I was just going to sugest re-soldering the pc board for the valve, but since you mentioned the burn marks, I would say replace it. I had the same problem on my fuel pump relay, after resoldering, it worked for a while but then started failing, this will repeat over and over again, but since the valve is expencive, I'd look for a used one, just go somewhere where they don't know what a benz is, grab some wires as a cover up, throw the valve over the fence and pick it up free on the other side
xp