Tete, I simply plugged the speed sensor plug "c" into the Acewell harness plug "C" as shown on the Ace schematic. Curiously, the Acewell schematic shows that the speed sensor WHITE connects to the Ace PURPLE, but on the actual wiring harness the speed sensor WHITE connected to BLACK which connects directly to ground (BMW brown). I thought I might have to connect white to the PURPLE but I tested it and it WORKS. I have not tried connecting white to purple because the speedo works, so I will leave it connected white to black. What does your schematic show? What does your wiring harness contain?