the black/yellow when grounded anywhere should shut off the motor. make sure your gound wire and that your black/yellow are good to the motor. you can take a wire and short the black/yellow to ground on the power head and see if it shuts down correctly. if so look at the wiring cable for issues. you can use a remote starter button like the one here in the pic i made. it works both as a starter button or clip on and use as a kill button.
