http://pages.prodigy.net/jforgione/MB_X11.html

I suggest bookmarking that site has some good info.

I have some documents that show what pins out of the 16 pin connector(in battery compartment) to connec to. To test the ecu connect the2 test leads from the test to pins 2 and 3.
To test mas for error codes connect it to 4 and 16(I think) might be wrong, woudl have to look at the document agian.
you probally have the cfi fuel injection system and would first the wiring harness for continuity then actually test the components of the fuel injection system.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)