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Old 04-18-2012, 07:08 PM
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Pulsating Vibration, need help

Maybe someone can help me with this, I have a 1988 k5 blazer with a very intense pulsating vibration, this vibration has been with the truck for years and I've just never bothered with it but I want to sell it soon so I would like to fix it.

Basically what its doing is whenever I get the truck above 50mph there is an intense pulsating vibration that pulses through the entire truck (makes a kind of WOO....WOO....WOO....WOO sound). It pulsates on acceleration and on deceleration its a solid harsh vibration.

The truck has a th400 that I rebuilt about two years ago with the direct clutch duel feed mod, a b&m race shift kit, and a brand new 2000 stall torque converter. I have put maybe 2000 miles on, and it has a np203 part time converted transfer case. I can't remember if I had this pulsating vibration when the truck had a 700r4 in it.

I know the tires are very old and the thread is pretty stiff but their not really chopped up at all and they seem to roll straight. I've rotated and balanced the tires but there's no change.

I drove the truck got up to speed and as the vibration was happening I shifted into neutral...no change. I then kept it in neutral and shut the engine off and let roll down the road at about 55mph...no change in the vibration.

All the u-joints are new, both front and rear axles have been rebuilt because I just swapped in 4.56 gears.

Heres the problem I've seen the rear drive shaft is not original to the truck and someone down the line shortened it but it looks like when they welded it back together they didn't pay attention to phasing because I cannot phase the drive shaft yokes parallel, they are always at least 20 degrees or so out of phase. Can this create a horribly intense pulsating vibration on acceleration over 50mph that turns into a solid vibration on decell? I have read that th400's can create a pulsating vibration but I can't confirm that.

sorry for the novel but any help would be greatly appreciated.

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Old 04-18-2012, 07:18 PM
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Quote:
Originally Posted by frickenbored
Maybe someone can help me with this, I have a 1988 k5 blazer with a very intense pulsating vibration, this vibration has been with the truck for years and I've just never bothered with it but I want to sell it soon so I would like to fix it.

Basically what its doing is whenever I get the truck above 50mph there is an intense pulsating vibration that pulses through the entire truck (makes a kind of WOO....WOO....WOO....WOO sound). It pulsates on acceleration and on deceleration its a solid harsh vibration.

The truck has a th400 that I rebuilt about two years ago with the direct clutch duel feed mod, a b&m race shift kit, and a brand new 2000 stall torque converter. I have put maybe 2000 miles on, and it has a np203 part time converted transfer case. I can't remember if I had this pulsating vibration when the truck had a 700r4 in it.

I know the tires are very old and the thread is pretty stiff but their not really chopped up at all and they seem to roll straight. I've rotated and balanced the tires but there's no change.

I drove the truck got up to speed and as the vibration was happening I shifted into neutral...no change. I then kept it in neutral and shut the engine off and let roll down the road at about 55mph...no change in the vibration.

All the u-joints are new, both front and rear axles have been rebuilt because I just swapped in 4.56 gears.

Heres the problem I've seen the rear drive shaft is not original to the truck and someone down the line shortened it but it looks like when they welded it back together they didn't pay attention to phasing because I cannot phase the drive shaft yokes parallel, they are always at least 20 degrees or so out of phase. Can this create a horribly intense pulsating vibration on acceleration over 50mph that turns into a solid vibration on decell? I have read that th400's can create a pulsating vibration but I can't confirm that.

sorry for the novel but any help would be greatly appreciated.
If the driveshaft job was homegrown- and botched to boot- there's every possibility the shaft wasn't even balanced. First thing I would do is get it corrected then see if the vibe persists.
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Old 04-18-2012, 07:26 PM
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Out of phase yokes will be the trouble .
The shaft vibrates as the velocity change from one end of the shaft to the other is not equally offset by the other end.
Yank it out and take it to the driveline shop and have it cut off and phased properly.
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Old 04-18-2012, 07:26 PM
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A "Out of Phase" shaft will cause that vibration. Have the Shaft put in phase and have it straightened and balance.

I bet you will noticed a difference.
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Old 04-18-2012, 08:14 PM
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driveshafts

When doing drive shafts I always look for one that does not have a big gob factory weight on it. I try to find one with NO factory weights to start with, a lot easier to get a true running shaft than one that was close to the reject specs.
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Old 04-18-2012, 09:13 PM
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I'm thinking that I'm going to pull the rear drive shaft tomorrow and drive it with the front axle since the transfer case it a fixed yoke. I tried using hose clamps on the rear drive shaft to get it balanced somewhat and after moving them around a few times I could tell that the vibration had gotten worse at time and very slightly better at other times. Hopefully I'll be able to confirm that it is just the drive shaft when I take it out tomorrow, that would be a pretty easy fix.
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Old 04-19-2012, 06:41 AM
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so I haven't driven the truck without the rear driveshaft yet but I took it out and measured the angles of the yoke and there is a 5 degree difference between the front yoke and the rear yoke, when you look down the driveshaft you can see that the yokes do not line up. I cant tell if the driveshaft is bent and obviously I don't know if its balanced or not.

can a just a 5 degree difference cause a really harsh pulsating vibration?
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Old 04-19-2012, 12:54 PM
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Quote:
Originally Posted by frickenbored
so I haven't driven the truck without the rear driveshaft yet but I took it out and measured the angles of the yoke and there is a 5 degree difference between the front yoke and the rear yoke, when you look down the driveshaft you can see that the yokes do not line up. I cant tell if the driveshaft is bent and obviously I don't know if its balanced or not.

can a just a 5 degree difference cause a really harsh pulsating vibration?
It might- especially if the angles between the diff and trans are severe enough (extreme example below). Or to put it another way, the worse the angle, the worse any out of phase vibration will be. The overall balance can cause a vibe regardless of the angles involved.



That the joints are out of phase at all tells me that this wasn't done by a driveline shop, and I would also seriously doubt it was ever balanced, as I mentioned earlier.

Last edited by cobalt327; 04-19-2012 at 01:00 PM.
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Old 04-19-2012, 03:49 PM
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yup I think you guys hit the nail on the head, I just got off work and went and took the truck for a drive without the rear driveshaft and the crazy pulse vibration was gone and the truck actually rode very nicely, just a little vibration from the mud tires it has on it but thats normal.

my u-joint angles aren't extreme, they look pretty normal to me since the truck isn't lifted or anything. I'll have to look around for a driveline shop and hope that my driveshaft can be fixed.
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Old 04-19-2012, 04:29 PM
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Here's a couple pictures of the shaft, as you can see its been cut and welded in a couple different spots:



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