Harmonic Vibration

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Frankenchevy

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Over the last few weeks I’ve noticed a vibration of sorts with my truck. It’s barely there, but I notice it and feel it’s getting more prevalent.

It’s only noticeable above 55. It isn’t in sync with engine or wheel speed. The best I can describe it, is that it goes in and out like when your phone vibrates with a silent alarm- on then off, on then off. Sometimes with a 1 second pause, but sometimes even shorter. It changes frequency, even when rpm and speed are steady. If going say ~65mph with the gas on, the vibration is as described, but it seems like when I lift(no load), the vibration significantly dissipates or disappears. If anyone has ever ridden a dirt bike at constant high rpm in a desert or dual sport situation that doesn’t have a balanced crank, it’s almost like the harmonic vibration you feel between your ankles when squeezing the case.

At first I thought maybe it was the driveshaft. I recently put new u-joints in when I had my driveshaft out to replace my pinion seal. Tomorrow morning, I’m having it balanced. The weird thing is they balance it with the transfer case yoke attached, but not the diff yoke. Why do people say to keep track of driveshaft to diff yoke orientation if they aren’t balanced together?

The other moving items I haven’t replaced:

—th400, feels fine & shifts well. New oil and filter a couple thousand miles ago. Oil that was drained out also looked brand new.

—bearings in transfer case, the splined shaft on the yoke side has no noticeable play with driveshaft removed and the seal never weeps oil as I’d suspect it would if vibration were present there

—any bearings in the 14bff. It has new fluid and again the fluid that came out looked really good. I have all new wheel/hub bearings, just haven’t had the time to install. I wanted to look at the condition of the shaft where the bearing rides as well as the splines when they come out.

Only other culprits I can think of might be wheel/tire balance, but both are brand new, just rebalanced when rotated and it’s not in time with revolutions. Not sure if that last point can rule them out or not. Also, haven’t gotten around to replacing tie rod ends or rebuilding my D60 kingpins. The truck has really low mileage and was maintained by the military, so I’m assuming they kept up on greasing the kingpins.

I did go through and recheck torque on all suspension and steering items since this truck was fully disassembled and reassembled by me and everything checked out.

Thanks Guys
 

shanegtr

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Firstly, in my line of work harmonics are just a convenient error in a mathematical calculation that can help to analyze vibration problems. Anyway sounds like it may be what I call a beating vibration - actually a combination of two separate, but close vibration frequencies (the time between each beat is the difference between the two frequencies)
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Sorry it dosent help you directly narrow down the fault, but just thought I'd share some info that the issue may actually be a combination of two (or more) frequencies that produce the end result
 

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Torque converter
 

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I've had that pulsating "rumble" when I had a diff go out on me
 

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Yoke(s)?
 

Frankenchevy

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Craig 85

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I had an '87 crew cab in the early 90's that had loose torque converter bolts when I bought it. I figured I could just tighten and run it. Unfortunately, it had hogged out the flex plate holes and caused similar harmonic vibration at 65 MPH +. If I pulled it out of gear at freeway speeds and let the engine idle down, it went away.
 

Keith Seymore

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Firstly, in my line of work harmonics are just a convenient error in a mathematical calculation that can help to analyze vibration problems. Anyway sounds like it may be what I call a beating vibration - actually a combination of two separate, but close vibration frequencies (the time between each beat is the difference between the two frequencies)
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Sorry it dosent help you directly narrow down the fault, but just thought I'd share some info that the issue may actually be a combination of two (or more) frequencies that produce the end result

shanegtr is correct; it's a "boom/beat".

That means there are two root causes. Eliminating one will eliminate the beating.

K
 

Frankenchevy

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I had an '87 crew cab in the early 90's that had loose torque converter bolts when I bought it. I figured I could just tighten and run it. Unfortunately, it had hogged out the flex plate holes and caused similar harmonic vibration at 65 MPH +. If I pulled it out of gear at freeway speeds and let the engine idle down, it went away.

I pulled the inspection cover last night in the dark when I got home from work, I’ll recheck them today when I get home. They seemed tight. Unfortunately, with the header collectors, I have to disconnect the trans cooler lines to pull the inspection cover all the way down.

shanegtr is correct; it's a "boom/beat".

That means there are two root causes. Eliminating one will eliminate the beating.

K
Okay. Two root causes could possibly be an imbalance on the driveline and something else? I’ll find out when I get home with my newly balanced driveshaft today.
 

Keith Seymore

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I pulled the inspection cover last night in the dark when I got home from work, I’ll recheck them today when I get home. They seemed tight. Unfortunately, with the header collectors, I have to disconnect the trans cooler lines to pull the inspection cover all the way down.


Okay. Two root causes could possibly be an imbalance on the driveline and something else? I’ll find out when I get home with my newly balanced driveshaft today.

Often the two are 2nd order of driveline (u joint angles) and fourth order of engine RPM (engine firing frequency). Since engine firing is a good thing that means you have to eliminate the other.

K
 

Frankenchevy

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Often the two are 2nd order of driveline (u joint angles) and fourth order of engine RPM (engine firing frequency). Since engine firing is a good thing that means you have to eliminate the other.

K
That would be sweet! I have to admit, the new u joints I installed felt pretty tight when I dropped the driveshaft off this morning—despite the fact that I used the thinnest circlips available in order to get them to actually seat in the groove. I think that in using a ball joint press to press out the old ones and press in the new ones, that I may’ve bent the ears in on the driveshaft—thus creating a tighter than factory groove to groove measurement. They reinstalled the u joints for me and they feel really nice now. They also installed a couple small counter weights on either end of the shaft.
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As for the angles, all of my rear axle to transfer case geometry is the stock cucv setup i.e. no suspension lift etc. I’ll actually put my angle finder on the tail housing of the TC and diff yoke and see if they match before reinstalling the driveshaft.
 
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Frankenchevy

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Something I can’t remember quite right...

On the rear drive shaft on an np208, should I put grease in the slip yoke splines or does the ATF make it up in there to lubricate it?
 

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Something I can’t remember quite right...

On the rear drive shaft on an np208, should I put grease in the slip yoke splines or does the ATF make it up in there to lubricate it?
Atf makes it up in there. There's a small drain hole in the tail shaft housing that lets the fluid drain back into the case. No grease needed, unless you have a new seal in there, then a small little dab around the inside of the seal to lube it
 

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Well it sounds like good news so far...
I got the same grief going with Big Blue.
 

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