I have a Euro 1985 280SL. I think my frequency valve may be dead. It is not buzzing at all. I have tested my controller as outlined in the Probst book, and it appears to be working. The only time I can get anything out of the frequency valve is when I ground out the oxygen sensor lead as the book says to do. Then I get a duty cycle in the 90% range, and it will start to buzz. Otherwise nothing.

Any suggestions?

Thanks in advance.