I had the same problem.
I found out by reading the mc2100 datasheet, that the controller uses the green wire to send small digital signals to the control panel as well as the tach pulse that is sent up the green wire.
You can verify that by reading the signal by manually rotating the wheel past the sensor and reading its pulse. If you start the motor running, you can completely remove the sensor and you get your crazy (additional digital data signals) readings.
I first made a low pass filter to remove those additional signals, but they seemed to be around the same frequency as ~5mph sensor readings.
In the end, I added a hall effect switch that uses the same magnet the tach sensor (reed switch) does. The output from the hall effect sensor runs to an optocoupler on my circuit, then to my board.
Dave