I guess you will need a different multimeter. Duty-cylce is an extended feature of frequency. Frequency is how many times an event happens. Duty cycle refers to the caracteristic of an event.
An example: Dwell. In old points systems a six cylinder engine running at 2000 rpm would have the points openning 6000rpm or 100 times a second or 10ms for each event. Duty-cycle would be the portion of the time that the points are open vrs the time they are closed. If the points open larger their dwell and duty cycle will increase.
If you watch the event with a scope the duty cycle can be measured on the screen. Good multimeters now have frequency, rpm, and duty-cycle.
The modern automobile uses many duty-cycle related functions for control. The frequncy valve in early Lambda control systems was a valve that leaked fluid to lower a pressure. It buzzed at a constant frequency. The amount of fluid it passed related to duty-cycle. If during one cycle the valve was closed 90% and open 10% it would flow much less than if it was open 50% and closed 50% etc.
In the case of on-off ratio, the signal is a square wave at constant frequency and the duty-cycle fluctuates. On earlier cars with K-jet the duty-cycle showed relative mixture setting, closed loop condition and showed self-diagnostic faults by holding a designated constant duty-cycle, ie: 10%, 20%, 30%, 40% etc. Each had a particular meaning. Closed loop was indicated by oscillating values up and down (not constant).
By 1992 self diagnostics had improved to where the car not only could read codes but actual values to a scan tool. MB has been very tight with their equiptment and software. The only scanner sold to the public that works from company programming is the Bosch KTS300. A number of aftermarket companies have reverse engineered scanners that do varying amounts of whats possible. None of them do a fraction of what the factory scanner HHT (hand held tester) can do. MB has recently made their newest laptop scanner available to the aftermarket for the small sum of $27,000. Not bad for an IBM thinkpad. BTW there also is an $8,000 yearly subscription fee (that is included in the $27k for the first year).
We use the Bosch KTS300, an Assenmacher AST Retreiver, and an Impulse counter. We are working on the $27k.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician