I have seen this happen 3 ways.
1. B2 brake band breaks. (Tranny rebuild time)
2. B2 piston breaks, you can do this in the car.
3. B2 pin falls out. Take off the pan and look for a silver pin that is about an inch or so long. IF you find it in the pan, you must reinstall it. It is not easy, but can be done by just taking off the filter. I have done this before.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS