These are really great cars and easy to work on. Search on eBay for "Xentry" or "Star Diagonsis", you should be able to locate a setup with a Dell D830 laptop, a multiplexer, and cables for about $6-700. This will give you the diagnostic system needed to troubleshoot anything electronic on the car, as well as the complete set of WIS (repair manuals), EPC (exploded parts diagrams), and Star Finder (wiring diagrams and part locator with photos).

I bought mine from a seller called "gordysgadgets" and have had great success with it.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech