Wiring a motion sensor -am iI missing something

[skunkman]

I am installing a motion sensor on my boat. It has 4 wires - red black white and green. The instructions say to connect the red to +12V, the black to -12V, the white to the security input on my command module and the green to a common ground. Why can't the green and the black both go to the negative post of my battery?
Thanks
K

 

[Henry Boyd]

Connecting the black to -12 (the negative terminal on battery) guarantees a complete electrical circuit, but not a ground. The green wire insures there will be a clear path to ground should a short occur. Without the green wire, and the device wired to the terminals only, a short would literally turn the device into an arc welder.

 

[hughespat57]

Most alarm sensors "switch to ground"
Meaning the white wire will go low (ground) to indicate that the sensor is active (sees motion)
The green wire should go to the alarm controller with the white wire and terminate to the ground or "common" sensor connection for motion.