining oscillation after installing the new front driveshaft might be due to the splined connection of the drive shaft to the output shaft of the transfer case. Once installed, I could wiggle the end of the drive shaft that coupled to the transfer case. If I had thought to get the dial indicator out, I would have measured it. A wild guess is that I could move it back and forth so that at the end of the transfer case output shaft it might have moved 0.040" total.

I did noticed some wear in the form of the surface metal being polished on the outer diameter of the splines on the transfer case output shaft. Short of taking out the whole transfer case, I can't think of a way of eliminating this free play.

With winter on its way and this being my winter vehicle, I think I'm done doing any major work on the truck for now.

Wish me luck with it.

OK, one final update in case anyone else finds this and is interested.

Per the previous post, I was concerned that there was still some vibration going on. I was wondering if it could be because of the bit of slop in the splined joint between the rear end of the front drive shaft and the front output shaft of the transfer case.

So I decided to measure this wiggle. I set a dial indicator on the frame and measured the amount of vertical deflection up and down by pushing up and down on the drive shaft. The dial indicator was on the front CV joint body near where the end of the front output shaft from the transfer case would be. I measured a total deflection from max up to max down of about 35 to 40 mils, 8-10 mils down and 27 to 30 up. I asked the local transmission shop guy and he said that this was exactly the way it should be. This was a relief! I had worried that the coupler on the replacement shaft was oversized or that the output shaft had been worn down.

Now I suspect that it is the new u-joint that might be a bit stiff and with a bit more driving the grease might loosen up and it will be smooth driving again. As it is I can live with it.

I still might replace the u-joint on the original shaft and perhaps in the spring reinstall it and see if it makes a difference.

Moral of the story? If you have a vibration, do not assume it is from the tires alone. I fell for that and should have known better. Tires imbalance will typically occur at specific resonant speeds instead of over all speeds. And if you can identify the frequency of oscillation at a given speed, you can identify if it is a wheel or drivetrain imbalance.

This looks like it could have helped me if I had found it earlier.
https://4xshaft.com/blogs/general-te...aft-vibrations

https://cimg6.ibsrv.net/gimg/blazerf...0efc9fd271.jpg

i do like your idea about changing out that u-joint :icon_dance-tap:

Definitely!!!