Troubleshooting Rear Axle Noise

Mechanics Hub - August 21, 2013
A few months ago our crew went out to the Mercedes dealer for some training and the instructor told us to bring out one of our buses with the MBE 900 diesel engine so we could use it as a test unit. One of the guys grabbed the keys for one of our Thomas HDX pushers and off we went.

When we hit the highway I couldn't believe my ears, the howling going on at the drive axle was deafening. This was a problem to be dealt with soon so when the time came we got into the troubleshooting frame of mind. We knew it was the differential in question but how much damage or wear was there?
The first thing we did was check the oil for fragments or filings of any kind. The drain plug is magnetic so that was fairly clean with the usual fine collection of filings. Draining the oil is another option but not totally necessary. Partially draining the oil works if you're looking for chunks.

Since there was nothing obvious there we tried our Snap-On camera on a snake tool that was fed into the differential housing. Nothing could be found there so it was road test time. The road test will tell me for sure if it's a major or minor repair that's needed.

I got one of the guys to drive while I sat at the back of the bus over the rear axle. It didn't take long to determine the problem. The howling noise was being caused by the crown and pinion gears. When under load the noise was very strong but when you backed off and coasted the noise went away. If you had a constant noise all the time it would be something like a separated tire or a bad driveline.

The faulty differential being in a pusher meant removal of the air bags and the mounting plates stretching across the frame. We also added blocking between the frame and differential housing since the air bags weren't there for support. These items were in the way of us getting to the diff gear. To save some time we used the shop hoist and one of the guys fabricated a lifting jig to grab the front of the gear and remove it from the housing.

Once removed we found a bad pinion gear with hard surfacing gone from one of the pinion teeth. The backlash was excessive as well. So I decided to go with a reman which came with a 30 month warranty and the price was not much more than the parts and labor for us to rebuild it. It's justified to go this route because of the heavy Summer workload we have going on right now.


Troubleshooting is the hard part in some cases since you could easily be led down the wrong path. This problem was 100% mechanical with a drive gear system so there wasn't any other determining factors to think about. I'll be sharing more troubleshooting and repair information in the future. I would appreciate it if you left your insights and similar experiences in the comment section.