Dark/Black Rear Differential Oil - UOA?

[The Critic]

2016 Mercedes-Benz E350 RWD.

This car is driven primarily on the highway. It currently has 85.3K miles.

The rear differential was first serviced at approx 42K by me. When I drained the factory fill it had a grey/green sludgy appearance. Fuchs Titan Sintopoid FE 75W-85 was used to refill since it is a MB 235.7 approved fluid. Differential holds about 1.3 quarts.

I just serviced the rear differential again and this time, the fluid was pitch black. The fluid smells fine but looks very nasty for only 43K. I am not used to seeing rear differential oil looking like this after 43k on a passenger car. Should I send in a sample for an UOA? Or should I just recommend more frequent services to the customer? Any ideas on what may be going on?

 

[Lubener]

Myself, I wouldn't get an analysis. It is likely just additives and I would be more concerned if there were metal particles present. I would stay on the present maintenance schedule.

 

[maxdustington]

1.3 qts ain't much.

 

[gathermewool]

Does this thing have a Torsen LSD?

 

[SatinSilver]

Maybe shorten the interval down to 30k miles and see what it looks like. Based on another thread for the same car that would equal about once per year.

 

[JLTD]

I'd get a UOA to allay suspicions. What is the recommended interval?

 

[The Critic]

Originally Posted by JLTD
I'd get a UOA to allay suspicions. What is the recommended interval?

MB states it is lifetime fill. Local dealer bundles a differential oil change with their 40K Major Service Package.

Originally Posted by gathermewool
Does this thing have a Torsen LSD?

Not that I am aware of.

 

[Kestas]

I changed the diff fluid in my Mercedes E320. After 30K it looked almost new (not original fluid). I wouldn't expect the fluid on your second change to be dark. Perhaps an analysis is in order. Water may have found its way into the fluid.

 

[Oro_O]

Originally Posted by Lubener
Myself, I wouldn't get an analysis. It is likely just additives and I would be more concerned if there were metal particles present. I would stay on the present maintenance schedule.


+1. Different additives can discolor in normal service intervals in a LSD. Have seen it many times. Since your vehicle has an LSD, it's the most probable reason.

Quote
Originally Posted by gathermewool
Does this thing have a Torsen LSD?

Not that I am aware of.


Since you are questioning fluid use and wear in your rear diff., it would be a good baseline to start with knowing what kind of differential it is. Different types have different characteristics.

 

[atikovi]

Originally Posted by The Critic
MB states it is lifetime fill. Local dealer bundles a differential oil change with their 40K Major Service Package.


Then the dealer is doing unnecessary service to squeeze more money out of the customer.

 

[zeng]

I wouldn't do UOA . OCI of 30 -40k should be fine.

 

[Onetor]

When I had the rear diff on my F150 changed around 55,000 the fluid looked brand new. Except for a little grey paste. But, I'm not versed in Mercedes either. I would ask a master mechanic at a dealership.

 

[The Critic]

Originally Posted by atikovi
Originally Posted by The Critic
MB states it is lifetime fill. Local dealer bundles a differential oil change with their 40K Major Service Package.


Then the dealer is doing unnecessary service to squeeze more money out of the customer.

Just to play devil's advocate here - there are many members on this site who replace their drivetrain fluids far more often than the OEM recommendation, or even 40K. And it is a "socially acceptable" practice. Why is it not okay for the dealer to make the same recommendation?

 

[atikovi]

Originally Posted by The Critic
Just to play devil's advocate here - there are many members on this site who replace their drivetrain fluids far more often than the OEM recommendation, or even 40K. And it is a "socially acceptable" practice. Why is it not okay for the dealer to make the same recommendation?

Lots of people doing it sooner because it feels good, doesn't mean it's necessary. I do it too before selling a car to show it's been fully serviced, but again, doesn't mean it's needed. Diffs rarely give problems during the life of a car, and even if they do, are much cheaper to replace than an engine or transmission. These same "members on this site" also promote 10K OCIs which is just silly. I mean, if you're so concerned that you change the diff every 40K, why aren't you so concerned about your engine, (which could cost 5 to 10 times as much to replace) and change the oil every 5K?

 

[4WD]

The only differential I lost was due to gear oil break down. It was $2k to install one with 20k more miles than what the truck had … I'll stick with early changes and my choice of lubes …

 

[DoubleWasp]

Originally Posted by 4WD
The only differential I lost was due to gear oil break down. It was $2k to install one with 20k more miles than what the truck had … I'll stick with early changes and my choice of lubes …


Indeed.

 

[BlueOvalFitter]

Originally Posted by Kestas
I changed the diff fluid in my Mercedes E320. After 30K it looked almost new (not original fluid). I wouldn't expect the fluid on your second change to be dark. Perhaps an analysis is in order. Water may have found its way into the fluid.

If water were in the oil it would be a milky color. Being it's pitch black, I would first smell it to make sure it isn't burnt. If it does smell burnt, you have BIG problems! Check the lowest part of the differential for metal particles. That's where they will collect.
Good luck!

 

[gathermewool]

Originally Posted by The Critic
Originally Posted by JLTD

Does this thing have a Torsen LSD?

Not that I am aware of.


The reason I ask, is because the gear oil IF in my previous '08 STI, with a Torsen rear LSD, was very dark gray. I sent it off for an UOA and the only thing out of the ordinary was high Fe. The magnet had a lot of goop on it, though. I don't have this saved on this PC, but I'll see if I can find and post later.

After doing some research way back when, I found that the dark gray appeared to be normal for these LSD's, maybe due to some sort of gear coating or how they break-in; I don't remember precisely.

Edit: found the UOA and pic: