You can buy a kit with test strips to test coolant, I keep them on-hand for testing. They are diesel-specific, the simple 3-way test: pH, Nitrates, and freeze-point, there are also 5-way strips.

The problem is that I don't know if these are accurate to the needs of a Mercedes-Benz automobile, so even if they indicate good there might be something that Mercedes diesels need that isn't being tested (or indicated to the proper Mercedes level). The kits only have about a 1-year shelf life, less than the life of the coolant in your car so if you do wish to play with the strips buy the 3-strip packet from NAPA instead of the more common 50-strip bottle$.

The easiest IMO is to just change at the factory interval with the proper G-05 coolant and distilled water (per FSM). On the 124s there is a new pressure tank available that has a silicate pack in it (aka: coolant filter in the H-D world) to extend the useful life of and stabilize the diesel coolant, I don't know if the same is available for your 126.

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Gone to the dark side

- Jeff