Vibration - Driveline Saga

Hi! New here. Happy to meet y'all.

I have a '79 Sierra Classic 1500 on a '83 2500 4wd chassis... with a dump bed.

The vibration feels like driveline and it must be so I balanced it. Vibrates. Balance again. No change. Rear axle housing had a wobble so installed new rear end and all the fixn's, yoke, etc. Vibrates. Replace worn transmission output shaft. Decided ex. unit was too long so designed and hung a 2-piece driveline. Less vibration. Sent it back for balancing. Less vibration. There's nothing left. I put a doghouse under the seat (4wd has bigger tunnel) for proper pinion angel. Disconnect and drive front wheels only and no vibration. Worst at 75 but you can detect it down to 35 or 40.

Well geniuses? Here's your riddle. What else can I do? Is my dump bed a possible cause? It's not fastened but by the hoist at the front. (That how to question comes later.)

Bent axle shaft?
Bent rear rim?
Flat spot on one rear tire?
Separating tread on one tire?

Is it wheel speed or engine speed? What happens if it is placed in neutral?

Worn suspension components will cause vibration, but that's more of "bouncing" feeling than a vibration.

Ask me how I know

Try flipping the driveshaft 180 on the yoke.

Try overinflating or underinflating the tires, see if that changes the vibration.

I feel for you have been down that rabbit hole, buy chance has someone fitted the wrong flex plate?

measure the pinion angle to the driveshaft and then measure the angel at the transmission to the driveshaft. They should be off set equally. If they arent then that is probably the issue at hand. If you have helper springs on it and the axle height has changed in the rear then at speed your offests are out of phase and that will cause a certain level of notchiness in the driveshaft u-joints and you'll feel that as vibration. Here is a video that explains the details of that condition.

Have you ever noticed a difference in the vibration when the truck is loaded vs. unloaded? This would be indicative of a pinion angle issue like @rpcraft mentioned. You probably need to get it squatting a bit to notice a change, which may be a little more difficult with K20 suspension.

Do you have access to a different set of wheels and tires to test? Like @Matt69olds suggested, wheels/tires are a relevant variable(s) that you didn't list in your first post.

scrap--metal said:

Have you ever noticed a difference in the vibration when the truck is loaded vs. unloaded? This would be indicative of a pinion angle issue like @rpcraft mentioned. You probably need to get it squatting a bit to notice a change, which may be a little more difficult with K20 suspension.

Do you have access to a different set of wheels and tires to test? Like @Matt69olds suggested, wheels/tires are a relevant variable(s) that you didn't list in your first post.

Click to expand...

Good thought on testing the squate dynamics.

Find a buddy with tires and wheels that will fit your truck, swap them out and see if you still have the vibration or not. I had a similar issue, found out that my 'new' tires were causing the issue.

The rear axle has been replaced and the BF Goodrich tires are new. The telling thing here is that it doesn't vibrate running with the driveline disconnected.

Engine speed doesn't matter, though I don't have a lot of choices at 75. It doesn't vibrate more at higher rpm at lower speeds though.

I purposefully ran it with a yard of compost and the vibration didn't change under load, though I did spread quite some compost on the highway at 70. We've licked the pinion angle to death and have done precise measurements but I will revisit along the lines of Matt's suggestion.

I don't think I ever tried popping it into neutral at high speed. I'll give it a try.

The flex plate idea has me thinking.

When customers come in with a vibration complaint first thing we do is verify the problem by driving it. Obviously, that isn't possible here. Can you say if it is high speed at driveshaft RPM or a slower vibration at wheel speed. 2 entirely different causes and diagnostic areas. If possible, another thing to try is jacking it up either on the ground but preferably on a 4 post hoist [that has jacks] and operating the truck at the offending speed. Looking at the rotating parts, it may be visible where the problem is.

The rear axle has been replaced and the BF Goodrich tires are new. The telling thing here is that it doesn't vibrate running with the driveline disconnected.

Engine speed doesn't matter, though I don't have a lot of choices at 75. It doesn't vibrate more at higher rpm at lower speeds though.

I purposefully ran it with a yard of compost and the vibration didn't change under load, though I did spread quite some compost on the highway at 70. We've licked the pinion angle to death and have done precise measurements but I will revisit along the lines of Matt's suggestion.

I don't think I ever tried popping it into neutral at high speed. I'll give it a try.

What transfer case do you have? Is it a slip yoke or fixed yoke output? If its a slip-yoke output on the rear, have you tried replacing the output bushing?

You said you have beat the pinion angles to death, what were the measured angles?

Fast 99 - we did put it on a hoist and ran it at speed, which is how we caught the wobble from the bent axle housing. Again, if it doesn't vibrate with the driveline disconnected (running on the front axle) doesn't this take those other considerations out of the picture?

When I sent the new 2-piece driveline back to be balanced I also had it timed, as it arrived slightly out of sync the first time.