I need to change out the differential in my 1981 300D. I see 4 bolts on a rubber mount in the back (easy to get to) and 2 bolts in the front. I tried reaching over the driveshaft to get to these front bolts, but it seems too tight for a socket. Anybody removed one of these things? Would appreciate any tips you may have concerning wrench size, techniques, etc. Searched this forum but couldn't find anything specific. I'm using a rental car right now and need to get it changed soon. Thanks!

I just did this job a couple weeks ago on a 1982 300D, so that should be just about the same as what you are facing. My differential was badly worn - whining and a large amount of looseness between "engine power" and "engine braking" conditions.

There are four 19-mm nuts holding the differential to the suspension crossmember. They have an aircraft-style nylon insert, so they are pretty tight all the way off on removal.

Yes, I can offer you some tips and some of my experience with the job.

First, get the rear of the car up on jackstands. I used wood blocks (2x4 cutoffs chiseled to match the jackstand) under the rubber subframe mounts in front of the rear wheels. You've got to get the driveshaft down and the half-axles out. I recommend removing the entire back section of the driveshaft. This gives you room to work on the front two differential mounting bolts.

When you release the rubber differential mount on the back (those four 13-mm bolts) this will allow the differential to drop a bit, so support it with a jack. (Oh, drain the gear oil first) Then take off the back cover of the differential, and pull the clips that hold the axle shafts. Now you need to pull the wheels and take out the bolt that holds the axle shafts in the hubs. The axles come out of the hubs first, then you can clear them from the differential.

I recommend putting the wheels back on at this point for safety. You can lower the differential some now - actually you can let it go. This will drop the suspension crossmember (or whatever its called) and provide more clearance to those four nuts. They will come loose with either a wrench and taps from a hammer, or with a 6-pt socket and a breaker bar.

If you have the replacement differential, you have an idea just how heavy that thing is to lift. The old one's gonna come down easily. Getting the new one up is much tougher. The four holes you have to get the differential bolts through are so closely toleranced (drilled) that you can't do two bolts first and then push up the other two. You've got to get all four lined up at once nice and straight, and push straight up. It's a two-person job or a lucky one-person job. I was lucky.

Assembly from that point will be easier.

A word on the driveshaft. On your 1981 there are probably no match marks to align the front and rear driveshaft sections. You must mark it yourself, because to maintain driveshaft balance it has to go back together on exactly the same spline as it came off.

Well, that was the short version of the story. Ask for any clarifications or details I left out.

My new differential came from a 1985 300D with 89K miles. Quiet and tight as new. Makes the car a pleasure to drive.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

So you're saying that once I remove the 4 bolts from the mount in the back and lower the differentail somewhat, I can get to the two front bolts?

Yes. Those four 13-mm bolts hold the differential (through the rubber mount) in place against the car's body. When you take them out (while the body is suspended with the wheels off the ground) then the differential and the suspension crossmember it is bolted to can drop down about four or five inches.

Kinda fun to watch the differential drop down and the wheels pull up. It's that wonderful MB rear suspension design! Nobody else comes close to that many moving parts in the back......

You don't need thread-lock on the 19-mm nuts that mount the differential to the suspension crossmember, but I used blue thread-lock on the four 13-mm bolts that hold the differential rubber mount to the body/frame.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Thanks! I'll let you know how I make out.