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Old 09-07-2010, 10:12 PM
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Default Help with my suburban please

I've never had a big 4wd before. I understand a little about what happens to engage the front drive system, but obviously not enough.

Something is wrong, or bad, or wearing out, or about to let go in the drivetrain system of my '96 GMC Suburban. I don't know what it is...

I can hear and feel a vibration, I really think two parts that should be coupled or uncoupled are grinding on each other. The first time I heard this noise, I took off in the rain and the rear wheel started spinning. Then this awful grinding noise. It sounded like it was coming from the transmission. The sound changed pitch with wheel speed, not engine or trans speed. I got to a place where I could stop and at least give a visible inspection for something hanging down or out of wack. When I slowed to a stop, I heard a clunk and the grinding noise stopped. After driving some more, it felt like it went in and out of manual shift on the fly 4wd just fine.

Today, it happened again. Then when I came to a stop, I felt the same clunk and it stopped.


Have you guys ever heard of anything like this? I'm outta my league. Thanks.
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Old 09-07-2010, 11:07 PM
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I just sold my 94 z-71 but once I got it serviced and had the rearend fluid drained and replaced and afterwards it did just as you discribed your rear. I had a limited slip rear that was doing all kinds of crazy slipping and grabbing because their replacement gear oil didnt have the limited slip additive .... I added some and the problem went away after 100 miles.
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Old 09-08-2010, 06:43 AM
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Had 4x4 burb and the rear axle locked up. Had the fluid replaced and the rear end cleaned out and it was ok after that.

If it's in the front drive there are a few possibilities. The switch can go bad if it's electronic shift, it will do a lot of weird things. The actuator motor can also go bad, it thinks it's in the wrong position and tries to correct it. The front axle is vacuum actuated but that shouldn't be a problem here. If it fails the front axle just won't engage even though the transfer case is in 4WD. Is it electric shift or manual? Does the service 4WD light come on?

Mine is screwed up right now. It goes into 4WD ok but the light comes on after about 1/8 mile. I think the forward output shaft sensor isn't working so it thinks it's not spinning.
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Old 09-08-2010, 06:52 AM
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The 4wd is lever-operated manual shift. There is a lever on the tranny hump. It isn't coming from the rear end, I am almost certain of that.
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Old 09-08-2010, 07:14 AM
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Is your transfer case chain driven. Could need a new chain. Also do you have manual or automatic hubs ?
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Old 09-08-2010, 07:50 AM
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I don't know if the front shaft is chain driven. It very well could be, and the racket it makes sounds like it could be a slipping chain. It is a 4L60E trans, and there is no front hub locking mechanism...so I guess they're always locked, and the front running gear is always supposed to be somewhat engaged. It has IFS and half-shafts or CV shafts or whatever. Visual inspection showed no wear or damage to the boots, and there was no real visible leaks where the t-case or front ring/pinion may have dumped its juice.

I believe the sound is coming from front drive system, possibly where it mates to the tranny. That makes me, ignorant as I am, think possibly drive chain...or whatever else couples with front shaft with the tranny.

I know it isn't the infamous use of the wrong posi or g80 gov-lock gear oil or lube, it has open carriers front and rear.
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Last edited by Highway Star; 09-08-2010 at 07:56 AM.
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Old 09-08-2010, 09:33 AM
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The front hubs are always locked and the axles are always spinning. Like I said there is a vacuum operated mechanism in the front differential that engages the front differential to the front driveshaft. I believe the transfer case is gear driven, could be wrong. Make certain the 4WD lever is all the way to 2WD and lube the linkage. It may be hanging up part way between 2WD and 4WD.
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Old 09-09-2010, 05:09 PM
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what's it do in reverse ?
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Old 09-10-2010, 12:09 AM
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I am going to take a real good look tomorrow, pull off both front wheels and calipers/rotors, do a complete thorough inspection of everything underneath, and see what I can find.

It won't stay in 4wd right now. The lever stays in 4HI, but the light on the manual shift cover plate blinks. It is supposed to stay on, and I'm pretty sure that when it blinks, something is wrong.
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Old 09-10-2010, 12:33 PM
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This is the noise. It is in 2 hi during the clip, he is outside right now on jackstands still. I pulled the front wheels. It looks to me like I can get the noise to stop by disconnecting everything necessary to keep the ring/pinion from spinning. Can I remove the CV shafts and the driveshaft?

I just made this video...driver side forward/backward, then same on passenger side.

http://s812.photobucket.com/albums/z...nt=4wdburb.mp4

Do you know what that racket is?
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