EHA power loss
 

I am getting a code 9 when I use my break out harness, and I am getting a reading of 2.04 on my digital meter, if I reverse the lines I get a negative reading of 2.04. this is a 190E 2.6 1991. Can the harness be holding the reading back some how?:confused:

Same symptom here
 

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.

ECU
 

I had not thought of the ECU being bad, I have no Idea of how to test it, I did remove the EHA and made an atempt to reset the fuel threw the two M/M adjustment but to no avail. the pot is new and so is the OVP. My wife took the car this morning and said it has stalled every time she lets off the gas pedal.

Manual adjustment
 

With the steady EHA current you're seeing, it can't do anything useful. I would suggest, for now, just make the manual adjustment using the main mixture hex-head screw and set it so that the output of your O2 sensor averages around .45 volts. That's what I did and at least the car is now drive-able until I can swap the ECU.