Mercedes-Benz W211 automatic transmission coolant (glycol) leak
Why does glycol get into the W211 automatic transmission oil?
The transmission oil is cooled by engine coolant in a small cooler. Its seal can fail, which lets coolant (glycol) mix into the transmission oil. The factory changed the sealing from September 2003, and the fault mainly affects the older Valeo coolers.
How can I tell if the W211 transmission oil contains glycol?
A typical symptom is a shudder under light acceleration, felt for example in the headrest, which disappears when you add throttle. Healthy oil is clear and dark red; cloudiness and a pink tint point to contamination. A glycol test on an oil sample confirms it.
How is glycol damage in the W211 transmission repaired?
The cooler and seals are replaced. An oil change alone is not enough, because glycol stays in the friction surfaces and seals, so the transmission is flushed several times. If the friction surfaces have already come loose, the torque converter and friction plates have to be replaced. Have a workshop assess the extent of the damage.
How can I recognise a cooler that has already been replaced?
A white paint dot on top of the cooler shows that a workshop has replaced it. The manufacturer's sticker is visible on top of the cooler. In some cases even a new Valeo cooler has started to leak.