What are the specs for current going through the EHA at idle or otherwise?
What is your procedure for setting it correctly?
(mine is a 1988 190E 2.3 8V)

If Lamda is adjusted properly??? there should be equal positive & negative ma readings.

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How's Lambda adjusted properly?

Here what I have been doing:
1. measure the current through the EHA
2. Measure and visualize using a scope the voltage between pins 2 and 3 on the test port on the driver side fender.
3. I attempted to adjust the EHA's hex screw clockwise for rich and counter clockwise for lean (chamber pressure differential in the FD) and tried to obtain currents within +1mA and -2mA. That's where I failed; I would get that for a second or two then it will either go to -10mA or +10mA depending how I fiddle with the fuel to air mixture screw (part 4).
4. I try to get a 50% duty cycle between pin 2 and 3 by adjusting the fuel to air mixture 3mm hex. That too drifts to either rich or lean as the EHA current drifts.

I need a better process that insures a lock!