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34970A/34902A Labview VI Frequency Setup Issue

Question asked by jimpiet on Mar 20, 2014
34970A with the 34902A module plugged in to slot 2 and communicating via RS232 using LabView 2012. Using the Agilent 34970.lvlib:EZ Freq and Period vi.   Input to the card is 12V pulsed DC from a fan tachometer output.

The VI works and I get data but I need to set the voltage range to 100V.  When executing the VI, the agilent always switches to auto range.  I check this by viewing the settings through manual access to the front panel.  The issues is I am getting a random frequency when the fan is disconnected or fails when it should read 0Hz.  If I manually change the voltage range to 100V the reading is 0Hz.  If I change back to Auto it is again random.  I suspect that the Agilent is seeing some small amplitude frequency but nothing apears on a scope.  The fan tach output is pulled up to +12Vdc which is required.  I checked the string/format sent to the agilent and it is correct per the manual.  

Why is the unit automatically switching ranges?