It now controls the fan light with two 3-way switches and fan speed with the slider. The modification required the elimination of a speed. I misunderstood the switch is meant for fan control only. I had mistakenly thought it would control the light using 2 switches on a combination ceiling fan + light and the slider incorporated an off position. The slider has 3 positions for fan speed: slow, med, fast. I modified the circuit board eliminating high speed making this position - off. The 3-way now switch controls the fans' light, the slider provides fan control: off, low and medium speed.I do not know why a controller is already made in this manner but with the third speed. It would be a very small manufacturing change.