Clunger,

What do you mean that the 240D is almost stalled out when you jump the 300SD? Was there a big spark when the battery cables were connected? This would tell me that there was a big short somewhere in the 300SD. Or did the 240D almost stall out when the 300SD starter was energized? If the overload doesn't happen until the starter is energized, it sounds like a short in the starter. If a large current is being drawn from the batteries, it is NOT a problem with corroded terminals or bad connections. You are drawing too much current, not to little.

Do the headlights dim when the starter is energized? How about when both batteries are connected? If lights dim a lot, this is another indication that too much current is going thru the starter and is probably a bad starter even if it is a rebuilt starter.

P E H