Converted NP203 grinding sound in 2WD

[FitzK25]

I have a converted 203 in my 75 K25 - truck is fairly new to me, never been inside the transfer case. It has a piece of threaded pipe so you can overfill it for the part time conversion. The truck had been sitting for a while when I got it so I changed all the fluids, including the transfer case, with straight 30w. First couple of thousand miles all worked just fine. In the last couple of months it’s developed a nasty metallic buzz/grinding sound in 2wd high. I already checked the shifter adjustment to make sure it’s grabbing the correct gear and not drifting into neutral or something, and checked the fluid level. The sound comes and goes, I can go a whole day without hearing anything, and the next it sounds like a can of marbles every time I use anything but the lightest touch on the throttle. The sound when it’s particularly bad causes the shifter to visibly vibrate forward and backward at a high rate. If I shift into 4 lock it’s perfectly fine, no sound, either with or without the front hubs engaged. Does this sound like a problem with the part time kit and/or differential assembly in the tail housing? It’s my daily and I’m trying to get an idea of what I need or probably will need before I tear into it. I’m looking at a couple of two piece cast differential housings to possibly order in hopes that the problem lies with that crown gear engagement rings. Thanks in advance for anyone’s thoughts.

 

[AuroraGirl]

Uh.. pretty sure the transfer case calls for 10w30.. its too viscous and not lubricating

you need to run it periodoically engaged to lubricate the front side even tho you overfilled.. im guessing the honey you put in there isnt doing well

 

[FitzK25]

It calls for straight 30w non detergent. And yes, I’ve been running it in 4wd regularly so the chain throws oil back into the rear housing.

 

[idahovette]

Checked the u-joints???

 

[77 K20]

Have you checked to see if it is actually going into 4 wheel drive when you move the lever? And not just 2 wheel drive?

 

[FitzK25]

U joints are fine, and yes it’s going into 4wd just fine. No noise in 4 lock, and it works great. I’ve had somebody listen/watch outside the truck and the sound is internal to the transfer case.

 

[77 K20]

I was just thinking maybe your chain broke. As after mine broke the shifter would rattle and make noise and there was noises from the transfer case. Then after the sound coming and going it "went away". Everything was nice and quiet. And then the day I went to go play in the snow I found that I didn't have 4HI or 4LO.

Guess pieces of the chain were causing the intermittent noise on mine until they all fell down into the case and quit moving.

 

[coolpup32]

new guy here.

I'm having the same problem (grinding/pinging sounds in 2wd). Restoring an old 76 k10 with PT conversion on the np203. Truck only has 60k mi, i was told the chain is stretched but i just pulled the inspection plate. It doesnt have too much slop in the chain, i dont think its enough to jump gears. Besides the fact the chain isnt under power when in 2wd. Judging by the smell, it had gear oil in the tcase.

The u-joints appear to be ok but the rear slip-yoke has a lot of slop and makes a similar pinging sound when i shake it by hand.

When i hear the 'grinding' sound i can feel heavy vibrations in the tcase lever that seem to match the sounds im hearing. I could not get 4hi to engage until today after oiling the shift linkage.

At this point i'm thinking a few possible causes (sanity check me please!)
- Shift linkage needs cleaning / oiling
- Rear drive shaft / yoke
- Rear output bearings in tcase from lack of proper lubing


other notes:
1976 k10 Scottsdale
th350 tranny
np203 with Rancho suspension conversion
original 350 cid engine
new 35in cooper M/T

 

[75gmck25]

It doesn’t sound like a u-joint noise; however, take a close look at the cv joint in the front driveshaft. It goes through a lot of angular movement while also spinning around and keeping two u-joints aligned. There are quite a few moving parts that can wear.

 

[coolpup32]

It doesn’t sound like a u-joint noise; however, take a close look at the cv joint in the front driveshaft. It goes through a lot of angular movement while also spinning around and keeping two u-joints aligned. There are quite a few moving parts that can wear.

In 2wd and hubs unlocked, the front shaft isnt under power??

I forgot to mention, my sounds are intermittent, lasting 1-2 seconds and sounding gear like

 

[FitzK25]

@coolpup32 That sounds exactly like what I’m experiencing. My only update is I pulled the rear driveshaft yesterday, and while the output shaft bearing feels tight in the transfer case, there is a ton of rotational slop. It feels like it’s in that rear differential assembly. I was going to take the cover off and take a look at it, but the massive 1 5/16 retaining nut for the rear yoke is both rounded off and frozen solid. I couldn’t get it to budge, it’s soaking now.

 

[coolpup32]

@coolpup32 That sounds exactly like what I’m experiencing. ...

I agree. I have slop in my rear diff too, i wouldnt say its excessive though. most of my rear drive shaft slop is in the telescoping piece near the rear yoke (whatever its called).

I did something really stupid this morning and jacked the rear-end off the ground and placed in drive, revved up to ~50 mph. I still had the pto / inspection plate off so the tcase is empty, i didnt do it long... anyway, with little stress on the drivetrain it didnt really make the same sound but something similar (maybe). It seemed to happen predictably when i let off the accelerator. I have a video i'll upload to yt in a moment if it helps, but hard to hear with exhaust leaks.

to me it seems like its coming from rear-output side of the tcase. I have limited experience with drivetrain issues so i'm not really sure... and i've never been in this case before

 

[coolpup32]

Here's the video, you can hear the chatter at 24 & 31 sec mark. its much more noticeable on the road under stresses, etc...

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[FitzK25]

Yeah that sounds very similar to what I have going on - although it mostly appears under acceleration or load, IE on a hill. It’ll chatter and rattle for a couple second and then go away, and vibrates violently up the transfer case shifter. It’s almost like the metallic buzz you get when grind gears in manual transmission if you don’t fully release the clutch. There is also an audible clunk when you let off the gas or get back on it above 15 mph or so.

 

[coolpup32]

Yeah that sounds very similar to what I have going on - although it mostly appears under acceleration or load, IE on a hill. It’ll chatter and rattle for a couple second and then go away, and vibrates violently up the transfer case shifter. It’s almost like the metallic buzz you get when grind gears in manual transmission if you don’t fully release the clutch. There is also an audible clunk when you let off the gas or get back on it above 15 mph or so.

YES! exactly the same for me. this is just in my shop on a jack so not under a load per se... but going up hills, accel / decel i hear it, although much louder and violent than in this video. I havent tried, but i've heard it will not happen when in 4hi