The spike is used to control the compressor. It is wired
to the relay outputs of the digital side car, these have the
row of LEDs. Refer to page 28 of the data sheet, there is
a picture.
http://www.usfirst.org/uploadedFiles...s-Rev-0-5a.pdf
The black line on the pwm cable goes to the gnd pin.
On the spike, the black line goes to the B pin.
You should be sure to run the compressor in the right
direction with your software.