85 720 4x4 Drivetrain Noise Grinding Rocks

[IZRL]

Have a noise coming from somewhere in the drivetrain that sounds like you're grinding rocks. I can't tell if it only does it in 4 low? Or if it also does it in 4 high and 2WD and it's so quite that I can't hear it in the cab? I was thinking best case scenario it was the U-Joints. It didn't look like there was any play or very little but they looked like it wouldn't hurt to swap them out either way. I swapped the rear driveshaft u-joints and it's still making the noise.

 

What I did notice while swapping u-joints is that either one of the flange yokes or the slip yokes. Didn't pay attention to which one, rear or front 🤦‍♂️. You could almost push the u joint caps in by hand with almost zero resistance. I was able to push the caps in about 3/4 of the way by hand. Then it took almost zero pressure to push them in the rest of the way with the press tool.

 

This is a video of the u joint going from the rear driveshaft to transfer.

 

 

 

This is a video of the rear u-joint at the rear Differential. In this video I started in 4 low and I believe that after I stopped mid video, I shifted to 4 high? Not 100% sure tho.

 

 

 

 

[Thomas Perkins]

Back in 12/15/2017.Mine was making a noise.I took it to the transmission shop.The yoke was bad.He got me a new one for 135 bucks.They raised it and ran it and seen the problem.I still have the old yoke.Also replaced the u- joint.But that was not bad.See the grooves.Causing it to vibrate.When I would get on the high way,it would not go fast.This was in 2wd.Don't drive in 4wd.I keep it on the road.

Edited March 21 by Thomas Perkins

[datzenmike]

It's 50/50 if it's transfer case or rear differential. Seems like a clunking sound near the end of the second video, so do it first. 

 

Check the rear differential oil level and then drain and look at the oil. Any chunks or flakes on the drain magnet? 

 

If nothing do the transfer case next. The oil will give you a 'look' inside both perhaps for a clue.

 

 

If due to change the oil, good time to do it. The oil should be changed every 30K mikes.

 

 

[IZRL]

3 hours ago, datzenmike said:

It's 50/50 if it's transfer case or rear differential. Seems like a clunking sound near the end of the second video, so do it first. 

 

Check the rear differential oil level and then drain and look at the oil. Any chunks or flakes on the drain magnet? 

 

If nothing do the transfer case next. The oil will give you a 'look' inside both perhaps for a clue.

 

 

If due to change the oil, good time to do it. The oil should be changed every 30K mikes.

 

I changed all the gear oil 2 months ago (transfer, tranny, and both diffs). I did find a few metal shavings that were about a 1/2 inch long and about double the thickness of a strand of hair in the oil of the rear diff. I don't remember seeing anything special in the transfer case.

 

So the rear diff is definitely taking a shit but what's weird is that the grinding happens on both acceleration and deceleration in only 4 low. Wouldn't it be making the grinding noise at all times, in 4 high and 2wd if it was the rear diff? I did a little searching around on the interweb and some folks are saying that it might be that 4 low is not fully engaging. And that the 4wd shifter linkage might need adjusting (other vehicle makes/models). I looked in the book and it mentions nothing about adjusting anything when it comes to 4wd shifter components. Is there a way to check if 4 low is fully engaging for these trucks?

 

 

[datzenmike]

4 low would indicate the transfer case, yes. The T-100 is known for being noisy when abused or worn. (no judgement here) but I think more of a growl from worn bearings, not those rattling sounds. I don't think you can be anything but engaged or not engaged like shifting into gear without using a clutch. There would be a constant clash or grind. Sounds almost like a rock inside a hub cap.

[ElliotV]

I haven't messed with the tcase much in my truck but the spare cases I have it looks like there isn't even a shifter linkage like you see on most trucks it looks to just have a threaded end for the shifter knob. I have heard these t-100s are pretty stout on one forum I guess there was a guy running double cases with a sbc and 35s on solid axles that would abuse the hell out of his truck and only broke a couple when he would roll over or something of the likes. My guess is perhaps a pinion bearing or something along that line. In the first video it sounds like I can hear a bearing whine at the diff and if it is in 4low the rear end would be getting more torque to it than normal and with the shavings having come out of it there could be something going on but I don't think you will really know unless you start digging in.

[Stoffregen Motorsports]

I'd pull the diff first. It's easier than t-case disassembly and you already found chunks in there so...

 

You could apply the old Tom and Ray Magliozzi theory and let it blow up. Then you'll be absolutely certain where the noise was coming from.

[datzenmike]

Turn up the stereo.

[ElliotV]

On 3/22/2024 at 12:44 PM, datzenmike said:

Turn up the stereo.

That's the spirit