mixture settings and EHA current

[simmo300e]

Following the excellent advice in this forum, I rigged up a harness to check the EHA current to check if my mixture settings were correct.

KOEO was +20ma and at idle +3ma, fluctuating slightly when I revved the engine.

These figures are OK, if I understand the information correctly, but I could not move the needle off +3ma when adjusting mixture either way, even taking it to the point where it was too rich or too lean to restart the engine.

Tried the X11 sockets 2 and 3 and had a steady, slightly fluctuating reading of around 2.9v, which with a battery voltage of 13.2v equates to a duty cycle of 77% on my calculator. Again, despite adjusting the mixture both ways, could not get any change in voltage. The figure I was after was 4v, or 70%

I resorted to the squirt of carb cleaner method to adjust mixture and the engine's running fine but would like to check this scientifically. Am I doing something wrong?

Car is a 1987 300e, no cat, no lambda and if there's an oxygen sensor I can't find it. Maybe its a pre diagnostic model but if so, must be a way of checking the mixture surely.