Quote: I never thought about the 9 volt battery. I might try that and see what happens.
Common the negatives of the 9v and your 12v car battery (as long as the fan controller is looking for a 0v reference), and you should then be able to use the positive feed of the 9v as the trigger to the fan. I'd assume you'd be able to hear the difference in speed, but a tach would be nice to place on the fan blade base and get a real appreciation for the difference, if there is any
