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Discussion Starter · #1 ·
1981 GMC 1/2 Ton 4x4 - 4spd manual

Had a u-joint bust and driveshaft fell out and busted up, got one from a 3/4 ton and got it shortened and got an adaptor U-joint to make it fit.

Recently on the highway home from the dump heard some noises coming from the transmission area. A few grinds of the gears and then that was it. When i made it to a stop light, after i dropped the clutch there was a big "clunk." I checked lots and asked around and people said worn spiders and such like that, or ujoints. U-joints are fine, i pulled the diff cover and checked the gears everything looked good. Driveshaft can be rotated, it has quite a bit of play, but so does the pinion and the spiders, they all move too. I thought the axle shafts might be worn but it doesn't seem likely. When the clutch is let out, the driveshaft moves to take up that slack, then as soon as it hits it goes clunk and the truck is shot forward. I've noticefd the back axle has a definite up/down movement when this happens, it will haev to be addressed (front spring bushings) but im sure thats not it. I still have lots of slack in the driveline, and every time i drop that damn clutch it goes CLUNK and takes up the slack, its fine when i am moving though. I suspect that the transfer case is the problem, but I am not sure I do not know a lot about the transmission transfer case or rearend. I use the truck every day for hauling and i will need to do some towing here so I must remedy this quickly. Any help is greatly appreciated
 

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If there is a lot of slack in the transfer case chain, it will clunk.

Is it doing it in 2 WD or in 4 WD?

In 2WD the transfer case should be out of the equation, as the power is not going through it at that time. I know that the automatics have full time 4 wheel drive, but I think the standards don't use the transfer case in 2 wheel. I could be wrong on this............
 

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The rear drive gets power through the transfer case at all times, no other way of getting power to the rear axle without more parts than you would ever need.

Axle wrap (the axle twisting on the springs in relation to power being applied or removed) is completely normal, In drive the axle should twist pinion UP, and in reverse it should twist DOWN. This is completely normal.

You WILL have play in the slip yoke of the driveshaft, less is better of course.
You WILL have play in the backlash of the ring and pinion gears, You should ONLY have a certain amount.
You WILL have play in the spider gears, less is better.
You WILL have minumal play in the spider gear to axleshaft spline area, anything more than 1 degree is too much if even that much.

The larger the size of tire you are running, the more you are going to notice this, I have seen as much as a full 1/4 turn of the driveshaft between shifting from reverse to drive with the brake applied to prevent ANY movement of the tire and wheel.

If you have a transfer case that is chain driven (203, 207, 208, etc.) the slop in the chain is really only noticed between the front and rear drivelines, and should not be noticed in 2WD.

Since you have the 4-speed (SM-465) transmission you have the NP-205 transfer case, it is completely gear driven inside, and you should not have much to worry about with that being a part of your problem.

I would venture to say your biggest problem is the rear driveshaft slip yoke, followed by your spider gear wear, and then your ring and pinion gears backlash, and then your axleshaft splines.

I ALMOST FORGOT, do not forget to check the universal joints, and KEEP THEM GREASED. They should have NO measureable play in any of them.
 

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Discussion Starter · #4 ·
hehe

Well today the chain blew out of the transfer case and blew a hole right through it. I left it too long, but it was the transfer case. It still clunks even without the chain, too late to fix anything with a fist-sized hole in the bottom of the t-case. oh well, fun fun
 
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