I can't puzzle this out, and I'd like to pick your collective brains.

I've got an annoying vibration developing. It's 100% tied to road speed (gear, grade, RPMs, clutching to neutral make no difference), with bad spots around 50-55 and 68-75. The entire cab vibrates wildly up and down, including the steering wheel, but I don't feel anything telegraphing through the wheel itself, so it feels like the drives. When I'm in one of the "sour spots" the vibration is not steady, but it comes and goes in pulses of about 20 shakes followed by that same time of smooth running, then 20 more shakes. It's hard to describe.

Weight on the tractor is the key to dulling the shakes. I pull about 45K down and 20K back. Vibration is worse on the return trip, and it's fricking horrible bobtailing, UNLESS I've got my tanks more than half full. The truck was shaking like crazy doing down today, and coming back was even worse, until I got fuel. Full load of fuel plus 20K in the box rode better than 1/3 tank with 45K in the box.

The way this comes and goes in pulses is what has me confounded most of all. Could it just be something easy like the drives need balancing? I fear something more sinister is happening, but can't imagine what.

Oh, the vibration isn't really new, I don't think. There's always been a chucka chucka spot around 70-75 since I've been in this truck, but it's all orders of magnitude worse now that I have new tires all around on the tractor. What used to be a little burbly annoyance now makes me think I might lose some fillings one day.

 

ride height.

I absolutely guarantee you that's what it is.

Fix it ASAP. You're wrecking the u-joints.

When you're unhooked, look at the driveline angles, especially the rear driveshaft. They should be dead straight.

Drive tires do not get balanced.

 

I used to drive a brand new mack vision that did that. The mechanic could not figure it out. Had an alignment new front tires and balencing. the truck only had 25k miles on it. I quit that job but I heard from the new driver it still does it. Mine did it regardless of what weight I had in the box though. Good luck

 

I forgot to mention that this was my very first thought too. I've had that stink in my nose for a couple of weeks, and I can't find a problem with ride height.

I did have trouble around two months ago, and ran for about a week with a bad height control valve before I figured out I was having a problem. I didn't figure it out due to ride issues, but weirdness with dropping and hooking, which is why it took so long to pin it down and fix it with a new height control valve that's still shiny now.

Given that the valve is so new, and I can't find a current problem, think I may have already toasted the U-joints? How do I check U-joints? Why would this have manifested itself when I got new drives?

 

A toasted u-joint is very easy to spot. It hangs very funny. You truck has 5 of them.

Also, you'll feel massive "throbbing" when starting off, especially with a heavier load.

 

I broke a U-joint recently. Until it failed it gave me the symptoms that you describe, vibration that varied with speed and weight. Check the U-joints ASAP. If it fails while running chances are it will damage other components, yokes, etc., the drive shaft can fly around and slap/damage other stuff under there.

 

Nothing wrong with the U-joints as far as I can tell (where is the 5th one?!) Transmission to front differential is straight and true. The shaft connecting the two differentials slopes down at an angle, but it looks for all the world like it's supposed to be that way. The U-joints don't look flexed in that orientation, like what the splines are going into on either end must be angled to match. I also played with dumping and pumping up the airbags to the extremes while I stood there and looked at the driveline (KW 8-bag suspension) and it seemed there was little to no movement in the angle of the driveline components even at these extremes. I guess there must have been some movement at the extremes, but I couldn't see it putting the U-joints into any easily perceptible kink even with this experiment. Given that the ride height checks out, I don't think this is my culprit at all.

Maybe it's the steers after all? They were supposed to have been balanced, but I don't see any weights. They do have Centramatics.

 

I was going to say drive shaft but I don't think that would cause the up and down you're talking about.
So I'll say check your air bags inflation

 

ah yes, kenworth 8 bag. You're correct, this is the most "driveline stable" suspension there is. The 5th u-joint will be at the back of the tranny.

 

Can't say for sure, but the way you described it, i think it has something to do with tires,- balancing, out of round, may be even bad drums or wheels... :roll: