A 78 shouldn't have the push button controller like it does. It should have a plate for a servo. Either the replaced it when they put the 85 engine in or the car is not a 78.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD