I finally found the coolant overflow tank on my 1995 S350D. OM603 diesel.

It is underneath the passenger headlight.

Get to it by removing 5-6 screws holding in a plastic cover in the right front wheel well. (Turn the wheel all the way to the right before shutting off the engine.)

When you pull the plastic cover up and out of the way you will see an odd shaped plastic bottle. It is actually TWO bottles. The lower one is the windshield washer bottle. The upper one is the coolant overflow bottle.

No, I don't know how to get it out.
No, I don't know where the hose hooks on to it.
No, I don't know how it vents.

Yes, I would like to know.

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

If it's like the W126 there is probably a 2nd tank in one of the wheel wells too

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

That is the second tank. The first tank is easy to find on the driver's side of the engine bay and has the pressure cap on it.

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.