Help me identify a vibration

da_raabi · Mar 20, 2018

I've had this for a while, but recently it started getting worse.

It only happens at highway speeds of 65+.

It feels/sounds like a vrrooooommm.... vroooooommmm.... vroooooommmm.

I can feel it in my butt, but not through the steering wheel.

I do also get a speed-specific vibration/shake a 45mph, but only at 45mph. At 40 I get nothing, 50 nothing.

I rebuilt the transmission last December. I had the vibration before, but it seems to have gotten worse since. With the transmission rebuild I also installed a new flexplate and torque converter.

I just had the driveshaft rebuilt and balanced.

All 6 tires were road-force balanced.

I'm at a loss. I've run out of stuff to check!

Truck is an 86 C30 crew dooley.

foamypirate · Mar 20, 2018

Pinion angle? I had a similar vibration in my crewcab. Adjusting the pinion angle fixed it (though I have a non-stock drivetrain).

Did they replace your center bearing when rebuilding the driveshaft?

DoubleDingo · Mar 20, 2018

Possibly the drifeshaft is out of time/tune? Meaning the alignment of the splines may be off. It could have been like that before the u-joints were replaced and they just put it back not knowing any better. I'm not a pinion angle expert, but that sounds like a valid thing to look at as well.

da_raabi · Mar 20, 2018

Center bearing was replaced, everything is bone stock. I'm not sure how I could adjust the pinion angle. Maybe a different transmission mount?

The rebuilder did notice that somebody had shoved the 2 pieces together and boogered up the splines. He said he cleaned them up then painted big white arrows on the pieces to make sure I get it back together correctly, which I did.

The problem was exactly the same before and after the d/s rebuild.

climb-101 · Mar 20, 2018

maybe try a different set of tires from a friend. I had a truck with a vibration, they had to balance the tires 3 times in order to get it right.

yevgenievich · Mar 20, 2018

I have similar vibration, around 70mph. Have all new steering, went through three different tire sets, rebuild/balanced driveshaft, the transfer case output in good shape and transmission is a gm goodwrench 4l80e. All is stock, but wondering if pinion angle could get affected enough by one rear spring being warped/sagged somewhat. My driver side rear spring is partially sagged

da_raabi · Mar 20, 2018

climb-101 said:
maybe try a different set of tires from a friend. I had a truck with a vibration, they had to balance the tires 3 times in order to get it right.

That's what my Dad was saying.

Too bad I don't have friends...

yevgenievich · Mar 20, 2018

the rear end or rear suspension are the only things I have left to blame, but not sure how to verify yet.

RecklessWOT · Mar 20, 2018

I have a similar vibration, I always thought it was my driveshaft. It was the same when I got the truck as it is now, maybe a liiiitttle worse after an additional 50k+, but not much. I have replaced u-joints, on my 2nd set of brand new tires (both same brand and tread pattern but 31x10.5 vs 33x12.5) and had them professionally balanced both times, replaced all springs and shocks with a 2" Rough Country kit. Improperly shimmed rear carrier/axle gear swap after a blown rear, head on collision that threw my thrust angle off by 2deg, and nothing has made a difference good or bad. I was sure it was the driveshaft until you said yours has been re-balanced and no change, now I'm second guessing.

Snoots · Mar 20, 2018

Just a shot in the dark....axle shaft?

DoubleDingo · Mar 20, 2018

Just read on stovebolt, a guy was having a vibration and replaced items and it turned out to be a pinion bearing going out. He just discovered it and took it in to be replaced, so he hasn't reported back, but that may be another thing to consider.

GMFORLIFE · Mar 21, 2018

Motor mounts?

yevgenievich · Mar 21, 2018

In my case good motor mounts, motor also appears to be a rebuild unit. Someone was joking about putting sensors to figure out where the vibrations are coming from, but I am thinking serious about it. First couple of cams to watch the front and rear suspensions, then if that does not return, start measuring intensity of vibrations at various points.
What is the best way to verify pinion bearing or axle shaft? Measure the runout on the axle flange and slop on the pinion?

1987 GMC Jimmy · Mar 21, 2018

yevgenievich said:
In my case good motor mounts, motor also appears to be a rebuild unit. Someone was joking about putting sensors to figure out where the vibrations are coming from, but I am thinking serious about it. First couple of cams to watch the front and rear suspensions, then if that does not return, start measuring intensity of vibrations at various points.
What is the best way to verify pinion bearing or axle shaft? Measure the runout on the axle flange and slop on the pinion?

Something auditory like this?

https://www.amazon.com/STEELMAN-97202-Wireless-ChassisEAR-Diagnostic/dp/B00123J79O

yevgenievich · Mar 22, 2018

1987 GMC Jimmy said:
Something auditory like this?

https://www.amazon.com/STEELMAN-97202-Wireless-ChassisEAR-Diagnostic/dp/B00123J79O

Thinking of measuring frequency of vibrations. And for a lot cheaper than that kit. Was thinking of something like a piezo disc, and within seconds found a lot of people are using them. Circuit stolen online, and a bit different. But really just feed to an opamp, amplify and store on the computer through an arduino or any other microcontroller.
https://www.sparkfun.com/products/10293

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Help me identify a vibration

da_raabi

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1987 GMC Jimmy

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