The Stovebolt Site Forums The Shop Area Engine Shop high speed vibration on 46 half ton

Half way home with a load of hay doing around 45 or so (speedo is broken) there's a sudden vibration. I put the engine in neutral and rev the engine--engine sounds smooth. I slow to 35 or so and it goes away. Back to 45 and it sounds awful if I tromp on it.
It does get me home and after unloading the hay I jack up one wheel. In high gear with one wheel spinning fast everything is smooth as silk.
Anyhow, I bought what Jim Carter calls a repair kit U-joint since they don't have a complete joint for a 4-spd. I took it to a local mechanic who thinks it could be the missing rear motor mounts instead. Jim Carter sells these too. So should I wait on them to come through the mail and replace motor mounts or have him work on the U-joint? Is there any way to diagnose this thing before going for the motor mounts vs. the U-joint?

pistachioted,

Are both rear motor mounts missing? Your mechanic must mean they are oil soaked, broken and need attention, but not 'missing'!

Whatever the cause, it appears that it doesn't happen until the truck is under a load. It could be a bad u-joint, and screwed up rear motor mounts would exacerbate this condition for sure!

I guess the bottom line is, your mechanic indicates you need rear motor mounts, so put them in and try another load of hay! if you get the same vibration, a u-joint might be the next step.

If the engine is in poor shape tune-up wise, the strain the load of hay put on it may have caused it to miss under load, which would shake things up, especially with bum rear mounts.

You have lots of things going on and more research is required to pin down the problem. Good luck.

Stuart

I don't want to sound over-simple, but have you checked the balance of your tires and made sure you didn't just throw a lead weight off the wheel?

Tire balance is a good idea, but that would have continued even when I put the clutch in, right? So I don't think that's it.
About the motor mounts, I took him to mean that the rubber was missing, but I'll check that. Maybe the mounts were just real worn. I don't think it was tune-up because before the vibration started there was no sign of any vibration and after I heard it start it never stopped. I hope it isn't the rear end or transmission.
Thanks to both of you for the ideas.

pistachioted,

So the vibration is still there..you can get it to do it when you want..right. If the engine were missing and flopping because of bad mounts, you would surely see the gear shifter flopping too.

A bad u-joint might fail and cause a vibration and this would stay. You would probably feel this sort of thing in the seat of your pants. Could the load have caused a tire to separate and start thumping. With the rig lifted off the ground you wouldn't notice this type of problem. Run your hand around the circumference of each tire and hunt for a lump or bump.

The frequency of a bad tire and the frequency of a bad u-joint would be quite different. See if you can recreate the problem going down the road empty then try to refine your troubleshooting to narrow it down.

Stuart

Yes, you're right, the vibration is still there going down the road empty when I get up to speed. I like your idea that the gear shift lever would vibrate if the problem was the bad mounts! Since I don't feel any difference there, it seems that bad mounts are not the problem. It isn't a tire either or the vibration would not stop by putting in the clutch. So it seems that it is likely the U-joint. Or I guess it could also be a problem in the differential.

Pistachioted,

Generally speaking...because the drivetrain is always powered from the rear forward, up to the clutch, putting in the clutch doesn't disengage the drive train. It takes it out of a loaded condition, but if you are buzzing down the road and depress the clutch, everything is still turning from back to front. Does this make sense.

Sometimes you get different dynamics because of this 'reverse situation', and what might cause a noise or problem when under power doesn't do so when driven from the other end.

Stuart

I bet you a rice dinner it's a u-joint problem.

pistachioted,

Check the ujoint GOOD. There should be no side or end play felt by hand. A lot of time the little roller bearings will come apart inside and cause the problem you describe. simple fix repair or replace the joints. the rear one is usually the culprit, but check them all.
Good luck
Brad

always thought a good way to test the u-joint vibration while moving would be to as you did. put it in neutral and if it goes away.. it is probably the u joint.......there you go..

let us know what you found so we all can learn something

Well, I owe you all a report. I took it to a shop with a Jim Carter repair kit for the U-joint (Made in Argentina). But the U-joint seems fine, good and tight. It centers up when in gear on the lift.
You are right, Stuart. One mount looked okay but the other one was broken in two. After I found the U-joint was okay I had to order new mounts, again from Jim Carter, and we got those installed. So now it is perhaps a little better, but not a whole lot. The mechanic is stymied and so am I.
My plan now is to drive it close to home and wait for it to get worse unless any of you have a better idea for me.

one thing that happened to my 42 was the u joints were not in alignment. the front was one spline out of line with the rear, this caused a bad vibration, went away as soon as we turned the shaft to align the front and rear u joints

Keith, does your 42 3/4ton have a two-piece drive shaft with a center support and rear half open???

If pista.... has a stock 1/2 ton then there is only the front u-joint to worry about.

I've had a similar vibration in my 1950, 3604. It was discussed at length in earlier threads. Right around 42 or 43mph I get a vibration, feels like the rear end is hopping up and down, at around 46 or 47mph it goes away. All new and I mean new driveline, three new u-joints (in phase), a new center support brg. and rubber. A new torque tube bushing in front. The rear end was disassembled checked for wear and set up according to the Shop Manual, the axles were checked for straightness and indicated for run out at the lugs and flanges, all ok. New front and rear motor mounts were installed as soon as I brought the truck home.
I've had the original 15" bias ply tires, meticulously balanced. Then replaced them on the original two piece rims stripped bare and re-sprayed with PPG DBI, with brand new Power King bias plys with new flaps and tubes and meticulously balanced on three different visits to the tire shop. Then replaced those with later 16.5 tubeless rims and Cooper tubeless radials and had them balanced at two different shops. \
With the truck up on jack stands in the garage and run up to and through the troublesome speed range there is no vibration.
All of this and, "I still get a vibration at right around 42 or 43mph that feels like the rear end is hopping up and down, at around 46 or 47mph it goes away."
At this point I'm of the opinion that there is some sort of harmonic vibration set up at that speed as the chassis flexes. If you accelerate steadily up to 55mph you don’t notice the vibration, but if you hold it at the low 40’s for a few seconds it starts to vibrate.
Good luck!!!
Denny Graham
Sandwich, IL

I've had this problem for years with my '46 1/2 ton. Do you have the 3 speed or the 4 speed? Couple of things to check. 1st, try to isolate the problem to driveline or engine. When the vibration gets going at road speed, what happens when you step on the clutch? Is it worse or better? A bad motor mount can make the engine "ground out" on the frame with metal to metal contact. If that's it, the vibration will lessen a lot with the engine idleing while you're moving down the road at speed. Another thing that can cause engine vibration is an unbalanced flywheel.

I've got the heavy duty 4 speed and I've found that the U joint gets inadequate lubrication. I pull the pipe tap on the top of the rear trans and put in about 4 oz of 140w straight weight gear lube every time I change the oil. This helps with the vibration and greatly prolongs the life of the U joint.

It's a vexing problem. Let me know if you solve it. Good luck

David Bush