Just over a year ago, I drained and refilled the differential on my '84 190D-2.2. The old fluid that came out was a shade darker than the new fluid, but nothing out of the ordinary. The new gear oil that I refilled it with was yellow and clear (like the color of cooking oil).

Last night, I decided to give the car a general inspection to see if anything needed attention. When I checked the differential fluid level, the filler plug was covered with some sort of black slime. I stuck my finger inside the filler hole to see if the differential was full (it was), and the fluid inside was extremely dirty; it looked as though more of the same black sludge was blended in with the fluid.

The fluid has only been in use for about 18 months, or maybe 15,000 miles, and the fluid that was in there before wasn't dirty like this.

I intend to replace the dirty fluid ASAP, but I would like to know how and why it became so dirty so quickly. Where did this black sludge come from? Is there something else I should be looking at?

As always, I really appreciate any help or advice that you can offer!

Thanks!

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD