Bad vibration...

[michael bustamante]

So me and my daughter took the 67 GMC on its made in Cruise last night and we cannot get it above 30 miles an hour as there is a vibration as soon as it hits 30. I checked the driveline and the carrier bearing and those seem to be fine. Do you guys have any idea of what could be causing this in my 3/4 ton?


8/9/2023 ok update!!! i put on new wheels and tires.. still got the bad vibrations.. lifted the rear end in the air and my rr tire is moving a little wonky.. new tire, mounted balanced blah blah.... swapped tires and it still moves wonky. like a bent rim would but its not that. its a floating rear end, could the axle be bent and causing this? axle seal is not leaking. driveshafts look true when my daughter hits gas.. no play, carriewr bearing look s good.
Even with the rear end in the air it still vibrates the whole truck at about 25 mph... any ideas?

[54blackhornet]

Sticking caliper ? It happen to me thought a wheel was loose.

[Getter-Done]

What is the condition of the wheels and tires?

If you have a place to safely jack up the rear axle and put it on jackstands.
That would eliminate some stuff.

[michael bustamante]

That's what I'm about to do now.. lift and check the rear end and the trans mount but yup you may be right about the sticking brakes too. (All drum)

[RustyPile]

If it's been sitting in one spot for a while, check for flat spotted tires.

[72c20customcamper]

My vibration was the rear ujoint at the pinion bad vibration at 35 and up

[guyryan100]

Mine was old tires. I've read some interesting troubleshooting ideas on this site, including narrowing it down to front or rear based on whether the rearview mirror in the cab shakes, indicating front end problem, or you feel it in the seat, suggesting its the back end. Also whether it only shakes when you are accelerating which suggests an engine-related issue. I jacked up one rear wheel as a test and found no vibration with the rear wheels spinning, which eliminated transmission and driveshaft issues.

[Bigdav160]

Driveshafts out of phase (is it even possible on a GM?). Frozen U-joint.

[michael bustamante]

well the tires are shot (cracked up on the sid wall) but hold air.. been flat for at least 10 years. too bad as they were new too. drive shaft seems solid.. im going to jack up the rear end after work today

[72c20customcamper]

Originally Posted by michael bustamante

well the tires are shot (cracked up on the sid wall) but hold air.. been flat for at least 10 years. too bad as they were new too. drive shaft seems solid.. im going to jack up the rear end after work today

Umm tires ASAP dont move or pass go till you do . Tires that sit flat tend to crack all the way through the sidewall

[michael bustamante]

yup no more cruzes until i replace the rims and tires... but im still gonna check the rear end and make sure the brakes arent sticking

[54blackhornet]

I have doubts drum brakes could cause it, Disc brakes more likely.

[71CHEVYSHORTBED402]

My first thought was an unbalanced wheel.

[michael bustamante]

to the top.. please read the update.. need help! i cant drive my truck!

[Bigdav160]

That speed (30mph) is indicative of a driveline vibration, not tires

[dmjlambert]

It sounds like you might have found it. To confirm it, what I would probably do is jack up the right rear and remove the wheel, and put a straight edge such as a carpenter's square on the mounting surface of the axle and mark the floor with a pencil where the straight edge points. Then rotate the axle and repeat several times.

[NGN]

Maybe a motor mount collapsed. Trans mount also.

[72c20customcamper]

Originally Posted by dmjlambert

It sounds like you might have found it. To confirm it, what I would probably do is jack up the right rear and remove the wheel, and put a straight edge such as a carpenter's square on the mounting surface of the axle and mark the floor with a pencil where the straight edge points. Then rotate the axle and repeat several times.

Pretty hard to bend a floating axle . If something is bent I would think the hub or axle tube to be bent since the axle itself doesn’t hold any weight the drum and hub do . I would look at the drum itself and check the bearings

[body bolt]

Perhaps some crud between the brake drum and the hub?

[michael bustamante]

Originally Posted by dmjlambert

It sounds like you might have found it. To confirm it, what I would probably do is jack up the right rear and remove the wheel, and put a straight edge such as a carpenter's square on the mounting surface of the axle and mark the floor with a pencil where the straight edge points. Then rotate the axle and repeat several times.

ok ill do this tonight and iull tear into the drums assy and make sure bearings are true as well. ive never worked on a floating axle before so wish me luck

[RonWT]

check bellhousing bolts, check fan and all belts, check the U joints, flexplaate/flywheel?

[michael bustamante]

All that looks good, took off the bell housing cover, nocracks in flywheel, all engine mounts are solid, ( no lifting on powerbrake) no play in u joints or the drive shaft and no wobble under acceleration

[RonWT]

Pilot bearing?

[HO455]

From what I have understood you didn't have any vibration and then one day you did. Does the truck get driven regularly?

Then several weeks later you put new wheels and tires on and you still have the vibration. Were they new tires or a different pair of used tires?

My old 09 2500 Ram 4x4 tires would flat spot if I didn't drive the truck at least every other week. It would take 10 or more miles for the flat spots to work out every time it happened.

Did you replace the front tires as well?

Have you inspected the drive lines to see if one of the balancing weights has fallen off?

Try holding a straight edge to the drive lines and see if you can see a bend.

Did you check the yoke at the transmission for play? The same goes for the input shaft on the axle.

Does the vibration at 30 mph the same no matter what gear you are in?

[tdangle]

I had a bad vibration that came in at about 40 MPH. C20 auto with 4.10. I swapped trans, new mounts, had driveshaft rebuilt and balanced twice, once troubleshooting the vibration, 2nd time shortening the driveshaft for the 7004r trans. Replace tires, had them balance multiple times, rebuilt dana 60 rear axle, spent 2 years chasing the problem. Finally on a hunch took the driveshaft (2 piece) to a different shop, then said the spline and coupler connecting the 2 shafts had plan replace them and no more vibration. 1st shop did junk work.