Pretty much the same symptom on my 1990 300TE here - code 9 and stuck at 1.8 mils, not matter what. Did you check to make sure the EHA has a resistance of about 20 ohms, and that the ECU is getting power from the OVP? After testing and ruling everything else out, I've concluded that the output driver from my ECU is probably blown. I rigged up a simple circuit using 4 resistors plus a pot that I'm going to use to manually apply variable current to the EHA to see if it affects the mixture while the engine is running. If it does, that pretty much confirms the ECU is damaged.