HELP - serious U-joint or CV vibration?

[misterp]

Hey all - last night was the first time I've really driven the SS since installing the McGaughy's drop and there is a serious rumble/vibration felt in the seat and floorboard between 67 and 77 mph. And I mean *serious*, I've had a u-joint fail in a past vehicle and that's very much what this feels like. The vibration gets much worse when you put more load on the truck (throttle or pulling a grade, worst = both), and I even saw the speedo jumping a little bit because of it, it's that bad. The vibration is not felt in the steering wheel at all, only the seats/floorboards; it's bad enough that my friend noticed and worried about it (I'd been feeling it all night). All week I've been hammering the throttle around town and not felt any such vibration so it does not happen at lower vehicle speeds, I may drive fast but not 75 in town!!! And I already checked the driveline angles with an angle finder and I thought everything was ok (the pinion angle is -2* compared to the transfer case output shaft). But something must be really wrong, because this vibration is really bad.

 

So is this vibration in the front (CV-joint), or rear (U-joint)? How do I determine this for sure? If it's in the rear, any ideas on how to figure out why? I'm grasping for ideas here so anything suggested I will consider...

 

Mr. P.

[Jonmalibuss]

I'd say get the truck on a level surface and put the rear on stands and then put it in neutral and get under it and try to shake the driveshaft and if it's a u joint you should beable to find it or find something lose. I would say if it was the front you would feel it in the steering wheel if it's that bad.

[Jonmalibuss]

Or maybe you lost a wheel weight somewhere.

[misterp]

Thanks Jon - will start there.

 

EDIT - but I'm pretty sure it is not tire balance, because if the truck is coasting at that speed there is no vibration; only when under power.

[PladdPezzPunk]

sounds like a cv cause the u joint will usually shake all the time. CV vibes are very tough to diagnose. try taking it to a garage where they can lift up all four tires and drive the car in the air. thats usually the easiest way to see a cv vibe. It's possible you popped out one of the cv's and when it went back in, it was in a different position.

[misterp]

sounds like a cv cause the u joint will usually shake all the time.  CV vibes are very tough to diagnose.  try taking it to a garage where they can lift up all four tires and drive the car in the air.  thats usually the easiest way to see a cv vibe.  It's possible you popped out one of the cv's and when it went back in, it was in a different position.

I thought of that at the time; I used chalk to mark the flanges when I removed the half-shafts, and made sure they were reinstalled in the same orientation. I am going to wrench on the torsion bar adjusting bolts and raise the front to see if that causes any changes; if it does that will tell me it's in the front, I think.

 

Mr. P.

[Bowtie70SS]

May be worth your $$ to take it to a spring/driveline shop or a really good alignment place to have your pinion angles measured. You have to ask one of the GM tech guys but they do have a device that isolates driveline vibrations.

 

Dave

[BSER]

I think you may be heading in the wrong direction Steve. If you did the rear 4 inch drop I'd bet you have a pinion angle problem. A too shallow pinion angle can cause the u-joints to bind and cause the entire drive shaft to vibrate. That is also the reason it vibrates at speed; the binding can cause it to start vibrating at some rpm ranges sort of like a feed back loop. And under load the pinion angle changes causing the vibration.

 

Try adding a 2 to 3 degree shims on the rear axle. They are cheap and only shound take a hour or so to install.

 

Of course if you didn't do the rear 4" drop none of this applies.

 

Bob