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Old 10-23-2007, 08:45 AM
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General U Joint Question

I have a question on the U joints for my 69 GMC 1/2 Ton (I have 1 long shaft with no carrier). Will I feel play/slop in the joint if it is bad? And if there is no play does that mean it is good?

The reason I ask is that I was experiencing what sounded like a semi's engine brake engaging when I let off of the throttle, this sound was at the back of the cab. There was clunking a little as well and a vibration when I let of the throttle slightly. But when I took the shaft off last night the joints look pretty new (I have only had the truck a couple of months) and there is no play or slop.

Also can a bad U Joint cause tranny failure? My tranny is fine but I was wondering if you drove long enough with a bad joint would the vibration wreck the tranny?

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Old 10-23-2007, 09:05 AM
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Possibly sounds like like a rod knock, if it sounds like a jake brake. Does it change speed with engine RPM? Louder when you let of the throttle and somewhat quieter when accelerating?
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Old 10-23-2007, 09:08 AM
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It seems Drive train related as it is only when the wheels are moving. When I run the engine in park or neutral it makes no sounds. And the sound is at the back iof the cab and not the front so I highly doubt it is a rod knock (which I have heard).

It makes no sound at all when increasing speed, whether it is slow or fast. It makes the sound when I completely let off of the throttle. It also makes a rumble type sound when I am going a constant speed.
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Old 10-23-2007, 12:19 PM
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Is it a standard auto trans?

gcrmcc
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Old 10-23-2007, 12:48 PM
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It is an automatic. TH350.
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Old 10-23-2007, 01:09 PM
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I was wondering if you drove long enough with a bad joint would the vibration wreck the tranny?
Yes, they can and do. Check for any endplay in the joints (side to side movement in the tube yokes and slip yoke) and that the snap-rings are all in place. If every thing feels good, check the tabs on the differential yoke for wear, these center the joint on the yoke, and the u-bolts. But if theres any concern about the joints, they're fairly inexpensive to replace. Hope ya find the problem!
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Old 10-23-2007, 02:14 PM
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I had a similar problem on my 66 chevelle when I first bought it. It was a loud "knocking" only during deceleration. I thought it was a bad u-joint at first but after looking into it further it turned out to be 2 broken teeth on the ring gear in the differential. Considering the seller said it was "just rebuilt" whoever put it together must have been drinking heavily because the gear pattern wasn't even remotely close to center... Anyway, check your u-joints first, if you are satisfied with them, put it up on jack stands, remove the rear diff cover, put it in neutral (with wheel chalks behind the front tires!!!) and spin the rear wheels slowly to look for broken teeth...

Check the PIC attached to see what mine looked like
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Old 10-23-2007, 06:01 PM
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Well I think i will look at the rearend next as you suggest 66. With the shaft out I started it up and ran it in gear and there is no sound at all at any speed, or will this only happen under more load?

The shaft/u joints themselves seems ok but I am far from an expert.
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Old 10-23-2007, 08:12 PM
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Well the sound was coming from my diff. I took the cover off and all teeth are perfect and there is next to know where on anything I see. Does that mean it is a bearing? is there a bearing on the front of the diff between the yoke and the first gear as that is where the sound seems to be coming from. Does anyone not reccomend NOT running the diff for a bit with out oil just to listen and watch?
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Old 10-23-2007, 08:39 PM
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I would not run it,Sounds like pinion bearing,its behind the yoke , you cant see it.you should be able to feel play between pinion and crown gear.Not sure what rear that is you might have 2 bearings.
The way you describe it doesnt sound like axle bearing,but check all the bearings be cause the noise can travel and be confusing
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Old 10-23-2007, 08:50 PM
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It is a 3.07 1970 12 bolt rear end. So would you access the bearing by taking the yoke off or by complete dis-assembly of the gears and axle bearings?
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Old 10-23-2007, 09:01 PM
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You pull the yoke off...and bearing should come out.
I'm not pro on rears,but you have to set proper clearances on those rears.If I remember right theres a crush collar in there and has to be torqued to specs,
I'm sure some one will come to your aid or check in the Wiki articles....good luck.
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Old 10-24-2007, 09:08 AM
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Now I have only done this one time, so I dont claim to be an expert - but if I remember right the bearing is pressed onto the pinion and you have to disassemble the entire rear end to get it apart. Disassembly is really no big deal, just make sure to keep all the parts together in the order they came out (left side and right side separated in zip-lock bags and clearly labled - this is very important for proper reassembly!!). Beware that putting it back together correctly does take some skill and special tools (dial indicator, bearing press etc) doing it sloppy can result in broken teeth and other associated horrors that are sure to put an even bigger hole in your pocket in the long run.

Realizing my own limitations I took the diff out of the car and paid a local racing shop $350 in labor to assemble my 12bolt. (I provided all the parts incl a new Eaton Posi, 3.73 richmonds, superior axles and a master install kit). Make sure you get references and dont just pick bob-joes clown car repair from the phone book. Proper differential setup is kind of a dying art in my opinion with the abundance of front wheel drive and fully assembled 12bolts right out of the catalog for the low low price of $1800 and up...
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Old 10-24-2007, 09:26 PM
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Since all the gears in the diff look like they have no wear at all would you guys just replace the 3 bearings? There was no metal in the oil either.
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Old 10-25-2007, 07:37 AM
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You don't need to replace much else if everything looks good. If its a posi you might want to inspect the clutches. I'm pretty sure you will need a new crush sleeve for reassembly and a new pinon oil seal. If you are doing it yourself summit has install kits that come with everything you need..

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