The duty cycle read at the diagnostic socket is actually reading the negative side of the frequency valve. Remembering that duty cycle is arbitraily referenced one would do the same thing at the negative side of the frequency valve for the same reading.

The scope/DVOM would be hooked with the positive lead of the instrument to the negative side of teh valve and the negative side of teh instrument to ground. With a scope one will see voltage high when the valve is not commanded, and voltage low when the valve is commanded.

In this set up duty cycle ON would be greatest when the reading was low most of the time. In the normal BOSCH frequency valve task the higher on time lowers the lower chamber pressure and thus richens the mixture. On "grey market" cars the frequency valve often had a different type of function so I would look carefully at where it is hydraulically attached. If it is spliced into the control pressure circuit you will be much better off without it if you can do so legally.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician