Yeah will do pics tonight, and diagrams once i get the rpm issue sorted.
I think its a voltage/current supply issue. When the motors are powered, the rpm counter is unreliable, but let the power button go, and it counts properly.
Unless its plugged in to USB, in which case it works fine with the motors powered. My guess is that when the motors are running, either the voltage drops too low for the sensor to read properly, or they draw too much current and the sensor cant pull enough from the arduino. But with USB plugged in it can draw from that.
Might be as simple as switching the sensor to get power from the battery instead of the arduino.