conanh,

When the starter is not installed, can you run power to the unit and check the solenoid operation? If it works ok when it is not installed, how about checking the Voltage at the connection to the starter to ground with the circuit is energized? There may be a problem with a section of the wire running down to the starter or even the connection to the starter (some are fittings on the end of the wire, fastened by crimped connections which can deteriorate over time - green copper corrosion products, etc.) that is dropping the voltage at the starter. It sounds like you are not getting the juice you need to the solenoid, or, there may be some issue with the starter mounting that makes the way the pinion gear hang up on its way out to the flywheel. If it was aimed too far inward, it might not be able to mesh the teeth on the pinion with the flywheel without a little extra degree of freedom that your rapping gives it. Just an idea. Good luck, and let us know what it is when you find it. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)