First I want to thank for the excellent advice I was able to find in this forum. I searched the forum for all relevant info and was able to do a systematic analysis of the problem. It was so satisfying to actually step-by-step verify what was working and not, and finally pin-pointing the problem.

I have a European 1990 300E with no diagnostic module (i.e. no check engine light) so I don't know for how long I've had the 50% duty cycle fault. I have had a irritating rough idle and stalling during hard braking for quite some time. Otherwise the car is a dream and it seemed to have good power under all conditions. I was initially set out to "throw parts" at the problem. It seems that a lot of the initial suggestions for fixing a problem like mine is to replace the OVP and the fuel pump relay. I'm glad that I took the time to check the system with all sensors and understand how everything is interacting instead.
I ended up with a defective O2 sensor and a sticking idle control valve. The valve was thoroughly cleaned as described several places in this forum (soaking it using carb cleaner and repeating the process until it worked satisfactorily). The O2 sensor heating was defective and the sensor itself would never give good values, just indicating 0,05V - 0,15V (occasionally higher values). It was replaced and I was able to adjust the new one without trouble. I connected a Voltmeter to the sensor wire and warmed the engine. I had to richen the mixture more than 1/4 turn to achieve 0,5-0,7V, so it must have been lean (which I guess was good because I've seen posts in the forum indicating that a rich mixture could destroy the catalyzator). I disconnected the DVM and connected the sensor to the computer and was immediatley able to see closed loop control activity on the diagnostic socket (pin 3). As recommended by Stevebfl I then adjusted the duty cycle to approx 45% to have it slightly rich (which will improve things during cold operating condition and warm starts). Back to normal!

BTW, the original O2 sensor was about 200$, I checked alternative sources and found one for 100$ (with wrong connectors). I cut off the original connectors and connected them to the new sensor. Seems to do the job as I understand the sensors are more or less the same (operating area 0-1V) regardless of car brand.

During testing I discovered two things. There is a small leak on the EHA. I connected a meter and checked the mA current and it seemed to operate as described in the forum (staying negative during cold start cranking and warming-up condition and the after getting warm and in closed-loop it showed fluctuating readings around minus 3-4 mA. My conclusion is that it seems to work correctly (any comments??) and as long as I can live with the smell it will stay in the car and I will monitor the leak.
However, I got a little bit confused when I checked with the MB dealer and he told that they have hardly sold any EHA's the last couple of years. Reading this forum I got the feeling that a lot of people indicate possibly leaking EHAs whenever someone mentions fuel smell up- front???

While checking the idle contriol valve I found that after switching on the ignition the valve would be activated for approx. 10 sec and then stay off. Why this behaviour????

Thanks again and enjoy your Benz!

1990 300E 320000km