I believe you are referring to the EHA. The duty cycle will vary depending on how much time the EHA is lean biasing versus how much time it is rich biasing the mixture to maintain and average stoichiometric mixture. (I think a higher than 50 percent duty cycle is an average lean bias, but I might have it backwards.)

The mechanical adjustment on the air flow meter should be set to achieve a duty cycle in a range of 45 to 55 percent at idle and 2500, no load, which places the EHA near the middle of its working range. It then will adjust fuel flow to maintain stoich. for varying air density, varying oxygen quantity in the fuel, etc.

Frequency and duty cycle are not the same thing. I'm not sure if the EHA frequency is constant (I don't think it is.), but the value we test from the X11 connector superimposes the postion of the EHA (rich or lean bias) on a 100 Hz signal generated by the ECU, which tells us on average if the EHA is biasing lean or rich.

I do not believe the EHA is serviceable. If defective MBZ says replace it, but enterprising individuals can sometime open up "non-serviceable" components and repair them.

There is an excellent writeup on testing and adjusting the duty cycle at:

www.landiss.com/mixture.htm

I highly recommend that all the DIYers print out these pages and file them with your service documentation or maintenance records. I sure did!

Duke