With that mileage, and good service, and not being beat on a daily basis, the overwhelming odds are that your transmission is not toast. As above, have fluid and filter replaced and see what happens. RE: checking the level, the absolutely most accurate way is to make sure the trans. fluid is hot (not the same as having the engine coolant hot). This usually entails driving at least 15 miles. The dipstick will be HOT to the touch if the trans fluid is at normal operating temp.

On the other hand, if the worst happens and you really do need a new tranmsission, an independent MB mechanic can put in a factory rebuilt for a lot less than the dealership, and you can be assured that it will have all the updates, and will come with a really nice warranty.

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Steve
'93 400E