99 1/2 Gmc 2500 4wd Vibration In 2wd

[Spurshot]

I posted this on the "ask a GM Tech" section as well, but thought it may be more appropriate here.

 

I have a 1999 1/2 New Sierra 2500 with a 6.0 eng, 4L80, trans and pushbutton NP246 transfer case 210,000 miles. I noticed a vibration in the driveline after replacing U-joints. I'm not saying it wasn't there before, just that I noticed it afterward.

 

The vibe occured in 2 wd at speeds above 45 mph and became pretty significant at freeway speed, enough to buzz the mirrors.

 

I isolated it to the front driveshaft which was turning while in 2wd. I did this by jacking the rear and removing the wheels/tires to run the rear driveline without wheel/tire vibe influence. During this exercise, I noticed the front driveshaft turning while in 2wd with the front wheels/tires on still on the ground.

 

I pull the front driveshaft and the vibe is gone. I wasn't aware that the front driveshaft should be turning while the TC is in 2wd, but others tell me it turns in 2wd by design. I still don't know if that's correct. The front driveshaft can be turned by hand while in 2wd, but the drag of the internal TC clutches is significant.

 

BTW, all the 4wd hi and low fuctions seem to work fine as tested in the dirt. 2wd seems to actually be 2 wd when driving.

 

I've chased this problem for a few months and couple thousand dollars so far. Here's the list of things done to date:

 

New TC rear DS yoke bushing

Replaced the new Jap autoparts store U-joints with Spicers & checked for free movement

Check front DS for straightness, balance by a DS pro

Rebuilt front diff, especially conscious of pinion bearings

Rebuilt TC with new clutches, chain, bearings etc.

 

After the rebuild of the NP 246 TC, the vibe is much reduced, but still noticable in the pedal and gets noisy at around 75 mph.

 

I have not as yet tested for DTC fault codes.

 

Questions:

1. Should the front driveshaft turn while in 2wd?

If not, could this be an encoder/motor issue of not properly/fully placing the TC in the correct position?

 

Even my 4wd pro and 4wd enthusiast brother are at their wits end on this one. Help!

 

BTW, the truck is all stock, no lift or any other modification. And all driveshaft angles were checked by a good 4x4 pro with experience in driveshaft angles.

 

Chuck

[green 788]

My 1996 3500 has its front wheel shafts turn all the time... I have assumed this is the normal way of things for these trucks. But I'm not sure about the actual TC front shaft. But since the front shaft from the TC case is connected all the way through to the front wheels (we don't have lock in-lock out hubs on our GM trucks)... I believe that the front TC shaft would have to turn also, even in 2wd.

 

The fact that the bad vibe goes away when you remove this shaft would seem to indicate that the problem is in that shaft, or in the carrier bearing in the TC. But you've already checked these things... hmmm.... this is a pencil chewer.

 

The only other thing I can think of might be that the suspension mounting bushings are worn to the point that the front end is pressing rearward on that shaft, thereby putting it into a bind, and setting up the vibration.

 

I clicked on your post because my truck also has some vibration that seems to be coming from the front end at certain highway speeds. I have assumed up to this point that I just need to balance the front tires. (I have already installed new rear tires).

 

My truck has 204,000 miles... so whatever your issue is, mine may also need the same fix. I'm thankful to be learning from you what has not worked.

 

Dan

[Spurshot]

Dan,

The front driveshaft has a long male spline that slides into the front of the TC output shaft. It is no where near bottomed out against the TC. I even replaced the output shaft of the TC with a new one during the TC overhaul to ensure the internal splines weren't worn out. I did not replace driveshaft yoke with the male splines as yet because it looks so good. Most of the black oxide is still on the splines.

 

As you may know, the front differential is electrically disengaged when in the 2wd mode. The front halfshafts (axles) are engaged with the front hubs all the time regardless of 2wd or 4wd mode. This means the halfshafts turn all the time the vehicle is moving.

 

You can try to isolate the vibration as I did by blocking the front tires, jack the rear tires (and remove them if desired), run the truck in the 2wd mode up to the speed you get the vibration. Then pull the front driveshaft (4 bolts on the front diff yoke, 10 min job max) then run again.

 

After searching the net, I'm pretty sure you'll find that you have the same issue I have as others on the net seem to have as well.

 

Chuck

[green 788]

10-4...

 

One thing I do notice with my truck is a tick tick tick coming from the left front wheel. I originally thought it was just something inside the chrome wheel simulator, but I noticed it doesn't tick when the truck is on a jack and the wheel is rolled.

 

I spoke to a knowledgeable guy this evening about that issue, as well as yours. He told me that I almost certainly have a bad wheel bearing causing that tick. He said that if it only ticks when the truck is rolling (and not when it's on a jack) that's a sure sign of a bad bearing.

 

I told him your vibration problem went away when you removed the front drive shaft. That seemed to stump him for a while, but he then suggested pretty much what I did and that's the suspension bushings might be worn to the point that something is misaligning just enough to set up the vibration. I guess it's worth checking on those bushings. I'll be checking mine for sure.

 

The other suggestion he had was a possible out of round rotor (but that wouldn't explain why removing the shaft stops the vibe, and why you still get the vibe even with the front wheels chocked and the rear end lifted).

 

Let me ask you this: Is the vibration you're getting similar to a front tire out of balance (where the thud thud thud seems to have the same timing as a tire revolution)? Or is it a higher frequency vibration, like a drive shaft or half shaft rattling?

 

Dan

[Spurshot]

Dan,

The frequency of the vibration is in tune with the driveshaft, roughly 4 to 1 of the tires. It reveals as a low frequency rumble. The front halfshafts aren't turning with the front tires chocked. I even pulled the rear tires off to ensure they didn't influence the diagnosis.

[green 788]

Okay, thanks...

 

The guy I spoke to this evening told me that some Chevy's will have the TC which allows the front shaft to turn in 2wd, and some don't. He wasn't sure about which year models, and which TC models were which.

 

I don't understand where the front shaft is disconnecting from the half shafts... if it is actually turning with the rear end lifted and the TC in 2wd. I thought if the front shaft was turning, it would have to be spinning the half shafts.

 

Where does it disconnect then? Maybe the clue is there...

 

My vibration at this point sounds more like tire issues, or possibly that bad wheel bearing I apparently have. It's more of a bounce (like a separated belt in the tire) than a rattle from a shaft.

 

Dan

[green 788]

Probably grasping at straws here...

 

But the Haynes manual makes it a point to mention that before you remove the front drive shaft (which of course would have been done to replace your U-joints), you're supposed to mark the position of the shaft and yoke, so it goes back in the same way it came out.

 

Apparently you can put it back in 180 degrees turned the wrong way.

 

Dan

[Spurshot]

Dan,

The front axle differential has an electrically driven servo that disconnects the differential from the front driveshaft. I'm not sure how it functions internally, just that it disconnects in there.

 

On the yoke to driveshaft, I marked it and reassembled the same position.

[green 788]

Not sure how similar it would be to this one...

 

http://www.truckinweb.com/tech/suspension/...l/photo_20.html

 

...but have you tried unplugging the 4x4 actuator, suspecting that it may not be withdrawing the engagement of the gear completely?

 

Hopefully this can be resolved for the sake of others who may have the same issue in the future...

 

Dan

[green 788]

Did you happen to see this thread?

 

http://www.gm-trucks.com/forums/lofiversio...php?t67539.html

 

The guy says he had the same problem as you... and a NEW front drive shaft fixed the problem.

 

Maybe even though you had it balanced there's something about it that's just a bit out of spec... and it didn't become a problem until it was removed in order to replace the U-joints. (?)

 

Dan

[Spurshot]

Dan,

Thanks very much for the reference link to that earlier thread. It sounds identical in every way to my problem. Same truck configuration and all parts should be the same. I'm going to get a driveshaft or at least the yoke with the splines asap. I'll probably just get a whole driveshaft if it's reasonable. I'll let you know results.

 

Chuck

[Spurshot]

Dan,

By the way, replacing the front driveshaft yoke (it has external splines that fit into the transfer case) was on the list of things me and my 4wd shop agreed to replace when he overhauled the transfer case. He replaced the output shaft that has the mating internal spline, but didn't replace the yoke. He told me he couldn't see any wear on it and I could replace it later easily. I agreed when I picked it up.

 

This guy in the thread had to have a spline fit problem since he had replaced u joints and checked the driveshaft for straightness and balance.. Either way, if the whole d-shaft is reasonable price-wise, I'll change it.

[green 788]

10-4... I hope that works.

 

Dan

[green 788]

This ticking sound I was getting was indeed from the chrome simulator. I guess the truck's weight had to be on the wheel before the wheel would flex enough to make the simulator scrape.

 

Still curious as to whether the front drive shaft solves your problem... keep us posted.

 

Dan

[Spurshot]

Dan,

I just returned home from Northern Calif to buy my new 2008 1500 4wd Crewcab with the 6.0L, 4L80, and I think the same T-case. Out of curiosity tonight, I ducked under the new truck to see if the front driveshaft is the same. It is very similar and may have just a superseded p/n. But..., and here's the interesting part, the driveshaft (front) on the new truck can be turned very freely by hand, whereas the old truck's front shaft is just barely turnable by using two hands. Clearly, the old truck has heavy drag of the clutchpack which is driving the front driveshaft even when in 2wd. The new truck doesn't have that friction/drag and therefore the front driveshaft is being driven when in 2wd.

 

I'm very suspicious that there's a worn or out of adjustment part in my old truck TC causing this drag.