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Long standing pulsating vibration issue that I need help solving

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Ramba_ral

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Hello all, I have a long standing pulsating vibration issue that I need help solving

It happens at highway speeds only(65+). When i drop it into neutral it seems to vibrate less but hard to tell cause my speed bleeds off quickly

So far I have:
rebuilt the rear end,
both driveshafts have been replaced,
rebuilt transfer case
gone through 3 sets of tires and 2 sets of rims(road forced tested several times on top)
replaced flexplate and converter

havent checked
harmonic balancer
clutch fan


running out of things to check and not sure what else to look at.
 
First thing, what year make model? Stock axles? What size wheels and tires? What transmission?
 
1979 k5 blazer
12 bolt rear end. During the rebuild the axles were replaced
16" wheels and 315/75r16 tires
th350
4" lift
Pinion angle has been confirmed as good from drive line shop
 
What t-case? Is it a slip yoke on the rear output?
Have you verified the front driveshaft isnt spinning going down the road?
 
np205
rear out put is a fixed flang with a double cardan joint driveshaft
from what i can tell the front spins slightly when going down the road
 
Ramba_ral said:
np205
rear out put is a fixed flang with a double cardan joint driveshaft
from what i can tell the front spins slightly when going down the road
Click to expand...
Ok, thats normal.
Only above 65 huh. So if its not rpm related, and only road speed, it seems it would be tire/wheel, axleshaft, or maybe air/wind?
Any lightbars or accessories like a roof rack?
 
I had a pulsing above 68 that was a loose transmission mount.

If it changes in curves start thinking steering parts and wheel bearings has been my experience as well.
 
I got a front brush bar and side steps if that counts.
Id hope its not tire/wheel cause I've gone through multiple sets. About 3 to 4 already
I see your in Henderson. I had Adams driveshaft handle the rebuild for the drive shafts and rear end.

also thought about air/wind but its a pulsating noise that you feel through out the cabin.

@Barrman Ill double check the mount. while at speed it doesn't seem to change in regards to curves/turns
 
@kennyw The motor mounts are brand new and appear good. I also had polyurethane motor mounts on previously and put rubber back on to see if it would help the vibrations to no affect. Ill double check the transmission mount
 
Ramba_ral said:
@kennyw The motor mounts are brand new and appear good. I also had polyurethane motor mounts on previously and put rubber back on to see if it would help the vibrations to no affect. Ill double check the transmission mount
Click to expand...
You should match the transmission mount to the motor mounts. Never mix polyurethane and rubber mounts.
 
Ramba_ral said:
1979 k5 blazer
12 bolt rear end. During the rebuild the axles were replaced
16" wheels and 315/75r16 tires
th350
4" lift
Pinion angle has been confirmed as good from drive line shop
Click to expand...
I am stuck on a pinion issue. Make sure the nut on the yoak is tight and not any play in the bearing,such as the crush sleeve did not get crushed all the way when diff was rebuilt. Also with the 4” lift ,it could be possible that the pinion angle is in spec when checked,but under a load it’s not. The rear axle will rotate up under a load,changing the angle. I would think the double ujoint (cardan joint) would be enough to over come that but maybe not .
 
Ramba_ral said:
I got a front brush bar and side steps if that counts.
Id hope its not tire/wheel cause I've gone through multiple sets. About 3 to 4 already
I see your in Henderson. I had Adams driveshaft handle the rebuild for the drive shafts and rear end.

also thought about air/wind but its a pulsating noise that you feel through out the cabin.

@Barrman Ill double check the mount. while at speed it doesn't seem to change in regards to curves/turns
Click to expand...
Well, adam’s isnt perfect. Ive seen two issues before on shafts. One time they didnt get an outer clip seated and my buddies motorhome driveshaft. It came apart on the road. Another one he had on a customers shaft that had inner clips, the clip was rotated and not in the flat machined area. The ujoint was off center. That would make it vibrate. They do good work, but anything can happen. I would double check.
 
Your pinion angle may be changing under load. If your really into the throttle at 65 it maybe twisting up beyond ideal. I have a bit of this happening at a different speed.

How much slip is showing on drive shaft.

Definitely look into the dampener and fan clutch, even if you find your vibration else where
 
For the slip this is what it looks like. Measured at about 3 1/2 in

20250204_073745.jpg
 
Thats a lot of slip showing.

IMG_20250204_123633.jpg

This shaft is similar to your set up. Pic is of pinion angle, before adjusting. You can just the amount of slip showing.
My finished pinion angle is just under 2° down from drive shaft slope
 
Ramba_ral said:
For the slip this is what it looks like. Measured at about 3 1/2 in

View attachment 496650
Click to expand...
Chock it, put it in neutral and push/pull on that slip shaft to see if it has any play in it. I can tell you from experience that the front ones always wear out and cause a lot of vibration on the road (hubs or T-case engaged). You can envision how that not being perfectly straight puts the mass of the tube in a bit of an orbit. The shop balancing it may have the slip in its proper (about 1/2-stroke) location and getting a different result than the vehicle does.

Have you measured the angles for yourself?
 
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