You could use either wire. Know that yellow or brown/white is the ground and whatever unit you are switching with them needs 12V to the other side. So yellow or brown/white to one black wire on the valve, the other black wire should have 12V when the ignition is on. If you use the brown/white you can also shift on time from when the brake is released. If you are running heads up classes, most guys do this. Go into the software and setup the rpm and activation. The 7531 is pretty complicated and requires a bit of delving into because it wants to know how far the rpm drops on the shift so it recognizes it only as a shift and not a problem or a pedal of the throttle. The ignition should be setup so that it knows what gear you're in. Again, the instructions are excellent so concentrate on one area at a time and you will be a pro with it!