Vibration. K20 '76 th350 NP203 and a 400 motor.

[Pavin]

Well guys I got a problem. It's a 76 square body. Yep this may never end. I bought this truck about 2 years ago and have been searching for a solution ever sense. Pleas don't say buy a Ford. Here's my story. I get this truck running after towing it home from the P.O. He let it sit for about 10 years or more. I know, I know but I fell in love with it. I get it up to speed and there is a vibration that is pulsing at around 56 miles an hour. You can really feel it on the 4x4 shifter. So I begin to change things out as I have the money. First thing up is the u-joints (Rear) then have the driveshaft checked for balance then motor and trans mounts, then harmonic balancer changed, (long list) flex plate and torque converter, Front fan, Alternator ( yep ) Distributor new, Spark plugs and wires, Pulled front driveshaft, pulled rear diff cover to see nothing odd here ( but I am a newb in that area), Tires were the first thing I had changed and they came off of my smooth running 73 then new tires and I balance them then had some ya hoo at a tire shop re-balance them (waist of money). Now for the current testing and where I am at: Jacked this thing up in the rear and put her in drive, put my head on the front brush guard and that vibration was very apparent. This is when I pulled the front driveshaft and found 3 bad u-joints. Leave that thing out and go for a test drive. I get to the freeway and get her up to speed. Now this is all down hill, literally. No vibration (It's fixed, happy dance in the seat) about 4 miles. Get off the freeway and head back home but now it is up hill. Crap just like before. The pulse of death feels like she will just come loose at any minute. Back home. Head down kicking the dirt. Next up ... Pull the rear driveshaft and check it again. Head on hood. Super minor vibration in drive at rpm. (No load) There are 4 shift positions for the 4x4 Engine off ... in 2H the front driveshaft connection spins very freely, (OH I am thinking the np203 is the problem so I bought another one and may install it but every thing except the shift positions is the same) Back to the 2H position thought, Seems right, 2L the 4x4 engages and there seems to be about an hour of the clock rotation slop. Hum same as the other np203 of which I took off the front drive rear bearing cover and can see that this is do to the chain slop / loose. The 4x4 doesn't really matter in 2H and traveling at 55 mph. This thing won't shift into 4L at all, (could be the linkage I think) It goes into 4H but not 4L almost has to be the linkage but you are the experts. Now I don't want to pull that NP203 unless I feel like a he man from the amazon. (Last resort so to speak) I weigh about 150 so that monster is twice my weight and I would be upside down. Ok ok ok so the transmission has a whistle in park and neutral and goes away in drive. Um, the darn thing is factory height but had a camper on it for it's entire life and pulled a trailer. Everything is stock, stock, stock. Last winter here in CA it seemed to run with no vibration until I got down the road a piece and it warmed up. It was 40 degrees out. the oil in the Np203 is oil and pretty thin. Most likely 10-30. It's about 1/2 inch above the fill hole as it has a 90 degree on it. The rear diff has slop in neutral might go an hour or so on the clock. It wouldn't be the tires as it does not vibrate going down hill like it does going up hill. Thanks for any input. Pavin.

 

[Frankenchevy]

I’ve never owned a truck with an np203, but I think they just have hi, lo and neutral. No 2wd, it’s a full time 4wd case.

Maybe some parts are mixed up and contributing to the issue, maybe not.

 

[Pavin]

Oh ya your right It is not as stock as I thought, it has the conversion kit to make it part time 4x4 and has the locking hubs. Ok so I think every thing else is stock. Ya know I don't even know what kit it has in it for the tail / out put / rear drive. I would bet that it has the cheap one in it as the "other" np203 that I have also has the cheap one in it. It is like 100 dollars for that one and I can not find the "good" conversion for the rear out put anywhere. Mile Marker I think.
Thanks.
Oh I also dropped the pan on the trans and found almost nothing in it but a bit of black muk. Not much if any metal so I put a magnet in it and a new filter and sealed it back up.

 

[Frankenchevy]

It sounds like you’ve covered most of the rotating stuff. Usually it’s obvious if it’s brake related, so no need in mentioning that. You said the engine revs smoothly in neutral and park.

Sounds like focusing on the rear end, then possibly trans. If you disconnect the driveshaft from the rear diff, is there axial play in the pinion?

 

[78C10BigTen]

axial play in the pinion?

You mean front to back?

 

[Frankenchevy]

Yeah. I felt a pinion recently that had slop like that. I was told it’s easier to feel for than radial play when it’s all loaded up in the diff. I think with a conical bearing shape, the two go hand in hand.

 

[78C10BigTen]

Yeah. I felt a pinion recently that had slop like that. I was told it’s easier to feel for than radial play when it’s all loaded up in the diff. I think with a conical bearing shape, the two go hand in hand.

Was curious as ive never heard axial play

 

[Pavin]

In neutral and park there is no vibration. There is whistle and with the stethoscope I have pinned it down to the hump of the trans right at the rear-ish area. The whistle is only in neutral and park. I just put it in neutral and checked the rear pinion slop and there is some maybe a half hour but one more note to add about that is when I put the truck in reverse the truck makes a clunk and ya it's a very noticeable one. I will drop the rear drive shaft tonight and check the play front to back tonight.

 

[Frankenchevy]

Ive been told an average clunk is not abnormal.

Sounds like you may have narrowed it down. Maybe look for a th350 to swap in. Worst case scenario you can sell the other one to recoup the cost if the problem is still around.

 

[Pavin]

Ok so I dropped the drive shaft and checked the front to back slop on the pinion and nothing. I did check the rear output shaft by turning it while it is neutral. It spins easier counter clockwise when looking at it from the back and harder clockwise. I then noticed that the whistle is there in the transmission just by turning the rear out put. I then noticed that there is a click in the transfer case every 7th hour of rotation. Fast or slow does not matter. Like that is not the same as the other transfer case I got. Is that whistle a pump in the transmission for pressure? What would click about every half turn of the output shaft inside the transfer case? I also just noticed that after a click then turning it the opposite way will make it click pretty quick like about an hour of rotation. Might be a bearing missing?

 

[Pavin]

I have a turbo 350 in my parts truck, does anyone know if a np203 transfer case takes a short shaft or a long shaft? Looks like my parts truck has a long shaft in it.

 

[bft305]

If you have the np203 then I would swap it and see if it helps. I read that it should have 10w30 or 40 in it. I use it in my np203 and had no issues. A moderate clunk is normal cause of the chain. Even if you have the part time conversion and hubs you should put it in 4wheel drive and drive it for a bit like that to splash oil in the back of the case. When I first got mine I thought it had a part time conversion but the PO just put hubs on it. 4low is hard to get engaged sometimes, I am planning on twin sticking mine once I fix the bigger issues. Have you replaced the tranny or engine mounts yet? That can help with vibrations if they are old and cracked. Since you are hearing a whistle from the tranny it might be time for a rebuild. I would also just replace all the u-joints while your at it, then you know they are good.

 

[Wildjoe330]

I did a lot for my Pulsating vibration (2 sources of vibration)
that was at around the same speed. I have a 205 waiting to
replace my 203. Getting rid of the TC with the chain seems
the best for me.

I'm gonna look in my records and double check see if
I'm running 10w-30w or 10w-40w

 

[Dooley]

Oh ya your right It is not as stock as I thought, it has the conversion kit to make it part time 4x4 and has the locking hubs. Ok so I think every thing else is stock. Ya know I don't even know what kit it has in it for the tail / out put / rear drive. I would bet that it has the cheap one in it as the "other" np203 that I have also has the cheap one in it. It is like 100 dollars for that one and I can not find the "good" conversion for the rear out put anywhere. Mile Marker I think.
Thanks.
Oh I also dropped the pan on the trans and found almost nothing in it but a bit of black muk. Not much if any metal so I put a magnet in it and a new filter and sealed it back up.

I did a lot of these conversions back in the day.
The 2 best ones were pricey at the time.
In my 1977 K10 Suburban I installed the Doug Nash Engineers kit that was a full replacement shaft.
In my 1979 K30 I used a SAE Engineering kit. it was a bit heavier than the Doug Nash kit.
The SAE even had a thrust bearing for the intermediate shaft to ride against.
I still have a pile of 203's around including one with an SAE kit in it from a 79 K20 Suburban.
Been saving that one as I have the mated T400 with it.
I even found a box recently that my Husky Selectro hubs for the K30 came in from Midwest Four Wheel Drive Center in 1980.

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[Snoots]

Slack in the chain of the NP203. Rebuild it.