On a two terminal connectors, if you know one of the terminals is going to be set to ground (when something else happens, say the climate control unit sets the monovalve's terminal to ground to open the valve) and the setting of ground is going to cause the device to operate, then one of the terminals would be this "control" terminal and the other terminal is likely to have power voltage or ground depending on the function and design of the device. So if you dont know which of the terminals is the "control" terminal, you could try both terminals with your black probe (one at a time) BUT only for one terminal you would get battery voltage when the ground is activated. For the other terminal you could get either battery voltage or no voltage, but will be fixed, would not change based on the external control (i.e. the climate control's setting). Hope this helps.

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Saumil S. Patel