I've been getting a subtle metallic howl from the front of my GL550 with 181K miles when accelerating from a stop for a few weeks. While driving up a parking garage yesterday, I could hear it better and could hear it was making a sound like a bike with a badly adjusted derailleur (where the chain alignment is off enough so that it starts to grab on the next gear but doesn't actually shift and makes a rattling sound). It isn't making the noise where the chain is obviously slipping and clunking though as depicted in the video somebody posted of a 320. I'm guessing the transfer case is in the early stages of starting to fail but I'm not sure how to confirm that is the issue rather than something else. I found one post that said that after removing the drain/fill plugs the chain was obviously loose. Is that the best way to diagnose, or is there another method? Thanks for any help.

 

To follow up, using a screwdriver, I can push the chain upward at least 1" through the fill hole. I'm assuming there shouldn't be that much slack even though the tensioner is on the other side?

 

drain oil. look for shavings. if present you know what is happening. if not present, refill and drive until noise gets much worse and you can tell exactly where it is coming from

has the oil been ever changed? the atf in the transfer case has a tough life. it should be changed every 50k miles or so...

 

Thanks for the help. Based on the fluid, it looks like I'm in the market for a new transfer case. The fluid was changed once by me, but I was following the MB maintenance schedule, and I'm embarrassed to admit it didn't occur to me to change it until I saw a post about it on here when it had about 150K miles. The fluid looked good at that point. The trans was out for warranty work @ 45K K miles, so it may have been changed then too if the fluid has to be drained/refilled when the xfer case is removed but clearly it should have been changed more often.

Is there any way to bench test used ones? Slop between the input and output shafts? Slack in the chain via the fill hole?

 

To close the loop on this, at the time I made the original post, there was no visible debris in the fluid so I just refilled. At 205K miles, it started making some crunchy drivetrain noises at low speed and made the vehicle jerk fairly significantly turning into a parking lot. I drained the xfer case fluid again, and found this:


I refilled it, and ordered a rebuilt unit from TCBMWX5 Inc for $1,350. After the drain and refill, all seemed to be back to normal, so I continued using it for short trips while waiting for the rebuilt unit to arrive. The day it arrived, about 5 miles from home, there was a loud noise under the car, I started leaving a smoke screen behind the car and a squealing noise started. I nursed it home and after removing the xfer case, I found this which would explain the noise and oil leaking onto the exhaust. It looks like the chain broke, which I'm guessing was caused by a bigger hunk of metal that didn't come out the drain getting lodged somewhere it shouldn't have.




The good news is I made it home without a tow, but the bad news is that I no longer have a core, so I won't be getting $300 back. The vendor was responsive and shipped quickly if anybody needs one in the future.

Swapping transfer cases took longer than I expected. The bolt on the top on the driver's side is easy to get a socket on, but really hard to get extensions/swivels to a place where there is enough room to swing a ratchet or get an impact to. I was able to get a butterfly impact on it, but it didn't have enough omph to get the bolt loose with multiple extensions and a swivel. I finally was able to use my IR impact after removing a bracket that connects the exhaust to the transmission. I couldn't get a jack under the transmission because my quick jack was in the way, but the transmission barely dropped after taking the transmission mount off. I had some wood stacked up on the quick jack to keep the trans from falling too far, and it didn't even drop enough to touch the wood that was within .75" of the trans pan.

 

Thank you for the information and the update.

cheers.