I attempted to use my Fluke meter as you described and I think my meter is slightly different than what you are describing. By pressing the "Hz" button twice the meter goes into the RPM mode for 2 cycle engines, by pressing the polarity button at this point does nothingand the reading is in the thousands.
My fluke meter has a %Duty/ms-Pulse button which goes directly into a duty cycle reading when pressed once, which is what I am doing. I was told that to check whether or not the reading is right pull off a coolant sensor and the reading should be a steady 30%, which is what I am getting after starting up a warm engine.
One thing I did notice when hooking my meter to socket #3 is the voltage is different when I have a constant 60% reading and ALL other times (even when I pull off the coolant sensor plug. I get a constant reading of approximately 5.5 volts before pressing the %duty cycle button when the percentage is 60% as opposed to approximately 9 volts at all other times. I checked the input voltage at the control box and in all cases it is 13+ volts (engine running).
I am testing the sensors as per the Mercedes manual and so far the following seem OK Full Load Contact OK Coolant Sensor OK Airflow Sensor OK O2 Sensor OK TD-Signal OK Altitude Sensor OK