FNG here.....Wasn't sure where to make the first post.

K5Devil

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I picked up a 1985 GMC Jimmy today.

This is my first Jimmy/K5 and man, I am excited. (I am NOT a novice to all things Chevy/GMC)
Mechanic/Machinist by trade and son to a 40 year career now retired Chevrolet/GMC Mechanic/Machinist. A lot of Chevy been through my beer scented sweat and blood.

This truck came from Florida all the way to Washington. Only Rust is on the tailgate which needs replaced. Floors are solid.

4:88 Gears (So I was told......and after the 60 miles back home it definitely feels like), 383 Stroker > 700R4. 3/4 ton axle swap with all suspension lift done very nicely. Acceptable paint. Got the steering strengthener kit installed.

The interior needs help. Ill be using the forums to get some info on some of the issues.

There is a catch, which is why the price was so good for me.

There is a gnarly rattle at low-rpms. To me it sounds like u-joints or possibly an unbalanced drive shaft.
I am really hoping it is not loose torque converter/flywheel. To me, it seems to be the drive shaft, as I have had a truck do something similar to this before. At low rpms it sounds like you're dragging a bus behind you but if you step on it (And this thing launches quick) it will go away. Only around 5-30 mph is it noticeable and as you increase in speed it goes away and is completely gone at highway speeds. I did notice a loose exhaust hanger but I am pretty sure this came loose from the vibrations of the driveshaft. Super fun to drive.

Anyways, I seem to have started rattling on. Here are some pics. I am looking forward to talking with some guys with the knowledge I seek.

Cheers,
Devil

IMG_0773.jpg

IMG_0774.jpg
 

CK5

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Welcome! Looks like it is in great shape, I would start with the easiest things first which would be the u-joints and go from there, I'm sure others will chime in with other things to check.
 

K5Devil

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Thanks for having me. Been looking for a rig like this for years. Also, been strolling this forum and others for a while. I knew I’d have to go premium once I found the truck.

Yeah, I am definitely putting some u-joints in first thing. Shotgunning with educated guestimation.
 

Wes Harden

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Welcome sounds like a nice rig you'll get lots of enjoyment

check he drive shaft angles at each yoke. they should be the same. also the t-case cross member maybe lowered this will mess with drive line angles
 
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K5Devil

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Already in need of some clarification:

I believe the joint I’ll need is a crossover 1310-1350 as these have 14 bolt axles installed. If you guys can tell just from looking or know how to measure for it with the drive shaft installed I sure would like to know.

Thinking 1310-1310 for the shaft trans end and 1310-1350 for the shaft diff end.

Is that a pretty common set up? I won’t be wheeling this much and I am more worried about getting it to ride smooth than breaking on some trails.
 

Justin V

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There are some good bits under there. Not the best of everything by any means, but definitely not the cheapest either. Rear shaft looks fine as long as the t-case side is double cardon.
 

muddybuddy

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Welcome! Great looking k5.

Pic of tcase side of rear driveshaft will help us.

If its just a single u- joint at the tcase end (which is how they are from the factory), the pinion pointed at the tcase is not what you want, you want it parallel, which could be achieved by removing the shims if there are any under the leaves, or flipping them around to shift the pinion down. Can you post pics of the shackles on the rear leaf springs? Cant tell if its a shackle flip (which could be causing the pinion angle) or if its just leaf springs. If its shackle flip, you could switch to shorter shackles, but that would reduce lift (which you probably need to match the front).

In short, a couple more pics and we can diagnose if dshaft angle is culprit of vibs.

If its a np208 or np241 tcase, it will be an s44 ujoint in driveshaft, so you need a s44/1350 conversion ujoint. If you search on here for that, a part number will come up to use.
 

K5Devil

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E0E44646-CA26-4E5D-8906-B133837FD85C.jpeg BBCAEBDB-ADC4-4103-AD5B-B4A984A7BCE1.jpeg It’s got a dual coupler at the T-case

All the exhaust was loose. Fixed that to help narrow down the vibes.
 

K5Devil

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Wow. Just making sure all the exhaust was tightened really helped. Definitely sounds and feels like u-joints.
 

SpeedlabDan

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Welcome and nice truck! Only thing I may have to add if I missed it is to make sure you have at least a couple degrees in the rear driveline as a strait on alignment will also cause bad vibes.
 

K5Devil

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Did a little cleaning up. Trying to get things working properly again.

Under the dirt this thing has trick flow aluminum heads. That was not mentioned to me and is a nice surprise.

Was told the A/C didn’t work. Simply tightened the belt and it blasts cold air .

There are wires coming and going everywhere in this thing. Luckily it mainly is just added wiring and the harness itself is in ok condition. Someone tried to put some crazy car audio set up in here and it looks like shit under the hood. 1 billion wires coming to the battery and ran over top of hoses and the like. Amateur stuff. Easily fixed.

C65E934E-F3CA-4A73-A981-7498386A1BC6.jpeg 362283B6-BB9C-4E83-A458-1FCFCB251716.jpeg 2EE96B2F-C752-406C-BDF2-5A5224173E81.jpeg
 

K5Devil

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im Pretty stumped on what combination of ujoints this is going to take.

It looks to be a custom drive shaft so I am wondering if the first two (diff to shaft - shaft to Cardan joint) are 1350’s and the one off the tcase is a 1310. That might be where the failure is. Might just take it in and get the shaft balanced at the same time. Only problem is I won’t have time for that for three weeks since I work 3 weeks and get 2 days off most the time.
 

muddybuddy

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Do you have an accurate way to measure the you joint cap diameter? Or even to compare whether all caps on the joint at the rear axle are the same diameter? That would tell you whether is a straight 1350 ujoint (in which case, just get a rear ujoint for an ‘85 k30) or if you need a conversion ujoint. Given its a custom driveshaft with cv at tcase, i would think its a straight 1350 joint. If you could just replace that joint that would give you a clear indicator if the cv joints are the issue or if that fixes it. The axle side ujoint is going to be much cheaper than the cv joint.
 

K5Devil

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Do you have an accurate way to measure the you joint cap diameter? Or even to compare whether all caps on the joint at the rear axle are the same diameter? That would tell you whether is a straight 1350 ujoint (in which case, just get a rear ujoint for an ‘85 k30) or if you need a conversion ujoint. Given its a custom driveshaft with cv at tcase, i would think its a straight 1350 joint. If you could just replace that joint that would give you a clear indicator if the cv joints are the issue or if that fixes it. The axle side ujoint is going to be much cheaper than the cv joint.


Oh I see what you mean. Just replace the one at the axle see if that fixes it. I just thought I might as well do all of them. I have dial calipers to measure them.

I’ll probably end up pulling the drive shaft after the wife goes to bed.
 
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muddybuddy

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Oh I see what you mean. Just replace the one at the axle see if that fixes it. I just thought I might as well do all of them. I have dial calipers to measure them.

Well if you have calipers measure the caps for all of them. If you can replace all then thats obviously preferred. I was suggesting the axle end one first because thats the easiest and can be change on its own.
 

K5Devil

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Right, I didn’t think I could measure the caps well enough with them installed. I also have to beat them out with a BFH and elbow grease so I’m just procrastinating about it. Haha
 
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