Biggest problem was hydraulic fluid leaking into the transfer case and mixing with transmission fluid. Most leaked enough that it caused problems. I bought a 91 sedan with 60,000 miles on it. With some effort on my part (mainly draining excess fluid from transfer case every 5,000 mile oil change) I drove it until I sold it with over 240,000 miles. Others weren’t as fortunate and had to work on the transfer case seals much earlier. Awesome car in the snow.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126