Bad diff ... W115
 

Seems my w115 has developed a bad differential. It's a 76 300D. What differential is this? 3.46? Any one have a spare? What should the tech charge me for a used replacement? they want 1500-ish for the whole repair.

In NYC. can pick up this week if you are in range if the tech has trouble finding one or wants to charge too much.

Thanks,
Henry

https://www.ebay.com/itm/Mercedes-W123-Rear-Differential-3-46-Gearing-Rear-End-Axle-300D-300TD-240D-300CD/324368862911?hash=item4b85e40ebf:g:1WEAAOSwgtBfqsCn

~3 hours labor should get it installed.

Thanks! Their quoted price seems a bit steep ...

I don't have access to jack or jackstands any more here and it seems too much of a hassle to do it on the street where I live now ... Ay.

-Henry

I swapped one of these out in my driveway in less than an hour.

Take into account I had the correct set of jacks, knew how to remove and install it since I had pulled one at a wrecking yard, and the weather was perfect so I decided to do it in the driveway.

This is not a difficult job. If you have the correct jack and two jack stands you could do this with a few sockets and two box end wrenches. And a great deal of care.

The $1500 sounds like the labor plus the cost of the diff plus a bit of profit on the diff thrown in.

Update: Shop is 90% sure its the diff but not 100% and they are hesitant to spend $500 on the diff if it doesn't fix the issue (their price apparently). I told them I didn't want to pay for a repair that was the wrong one much less $1500.

They said if I give them a diff they can do it for 3 hours labor. So that seems like a good deal and it takes the risk off them and the cost lower for me.

Can anyone else here confirm that the 3.46 linked by Frank is the right one for a W115? Or perhaps Frank, could you provide more justification for that? I don't know you and haven't read your posts, so I just want to be extra careful before spending 200 bucks and wasted effort going to the shop with the wrong part.

Thanks.

-Henry

Thanks for the tips! Great to know. If I still lived next to my friend with a jack and jackstands I'd do it.

Henry

I can assure you that when it comes to technical information Frank Reiner is "The Bible". He can be a bit acerbic at times but his information and advice is always spot on.

What are the symptoms?

On the sides of the diff housing where the inboard stub axles enter there are two versions. Your original diff has the first version with bolted on covers/bearing carriers. The one that could serve as a replacement is of the second version which has the bearings retained by snap rings. The two versions are visually different in that area, but functionally the same.
The shop that does the swap needs to be aware that there are selective shims/spacers that are used to set the end play of the stub axles in the side gears. Those spacers would be encountered irrespective of the version of the diff housing.

Great to know! Thanks.

Symptoms: been happening since we drove it from Atlanta to New York so in the last 1000 miles and seems to steadily get worse in that. Still drives fine and issue does not affect drivability at any speed. On the highway or accelerating above 30 mph it is unnoticeable. When decelerating between 10 and 20 ish mph it sounds nasty, but not so bad on acceleration (still noticeable). Gear scraping noise that is 3-4X wheel speed, which does not change when the gears shift. So it's not on the axles or on the wheel bearings or on the engine side of the trans. When you decelerate between 10 and 20 but put the car in neutral you it almost goes away, which means it is effected by the engine load. Mechanic does not hear anything on either trans or diff when the car is on the lift. It makes a different timbre of scraping but still nasty and same speed (3-4X wheel) when backing the car up.

So it leaves it to something on the end of the trans or the differential. It "sounds" like its coming from the back of the car, and with the gear type noise that does not change with gear shift I think it is the diff and not the back of the trans. To clarify this does not sound like a bearing noise it sounds like a gear. I had thought it was the diff oil too low. But both the diff oil and trans oil were changed 2 years ago and the mechanic confirmed they looked fine. So it's not the diff or trans oil. Mechanic has been in business doing MB since 1974 -- strikes me as very experienced -- and says he's never seen an issue like this. Perhaps other people would have just waited until the diff failed catastrophically, but I took it in thinking it was oil too low and would be an easy but immediate attention needing fix.

Any other thoughts ?

I will discuss this spacer concern with the shop! Thanks a ton for the info.

-Henry

Given the frequency (3-4X wheel speed) and the perceived location, your description would have many of us look first at the driveline in total, i.e., flex discs, center bearing & mount, centering bushings, and u-joint.

This x2!


If the diff were making all the racket, I'd expect you to find metallic oil or metallic filings in the oil. Unless the oil in the diff were JUST changed, clean oil would suggest the noise is coming from the driveline rather than the diff. A bad U-joint can make a heck of a racket and vibration.

This is very good food for thought. I will not just up and replace the diff yet. Mechanic emphasized how hard these issues were to track down.

I had a bad U joint once on a BMW 635 and changed driveshaft out myself as it wasn't serviceable. But in that case it was an rumble not a gear grind as the u joint was getting stuck and the driveshaft was destroying the center bearing. I don't think its the u joint. But who knows its a totally different car.


Ergh .

Henry

I went to pick up the car. I found out something totally unexpected. It has a 240D differential and the driveshaft input is like 5mm eccentric from the 300D one. So it's like my new york apartment, it's all crooked when you look at it down the line. And it's not the right gearing. What the hell!

Perhaps something got worn after the last 20K miles I've driven it with the wrong diff. Must have been installed by the original owner in the 1980s or 1990s before he died (i got it from a family friend after it sat in storage for two decades). So, I'll plan on getting the right diff. Should I expect any issues with the driveshaft not matching up? Like, could I also have the wrong driveshaft?

-Henry

And I found out that the owner of the shop had COVID a few months ago and it's affected his mind. I thought he seemed a bit looney. COVID sucks, poor guy. So I talked with one of the other mechanics there instead today. Ay. I can't wait to get vaxxed.

-Henry

The only difference in all of the MB diffs (of that basic size) is the ratio of the ring & pinion gears. Externally they are all the same, the above mentioned early and late versions notwithstanding.
Any eccentricity/runout of the driveshaft at the diff input could well be the cause of the described symptoms.
5mm of mis-alignement suggests that the centering bushing in the back end of the driveshaft has gone tits up. The back half of 114/115 driveshafts are all the same length; one that came with a 3.46 is the same as that which came with a 3.92.
While changing the diff pony up for a driveline rebuild.