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NP208 Transfer Case Causing Vibration?

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PaulZ

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Got a pretty good vibration going on at highway speeds, comes on about 45-50 and continues. U-joints good. My '83 K5 had a 4 inch lift when I got it, back to stock now, rear pinion and trans are parallel.

So, to diagnose, I removed the rear driveshaft and drove it in 4WH on the front shaft, it was smoother but still some vibes, especially when letting off the gas. Just today I removed the rear shaft again, started it up and put it in drive standing still. Vibration again comes on. So I think I've eliminated the rear end, rear shaft and u-joints. Revved up in park the vibration is gone, which is leading me to think it's either something in the tail end of the trans or the transfer case. With the drive shaft out I can move the output shaft in and out of the transfer case just a bit, maybe .050", and up and down just a bit too. With the shaft in there is a touch of up and down wobble but not enough to cause a leak.

Where to go from here? Is it worth pulling the tail housing off the transfer case to check anything? Oh and when I got this truck the transfer case was pretty low on ATF, over a quart iirc. But it works fine in all ranges and makes no other noises.
 
I wouldn't rule out another cause since it does it when standing still. There's always going to be more vibrations in drive than in park because any vibration coming from the engine is now transferred to the whole drivetrain. There are lots of causes of NVH-vibration in motion which would be caused by things like drive shafts, out of balance wheels, etc, and stationary vibration which would point to broken/worn body and engine mounts, misfiring, poly mounts, etc. Check your tail shaft bearings for slop, but I've yet to see a TC be much of a contributor to vibration unless it has a bad mount, trashed bearings or it's messing up driveline angles. Others may have different experiences.
 
Thanks Paratrooper. This is my first 4x4 so not much TC experience. I have considered, and checked the motor and trans mounts. But since it comes on only at speed and doesn't seem to be related to torque I haven't given that too much thought. Engine induced vibration I would think would also show up at the relevant RPMs in the lower gears. But yes, it could be some metal to metal contact that only happens at highway speed for some reason. Thanks again.
 
How do the trans Mount bushings and bolts look? You might check the bolts on the trans cross member to the frame as well.
 
Hmm, mount bushings, I don't know about those, all I did was put a jack under the transfer case to see if it would move up, seemed tight. I think my '83 mount is different than the earlier ones. I'll pull it apart for a better looks. Definitely has been soaked with oil. Thanks. Oh, and my crossmember to frame bolts have frequently come loose, think they are pretty tight now.
 
PaulZ said:
Hmm, mount bushings, I don't know about those, all I did was put a jack under the transfer case to see if it would move up, seemed tight. I think my '83 mount is different than the earlier ones. I'll pull it apart for a better looks. Definitely has been soaked with oil. Thanks. Oh, and my crossmember to frame bolts have frequently come loose, think they are pretty tight now.
Click to expand...
If they are like mine, they are all eaten up. This is what mine looked like before I replaced them. The one next to it is the new energy suspension mount.

9BC20923-A898-471B-9FD3-BC2570FD6503.jpeg
 
If it vibrates while the vehicle is standing still, the problem has nothing to do with the T-case, driveshafts, axles or wheels. Most likely the motor mounts and/or trans mount are bad, letting the engine vibrations get into the frame. This will sound & feel different in Drive and Park because the torque puts the engine in contact with something.
 
A slipped harmonic balancer can make it shake too,if its an externally balanced engine ..a bent fan blade can make some nasty vibes also,and will eventually ruin the water pump bearing & seal..
 
Thanks again for the replies guys. Well yesterday I did more testing. I pulled the TC mount bolts, bushings look fine, no metal to metal contact.

Prior to that, on my way home from the cabin I had to stop for gas, on empty. I put in 25 gallons, got back on the highway and it was much smoother. Hmm. Anyway got home, and when checking the rear mount notice the transfer case being very close to the floorboard on the passenger side, maybe 1/4", and the floorboard a little shiny where it might be hitting, but not scarred or dinged in any way like would be expected. The crossmemeber has slots where it can be slide over a bit but it's already all the way on one slot. I decided to loosen the bolts and shim it down about 1/4" which gave it more clearance and went for a test. It's smoother than before but I'm not sure if the tank of gas changed the u-joint alignment enough to make a difference, whether the case was in fact hitting the floorboard, or whether my body mounts maybe be sagging or hitting something. Is there a spec for the distance the body should sit off the mounts?

BTW I replaced the harmonic balancer recently which had a stripped pulley thread. No change. All very confusing I know.. I still think any engine related imbalance would show up in the lower gears. It's smooth all the way to 50 or so no matter what gear.
 
Blue85 said:
And how are the motor mounts?
Click to expand...

Whoa! I was just under there checking the converter bolts and noticed the drive side motor mount is making metal to metal contact between the lug where the long bolt goes through and the frame part of the mount! Other side has 1/4" clearance at that point. Going to be a few days before I can change it. Hope that's it! thanks!
 
replace the motor mounts and tranny mounts together..that way the new ones dont help wear out the old ones.
 
Thanks! Still haven't been able to get to the mounts but in the mean time this weekend is half price weekend at Pick and Pull, I'd like to go grab a transfer case just to have a spare, maybe rebuild it without having to have my Blazer down. About $100 out the door. Is it just a matter of dropping the drive shaft, cross member and unbolting from the trans, or is there more too it? Special tools?
 
Should be able to remove one without taking off the cross member..
Other than dropping the drive shaft,its just some bolts and the speedo cable ,shift linkage and torque rod that has to come off..
You may need to use a regular wrench on the trans to t-case bolts,a ratchet and socket might not fit in there..

I'd take the adapter to the trans too,they are not getting any easier to find and unless they charge you a lot more to keep it,its worth having a spare..ditto for the torque rod ,one bell housing bolt holds it on..
 
I would try to find an np241 for a spare instead of another 208. That way you could throw a slip yoke eliminator on the 241 and get rid of the biggest prob with the 208.
 
And don't buy cheap motor mounts. Either get the poly mounts from Energy Suspension or the DIY4x mounts. (I recommend the DIY4x ones) :)
 
Thanks Guys, unfortunately I didn't notice the 241 advice until just now, and I just got home from PicknPull. Ever notice, when you're out in the rain, laying in 3" of water on top of 3/4 crush, soaked to the bone, with one hand over the tail shaft holding the socket on, straining your neck to keep your head out of the water, the other hand on the ratchet which only has room for one click at a time, how f'ing long those bolts seem? Anyway this is what I got, out of an '89. I noticed a newer one but the front drive shaft was on the driver's side? I did grab the torque rod, my K5 doesn't even have one. Is it really necessary?

bYxRPUF.jpg
J1gG3q2.jpg
 
PaulZ said:
Thanks Guys, unfortunately I didn't notice the 241 advice until just now, and I just got home from PicknPull. Ever notice, when you're out in the rain, laying in 3" of water on top of 3/4 crush, soaked to the bone, with one hand over the tail shaft holding the socket on, straining your neck to keep your head out of the water, the other hand on the ratchet which only has room for one click at a time, how f'ing long those bolts seem? Anyway this is what I got, out of an '89. I noticed a newer one but the front drive shaft was on the driver's side? I did grab the torque rod, my K5 doesn't even have one. Is it really necessary?

bYxRPUF.jpg
J1gG3q2.jpg
Click to expand...
Yep, that's the holly grail 241 mech speedometer.
It's funny how this happens.
I bet if you went looking for it you wouldn't find it
 
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