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NP241 Grinding/Rattling Noise

T

TexAg Troop

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Austin, TX
I have a grinding noise which I'm 99% sure it's coming from my transfer case. It happens when I'm coasting or lightly accelerating, and it's very obvious above 25mph. Any help to try and figure this out is much appreciated.

Here's some of the info:
- NP241C from a 1990 Suburban (passenger drop)
- 27 spline input
- SYE kit from JB
- mated to a 4L60E
- filled with the AC Delco AutoTrac II
- front and rear driveshafts from Tom Woods (very recent)
- this isn't my daily driver but the noise has just started recently

What I've done already to try and figure this out:
- rebuilt with all new bearing and seals
- replaced rear output bearing with a new bearing (John at JB was very helpful)
- replaced pump

I tested the truck on jack stands and:
- transfer case in N it doesn't make noise, or at least anything noticeable
- 2wd makes noise
- 4wd makes noise with both driveshafts attached, or front driveshaft only, or rear driveshaft only

It has made the same noise before and after all of the replacement parts so the only moving parts left that I can see are the planetary and the chain, and since it happens in 2wd with the chain not moving I'm assuming I could rule out the chain (it looks fine with no sag).

I'm looking to buy a new planetary, but it seems fine, there isn't any play in the six pinion bearings although there is up-down play.

Here's a video of the noise:
 
What condition is the rear driveshaft?
 
Rear driveshaft is in perfect condition, U-joints are good. But the noise happens when in 4wd with no rear driveshaft attached and vice-versa with no front driveshaft.
 
If you do decide to replace the planets, make sure to get the right generation. They made a change to the gear cut in 1994, the later planets won't work with your early ring and input gear.
 
Any issues shifting the case?

Mine didn't make noise that I recall, but it didn't like 4 low. Figured out the snap ring retaining the input shaft was missing. Since that allows the input a fair bit of movement, a possibility?
 
Thanks, yeah from what I've researched I have the wide teeth (I think?). Also, I have the double 15/16" row bearing.

I haven't had any issues shifting into 4lo although I don't really have a reference point. Sometimes it 'might' seem a little difficult to shift between 2-N-4h-4l , and I just checked but I have all of the snap rings (input shaft, input bearing, planetary).

I'm wondering though if I might have somehow damaged the planetary by re-installing the NP241. It's not a crazy heavy case but I've always just bench-pressed it into place and slid it onto the transmission's output shaft. Of course I don't let it hang there for too long but as long as it takes me to get the 6 bolts in and tightened. Most recently this all happened because my adapter between the 4l60e was leaking. I believe it was cracked because I had tried to replace the gasket multiple times and it still leaked so I ended up replacing it. For anyone that's interested I've found that the best way to install this gasket (AC Delco 15642510) is by coating each mating surface with Permatex High Tack Gasket Sealant 80062.

Oh! something I forgot to mention which I found odd was that my electronic speed sensor which screws into the output case was barely contacting the speed ring gear teeth when I would screw it all the way down, making an additional noise. In my trouble-shooting I ended up replacing the O-ring with a fatter one and this didn't allow it to bottom down all the way. Of course now I realize that I didn't exactly fix the original problem.
 
It's been quite a while since I did a SYE kit on one of these, but I seem to remember something about a snap ring needing to be in the correct groove. Do you think that this could be a possibility?
I can't convince myself that it would make noise in 4wd without a hands-on refresh of the assembly.
That sound makes me think that there should be some signs on some of the parts to help lead you to it by now. Just sucks to have to pull it for exploratory surgery.
 
something loose and jingling on the shift mechanism???

is there any feeling of vibration that you can associate with the noise? feel anything on the floor? anything on the shifter shaft? anything on the transfer case housing? or the transmission housing?

time for a cheap mechanic's stethoscope and some probing?
 
barf!

i just listened to the audio- not a loose shifter! that sounds like something is eating bearings or falling off a shaft. are you sure there is oil in it?
 
after doing the SYE on my blazer, i noticed that with the truck in neutral, with driveshafts off... the flanges on the TC will spin. i can stop them with one hand, but something is creating drag and causing them to spin, and i assume this has to be in the transmission (i have manual sm465) - countershaft? bearings going out? low oil/wrong oil? with the truck on the ground and driveshafts on it is not enough torque to make the truck move, but either way- no noises like that video you posted!
 
Any chance you ended up with a 32 spline input on the t-case that is now semi-engaging.
 
Thanks for all the ideas (I've been knocked out this week having fun with the flu).
I have the snap rings in the correct locations AFAIK. On the shaft that I received from JB it only had two grooves for the tone wheel anyways. Previously, I've taken a stethoscope and listened and other than it all being loud it definitely seems like more noise is coming from the transfer-case case.
Input/output shaft is 27 spline.

I went ahead and drained and pulled the transmission pan just to check. The fluid is still red and seems to be clean enough although there was a good amount of "silt" at the bottom of the pan. Figuring this is break-in residue from the clutches that I recently installed from a rebuild kit.
I wiggled the output shaft from the 4L60e and it seemed to have some play but possibly within spec, whatever that is.
 
Been a little bit, but I have since pulled another NP241c from the junkyard. This one is from a 92 K1500 so it's driver side drop but still has all of the same internals. I've swapped over the input shaft, 6-pinion planetary, shift collar thingy, chain and the range gear (I know those aren't the real names but whatever). Basically everything that I can swap over I did. I've rebuilt everything and will be finalizing everything this weekend and test driving it.

I put the original planetary back on the output shaft of the transmission and it makes a "clunking" noise (see video). Although when I checked the planetary from the '92 it did the same thing. I'm thinking this might be normal? and just the sound that the gears make.


As far as it goes locally, I'm on my own for diagnosing this. I called the offroad shop and they said to call a specialty driveline place they suggested, called that place and they just don't want to touch it.

I'm worried it might be the rear planetary in the transmission.
 
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